1. law, govt and politics
2. law enforcement
3. fire department

FIRE HISTORY AND CLIMATE-FIRE RELATIONS IN JASPER NATIONAL PARK,
ABERTA, CANADA
by
Raphaël Daniel Chavardès
B.A., The University of British Columbia, 2011
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
in
THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES
(Forestry)
THE UNIVERSITY OF BRITISH COLUMBIA
(Vancouver)
April 2014
© Raphaël Daniel Chavardès, 2014
Abstract
In mixed-conifer forests of western North America, fire ecologists and managers are
increasingly recognizing the prevalence and importance of mixed-severity fire regimes.
However, these fire regimes remain poorly understood compared to those of high- and lowseverity. To enhance understanding of fire regimes in the montane forest of Jasper National Park
(JNP), I reconstructed fire history and assessed forest composition, age and size structure at 29
sites (Chapter 2). Historic fires were of mixed severity through time at 18 sites, whereas the
remaining 11 sites had evidence of high-severity fires only. At the site level, mean importance
values of canopy trees were more even among coniferous species and greater for Pseudotsuga
menziesii at mixed-severity sites. The greater numbers of veteran trees and discontinuous age
structures were also significant indicators of mixed-severity fire histories.
In a second study, I crossdated tree ages and fire-scar dates for 172 sites and tested
whether historic fire occurrence depended on inter-annual to multi-decadal variation in climate
(Chapter 3). Eighteen fires between 1646 and 1915 burned during drought years, with a weak
association to El Niño phases and the negative phase of the Pacific Decadal Oscillation. Fire
frequency varied through time, consistent with climate drivers and changes in land use at
continental to inter-hemispheric scales. No fire scars formed since 1915, although potential
recorder trees were present at all sites and climate was conducive to fire over multiple years to
decades. Thus, the absence of fires during the last century can largely be attributed to active fire
suppression. Improved understanding of the drivers of the historic mixed-severity fire regime
enhances scientifically-based restoration, conservation, forest and wildfire management in the
Park and surrounding montane forests.
ii
Preface
My research in Jasper National Park is part of a research program on fire history and
climate-fire interactions in the western Canadian Cordillera that involves professors and students
from the Universities of British Columbia, Guelph, Western Ontario and Brock. For Chapter 2, I
resampled 29 sites established by the Foothills Research Institute (Rogeau 1999); however, I
held primary responsibility for the site selection, sampling design, the collection and analysis of
data. For Chapter 3, I recovered the data and samples from 172 sites established by the Foothills
Research Institute (Rogeau 1999). I analysed the samples using crossdating to enable original
climate-fire analyses. I prepared first drafts of all chapters. Throughout my research, I received
constructive feedback and support from my supervisor, Dr. Lori Daniels, and editorial advice
from all of my committee members: Drs. Peter Marshall, Sarah Gergel and Ze’ev Gedalof.
iii
Table of Contents
Abstract .......................................................................................................................................... ii
Preface ........................................................................................................................................... iii
Table of Contents ......................................................................................................................... iv
List of Tables .............................................................................................................................. viii
List of Figures ............................................................................................................................... ix
List of Abbreviations ................................................................................................................... xi
Acknowledgements ..................................................................................................................... xii
Dedication ................................................................................................................................... xiv
Chapter 1: Introduction ................................................................................................................1
1.1
Fire as a Disturbance Agent ............................................................................................ 1
1.2
Fire Regimes ................................................................................................................... 2
1.3
Research on Fire Regimes .............................................................................................. 4
1.4
Drivers of Fire ................................................................................................................. 6
1.4.1 Bottom-up controls ..................................................................................................... 6
1.4.2 Top-down controls ...................................................................................................... 8
1.4.3 Teleconnection interactions ........................................................................................ 8
1.4.4 Relative importance of bottom-up and top-down controls ....................................... 13
1.5
Fire and Forest Dynamics ............................................................................................. 14
1.6
Human Impacts on Fire Regimes .................................................................................. 15
1.7
Climate Change Impacts on Fire Regimes.................................................................... 18
1.8
Fire Regime Research in Jasper National Park, Alberta, Canada ................................. 20
1.9
Research Objectives ...................................................................................................... 22
iv
Chapter 2: Historical Fire Frequency and Severity in Jasper National Park, Alberta,
Canada ..........................................................................................................................................24
2.1
Introduction ................................................................................................................... 24
2.2
Study Area .................................................................................................................... 28
2.3
Materials and Methods .................................................................................................. 29
2.3.1 Site selection ............................................................................................................. 29
2.3.2 Fire scars ................................................................................................................... 29
2.3.3 Forest composition and structure .............................................................................. 30
2.3.4 Cohorts and veteran trees .......................................................................................... 31
2.3.5 Fire frequency and severity....................................................................................... 32
2.3.6 Fire history and stand dynamics ............................................................................... 33
2.3.7 Successional trajectories ........................................................................................... 34
2.4
Results ........................................................................................................................... 35
2.4.1 Forest composition and structure .............................................................................. 35
2.4.2 Fire history ................................................................................................................ 35
2.4.3 Fire frequency and severity....................................................................................... 36
2.4.4 Fire history and stand dynamics ............................................................................... 37
2.4.5
2.5
Successional trajectories ........................................................................................... 39
Discussion ..................................................................................................................... 40
2.5.1 Frequency and severity of historic fires .................................................................... 40
2.5.2 Changes to the fire regime during the 20th century ................................................... 41
2.5.3 Forest dynamics and influence of the historical fire regime ..................................... 43
2.5.3.1 Stand dynamics ............................................................................................... 43
v
2.5.3.2 Impacts of fire exclusion on successional trajectories .................................... 45
2.6
Conclusion .................................................................................................................... 48
Chapter 3: Temporal Climate-Fire Relations ...........................................................................60
3.1
Introduction ................................................................................................................... 60
3.2
Materials and Methods .................................................................................................. 65
3.2.1
Research design ........................................................................................................ 65
3.2.2
Field sampling ........................................................................................................... 65
3.2.3
Laboratory analysis ................................................................................................... 66
3.2.4
Fire history ................................................................................................................ 66
3.2.5
Local Pseudotsuga menziesii chronology as a proxy of drought .............................. 67
3.2.6
Climate-fire relations ................................................................................................ 69
3.2.7
Twentieth century climatic conditions and fire occurrence ...................................... 70
3.3
3.3.1
Results ........................................................................................................................... 71
Fire history ................................................................................................................ 71
3.3.2 Pseudotsuga menziesii chronology as a proxy of drought ........................................ 72
3.3.3 Climate-fire relations ................................................................................................ 72
3.3.4
3.4
Fire occurrence and drought ..................................................................................... 73
Discussion ..................................................................................................................... 74
3.4.1
Climatic variation and historic fires in Jasper National Park ................................... 74
3.4.2
Multi-decadal variation in fire frequency ................................................................. 77
3.5
Conclusion .................................................................................................................... 81
Chapter 4: Conclusions ...............................................................................................................95
4.1
Summary and Contribution to Research ....................................................................... 95
vi
4.1.1 Importance of dendrochronological analyses ........................................................... 95
4.2
Management Implications ............................................................................................. 96
4.2.1 Fire regimes and management .................................................................................. 96
4.2.2 Importance of mixed-severity fires for at risk Rangifer tarandus and Ursus arctos
populations ............................................................................................................................ 98
4.3
Future Research ............................................................................................................ 98
4.3.1 Importance of human versus lightning ignitions ...................................................... 98
4.3.2
Monitoring regeneration and fuel ............................................................................. 99
4.3.3 Climate-fire interactions ........................................................................................... 99
References ...................................................................................................................................101
vii
List of Tables
Table 2.1 Site attributes and species composition of the 29 study sites in the montane forests of
Jasper National Park ..................................................................................................................... 49
Table 2.2 Fire record descriptive statistics of the 29 study sites in the montane forests of Jasper
National Park ................................................................................................................................ 51
Table 3.1 Summary of the fire records for 115 sites west of the Athabasca River ...................... 83
Table 3.2 Summary of the fire records for 57 sites east of the Athabasca River.......................... 85
viii
List of Figures
Figure 1.1 Fire history study areas in Jasper National Park, Alberta, Canada ............................. 23
Figure 2.1 Fire history study area in Jasper National Park (top). The 29 fire-history sites in the
Athabasca River valley, 15 km north of Jasper townsite (bottom) ............................................... 52
Figure 2.2 Size-structure histograms of trees (dbh ≥ 5 cm) by species for the 29 fire-history sites
located along eight transects ......................................................................................................... 53
Figure 2.3 Age-structure histograms of trees (dbh ≥ 5 cm) by species for the 29 fire-history sites
located along eight transects ......................................................................................................... 54
Figure 2.4 Fire history from 1550 to 2012 at all 29 sites.............................................................. 55
Figure 2.5 Species-specific lags in tree establishment following (a) predominantly less severe
fires causing scars (left column) and (b) high-severity fires initiating cohorts only (right column)
....................................................................................................................................................... 56
Figure 2.6 Comparison of age- and size-structure continuity (top), evenness (middle) and
diversity (bottom) between fire severity classes ........................................................................... 57
Figure 2.7 Mean and standard errors of importance values by coniferous species and deciduous
trees according to fire history class (columns) and canopy layer (rows) ...................................... 58
Figure 2.8 Mahalanobis distances between canopy and fire history classes. Shorter (longer)
distances imply greater similarity (difference) between classes ................................................... 59
Figure 3.1 Fire history sites (n = 172) in the Athabasca River valley in montane and subalpine
forests of Jasper National Park ..................................................................................................... 86
Figure 3.2 Fire history records from 1465 to 2012 at 172 sites .................................................... 87
Figure 3.3 Composite fire records from 1600 to 2012 for a) the 115 sites west of the Athabasca
River, b) the 57 sites east of the Athabasca River and c) all 172 sites in the study area .............. 88
ix
Figure 3.4 Climate-growth relations for Pseudotsuga menziesii from 1902 to 2009 ................... 89
Figure 3.5 Fire occurrence (black dots) from 1646 to 1985 relative to inter-annual to multidecadal variation in climate .......................................................................................................... 90
Figure 3.6 Departure (%) from the mean of fire years (year 0) relative to the four years preceding
and two years following fire for the a) residual chronology, b) ENSO, c) PDO and d) AMO
climate patterns from 1642 to 1917 .............................................................................................. 91
Figure 3.7 Tests of association between fire (black) and non-fire (white) years and climatic
variation ........................................................................................................................................ 92
Figure 3.8 Annual ring-width indices by fire years and periods from 1646 to 2009 .................... 93
Figure 3.9 Annual precipitation (top), maximum temperature (middle) and heat-moisture indices
(bottom) by period from 1916 to 2009 ......................................................................................... 94
x
List of Abbreviations
AMO – Atlantic Multidecadal Oscillation
BC – British Columbia
BNP – Banff National Park
ca. – circa
dbh – diameter at breast height
ENSO – El Niño-Southern Oscillation
IPCC – Intergovernmental Panel on Climate Change
JNP – Jasper National Park
m.a.s.l. – meters above sea level
PDO – Pacific Decadal Oscillation
SLP – sea level pressure
SST – sea surface temperature
TSLF – time since last fire
USA – United States of America
xi
Acknowledgements
For technical support, I acknowledge: Dr. J. Wilmshurst and D. Smith at Parks Canada, Dr. R.
Bonar at Hinton Wood Products and Dr. D. Andison, Healthy Landscapes Program at the
Foothills Research Institute.
For funding, I acknowledge: Hinton Wood Products and the Natural Sciences and Engineering
Research Council of Canada for supporting my Industrial Postgraduate Scholarship.
For leadership, I acknowledge: Dr. L. Daniels.
For direct support, I acknowledge: Dr. D. Andison (Healthy Landscapes Program at the Foothills
Research Institute); Dr. L. Daniels, G. Greene, S. Desroches, T. Martin, A. Dobko, O.
Villemaire-Côté, (Tree-Ring Laboratory at the University of British Columbia); V. Leung, G.
Lee, L. Gunther (Centre for Advanced Wood Processing and Timber Engineering at the UBC
Faculty of Forestry); C. Johansson, N. Hodges, D. Aquino (UBC Forest Sciences Information
Technology); H. Purves (Parks Canada GIS technician); Drs. S. Aitken, Z. Gedalof, S. Gergel,
V. LeMay, P. Murphy, H. Nelson, M. Pisaric, T. Sullivan; T. Wang, S. Watts (university
faculty); Drs. J. Innes, P. Marshall, C. Prescott, (Deans of the UBC Faculty of Forestry); B.
Bresnahan (chainsaw instructor); G. Kosh, D. Naidu, R. Poirier-Vasic (UBC Faculty of Forestry
officers and administrators); C. Seto, R. Cheng, C. Mutia, A. Chan, N. Thompson (UBC
Department of Conservation and Forest Sciences); Drs. C. Hudecek-Cuffe and J. Elliott
(archaeologists); UBC Faculty of Forestry custodians.
For indirect support, I acknowledge: Elder L. Grant, Dr. P. Shaw (First Nations Languages
Program); Dr. T. Jones, Dr. M. Amoroso, Dr. M. Baradeih, Dr. A. Srur, Dr. E. Arbellay, A.
Innerd, T. Maertens, E. Jones, H. Marcoux, J. Amerongen Maddison, A. Pogue, A. Hussain, M.
xii
Labonté, H. Erasmus (Tree-Ring Laboratory at UBC); Drs. C. Breuil, A. Carroll, P. Evans, A.
Kozak, R. Guy, R. Hamelin, J. Richardson (UBC Faculty of Forestry).
For spaces and places, I acknowledge: the Tree-Ring Laboratory @ UBC; Association for Fire
Ecology; UBC Acadia Family Housing, Faculty of Forestry Office of the Dean, CAWP,
Department of Conservation and Forest Sciences, Faculty of Graduate and Postdoctoral Studies;
Tree-Ring Society; Wildland Fire Canada; Musqueam First Nation; UBC; provinces of BC and
Alberta; country of Canada.
I also acknowledge the following: Gregory G. and Elizabeth W.; Anusheh and Rumana M.;
Marvin B. and Miles P.; Marina R.; Alexandra P., Paul P., Ryan P, Norma W., Stephanie T.,
Jordan B., Anthony R., Arnaud D.G., Joane E., Ting P., Jiayin S., Fernanda T. (UBC Faculty of
Forestry graduate students); David A., Lori D., Sarah A., Jacob A., Marshall and Aby; David R.
and Samuel R. (East Creek, UBC Haney-Malcolm Knapp Research Forest); Patrick W. and
Christine B. (Madagascar Conservation and Development); Yuli V.; Alice C.; Futao G.; Michel
S.; Ska-Hiish M.; Michael T.; Sarah D.; past and current friendly neighbours and staff in UBC
Family Housing; Kathleen L. and Catherine T. (UBC Gymnastics); Master Paul F., Michael E.,
Kenneth L. (World Taekwondo Federation); Luis P. (Jogo do Pau); Riyad Z. (UBC Kinematics);
Niel D., Andrew D., Julian M., Marco N., Thomas P., David S. and their families; Francis N.
(Shotokan); friends who shared spaces in different countries and academic places.
I acknowledge my family, including my extended family; I hug each one of you, or remember a
moment when I did so.
In particular, I acknowledge Adam Chavardès for consistent patience, understanding and
encouragement. Thank you.
xiii
Dedication
To the Athapascan, Algonkian and Salishan cultures,
Jasper National Park,
Seuk Jin Ko
and Adam.
xiv
Chapter 1: Introduction
1.1
Fire as a Disturbance Agent
In North American forests, fire is the dominant causal agent of disturbance (Swetnam and
Betancourt 1990; Dale et al. 2001; Pierce et al. 2004; Schoennagel et al. 2004; Westerling et al.
2006). A disturbance is “a relatively discrete event in time which disrupts ecosystem, community
or population structure and changes resources, substrate availability, or the physical
environment” (White and Pickett 1985, p. 7). According to this definition, disturbances alter the
state and trajectory of ecosystems, drive spatial heterogeneity among forest patches that are
disturbed at different times, and drive temporal changes in forests (Franklin et al. 2002; Turner
2010).
Individual disturbance events are characterized by a set of descriptors organized into five
dimensions including the causal agent, spatial factors, temporal factors, magnitude and
synergism (Heinselman 1981; White and Pickett 1985; Turner et al. 1998; Morgan et al. 2001;
Turner 2010). Spatial factors include the location and extent of fire including size and shape of
the area burned. Temporal factors include the start and end dates of the fire event yielding the
timing and seasonality of fire. Magnitude is measured in two ways. Fire intensity is the direct
physical energy per unit area per unit time generated by the fire and is often measured as
temperature or height of the flaming front. Fire severity is an indirect measure of the impact of
fire on vegetation mortality and survival and depends on species adaptations and susceptibility to
fire. Synergism includes the influence on and interaction with other disturbances such as an
insect outbreak or a blowdown, which can increase the amount of surface and ladder fuels and
1
influence fire size and magnitude. Interactions can be immediate or lagged, depending on shortand long-term disturbance effects and legacies.
1.2
Fire Regimes
A disturbance regime describes the spatial and temporal dimensions of successive
disturbances affecting a delineated landscape over a determined time frame (White and Pickett
1985; Agee 1993; Turner 2010). Spatial dimensions include the distribution of events in a
landscape and the associations with topography (e.g. elevation, slope aspect, steepness, and
position and landscape configuration). Temporal descriptors vary depending on fire severity. If
the fire regime is of low severity then the fire frequency is quantified as the number of fires per
unit time or its inverse, the interval(s) between fires. If the fire regime is of high-severity then the
fire frequency is quantified as a fire cycle or rotation, the years required to burn an area equal in
size to the study area. Both approaches measure variation in frequency to estimate predictability,
the scaled inverse function of variation in the fire frequency.
Of the disturbance descriptors for fire regimes, frequency and severity tend to be
inversely related. These two descriptors are commonly used to identify a particular fire regime
(Flannigan et al. 2003; Schoennagel et al. 2004; Wright and Agee 2004; Halofsky et al. 2011);
although, fire intensity (Heinselman 1981; Johnson and Van Wagner 1985) and dominant or
potential vegetation of an ecosystem (Agee 1981) have also been associated with fire regimes.
Traditionally, research on fire regimes has focused on low- and high-severity fire regimes, at
opposite sides of the fire-severity spectrum. Low-severity fire regimes are represented by
frequent surface fires, which have minimal mortality effects on trees and reduce surface fuels,
2
hence the name stand-maintaining regimes (Agee 1993; Swetnam et al. 1999; Heyerdahl et al.
2001; Schoennagel et al. 2004). Conversely, high-severity fire regimes are represented by
infrequent crown fires which have high mortality effects on trees and significantly reduce surface
and canopy fuels, hence the name stand-replacing regimes (Agee 1993, 1997; Turner et al. 1994;
Sibold and Veblen 2000).
Mixed-severity fire regimes describe the complex patterns and effects of heterogeneous
fires across a range of spatial and temporal scales (Lertzman and Fall 1998; Perry et al. 2011). At
the landscape scale, fires can burn with a range of severities simultaneously or through time
(Halofsky et al. 2011). Temporally, successive fires at one location burn with a range of
severities resulting in compositionally and structurally diverse stands (Heyerdahl et al. 2012).
Fire severity fluctuates spatially and temporally due to the control by and interactions among
weather, climate, topography, vegetation composition and structure, and historical disturbances
including anthropogenic activities (Turner 2010; Perry et al. 2011).
Over the past 15 years, mixed-severity fire regimes have garnered increased recognition,
particularly in mixed-conifer forests. However, the disturbance dimensions of mixed-severity fire
regimes remain inadequately understood in comparison to low- and high-severity fire regimes
(Halofsky et al. 2011; Perry et al. 2011; Marcoux et al. 2013). Improving understanding of
mixed-severity fire regimes represents an opportunity for landscape managers to develop
strategies for more sustainable harvest of timber, better conservation of biodiversity and
supporting forest resilience to environmental change.
3
1.3
Research on Fire Regimes
Traditionally, different research approaches and methods have been applied to
reconstruct fire frequency, depending on the dominant fire regime and its distinctive impacts on
forest structure (Amoroso et al. 2011). With the shift in research toward fire severity as a key
attribute of fire regimes, new methods have been developed to measure residual living trees,
propagules and structural legacies like snags and coarsewood after a fire to quantify severity. For
recent fires, direct measurements can be made using remote sensing techniques (Soverel 2010).
Remote sensing combined with field verification is used to detect fire severity through
delineation and classification of disturbed patches using percent crown cover and canopy closure
which indicate different levels of mortality and post fire remnants (Hessburg et al. 2007). It can
be used to differentiate variable effects within a fire that leads to patches of low, moderate and
high tree survivorship inside the fire perimeter (Lentile et al. 2006; Soverel 2010; Andison
2012).
For distinct past fires, severity can be indirectly measured using dendrochronological
reconstructions of tree age structures including living and dead trees (Sherriff and Veblen 2006).
Low-severity fires are represented by age structures in which ≥40% of trees survived the fire and
the post-fire recruitment cohort includes ≤20% of trees in the stand (Sherriff and Veblen 2006).
For high-severity fires, ≤19% of trees survive and the post-fire recruitment cohort includes ≥71%
of trees (Sherriff and Veblen 2006). For moderate-severity fires, 20-70% of trees survive and the
recruitment pulse includes 20-70% of trees (Sherriff and Veblen 2006).
4
Frequent, low-severity fires in which a high proportion of trees survive but form firescars often result in rich fire-scar records (Swetnam et al. 1999). To infer fire frequency in this
type of disturbance regime, fire-scars preserved in the boles of trees are used (Agee 1993; Brown
and Swetnam 1994; Heyerdahl et al. 2001). Fire years are determined by crossdating annual
rings to ensure the exact calendar year is assigned to each fire scar (Dieterich and Swetnam
1984). For long-lived trees, fire-scar records provide fire event inventories over decades to
centuries (Fulé et al. 1997; Brown et al. 1999). However, individual trees are not perfect
recorders of fire. Once scarred, trees are more susceptible to subsequent scarring but not all fires
will produce a scar on all trees (Swetnam et al. 1999). To alleviate this limitation, multiplescarred trees are sampled within a site to provide a complete fire record at the site or stand level.
To understand historic fire size as well as frequency specific methods are required. Since
fire frequency depends on the size of the area sampled, i.e., more fires are likely to be detected as
the sampled area increases, the area sampled at each site should be consistent to directly compare
fire frequency. Reconstructing fire size is challenging since all trees within the fire do not form
scars and fire perimeters do not result in distinct boundaries that are persistent and visible in air
photos (Swetnam et al. 1999). Instead, spatially explicit networks of sites can be sampled and
burn likelihood models are used to estimate size (Jordan et al. 2005, 2008; Hessl et al. 2007;
Kernan and Hessl 2010; Swetnam et al. 2011).
In contrast to low-severity fires, high-severity fires lead to even-aged forests in patches
with distinct boundaries (Agee 1993). To quantify a high-severity fire regime, fire history across
a landscape is summarized using stand-origin (Heinselman 1973) or time-since-fire (Johnson and
5
Van Wagner 1985) maps. The forests are mapped to delineate discrete forest stands (Johnson and
Gutsell 1994). Stand tree ages and fire scars located at stand boundaries may be used to derive
stand origin and in conjunction the time since last stand-replacing fire (Johnson and Gutsell
1994). The data are then summarized in a stand-origin map where the age class distribution is
used to estimate the fire cycle and fire frequency (Heinselman 1973). However, this approach
assumes all fires are high-severity, stand-replacing fires with distinct and persistent boundaries
(Van Wagner 1978; Van Wagner et al. 2006). A modification of the stand-origin approach is
obtained via time-since-fire maps. Time-since-fire maps delineate stands by the last fire event,
which was not necessarily a stand-replacing fire (Johnson and Van Wagner 1985).
Given complexities inherent to mixed-severity fire regimes, a range of research
approaches are necessary to quantify spatial and temporal variation at stand to landscape scales.
To reconstruct fire history at the stand scale, tree-rings are used to find evidence of past fires of
different severities through time (Amoroso et al. 2011). Evidence of low-severity surface fires
are found in fire scars and in stands with uneven age structures and veteran trees (Swetnam et al.
1999; Heyerdahl et al. 2012). Conversely, high-severity stand replacing fires result in few or no
veteran trees, even-aged stands and fast-growing shade-intolerant species (Turner 2010). Thus
fire-scar records combined with detailed forest age structures can help to reconstruct fire history
and quantify fire frequency and return intervals (Sherriff and Veblen 2006; Amoroso et al. 2011;
Heyerdahl et al. 2012).
6
1.4
1.4.1
Drivers of Fire Regimes
Bottom-up controls
Fire regimes are driven by bottom-up and top-down controls (Agee 1993; Cyr et al. 2007;
Heyerdahl et al. 2001, 2007; Lertzman and Fall 1998; Perry et al. 2011). Bottom-up controls
influence fuel abundance and combustibility, which determine the behaviour and spread of
individual fires and cumulatively determine the fire regime through time (Rothermel 1983;
Heyerdahl et al. 2007). Topography is a strong bottom-up control (Agee 1993; Sibold and
Veblen 2000; Taylor and Skinner 2003). Due to orographic effects, an increase in elevation is
related to a decrease in temperature and an increase in precipitation such that low elevations are
warm and dry and high elevations are cool and moist (Agee 1993). The differences in
microclimate affect productivity along the elevation gradient and influence forest composition,
structure and fuel abundance (Heyerdahl et al. 2001). Growing seasons and fire seasons are
longer at low elevations compared to high elevations, resulting in increased fuel combustibility
and more frequent but less severe fires (Agee 1993; Heyerdahl et al. 2001).
Local-scale slope aspect, steepness and position also affect fire behaviour and spread
resulting in different fire effects and regimes (Agee 1993). In the northern hemisphere, westerly
and southerly aspects tend to have warmer and drier local climates, whereas easterly and
northerly aspects tend to be cooler and more mesic (Barrett 1988; Taylor 2000; Heyerdahl et al.
2007). Steeper slopes lead to accelerated fire spread (Heinselman 1981; Barrett 1988; Agee
1993). Fires igniting at the top of a slope tend to spread downhill or laterally, described as
backing and flanking behaviour, respectively (Agee 1993). More common, fires igniting at the
bottom of a slope tend to have head fire behaviour with greater effects (Agee 1993). Landscapes
7
configured with narrow valleys and mountain passes can funnel winds which drive the
movement of fire, whereas streams and unproductive soils can hinder fire spread (Romme and
Knight 1981; Agee 1993; Taylor and Skinner 1998, 2003). Landscapes with complex topography
are often documented as driven by bottom-up controls influencing fire effects and regimes
(Veblen et al. 2000; Heyerdahl et al. 2001; Schoennagel et al. 2004; Heyerdahl et al. 2007).
Site-level disturbance history is another bottom-up control (Agee 1993; Taylor and
Skinner 2003; Schoennagel et al. 2004). Disturbance history includes, but is not limited to, past
fires, bark beetle epidemics, extensive blowdowns, and human impacts such as fire exclusion
(Turner 2010). The disturbance(s) can lead to changes in fuel accumulation influencing fire
behaviour, effects and regimes (Agee 1993; Taylor and Skinner 2003; Schoennagel et al. 2004).
1.4.2
Top-down controls
Weather and climate are top-down controls which limit fire occurrence and directly affect
fire regimes at regional scales (Cyr et al. 2007). Weather, the condition of the atmosphere at a
given place and time, influences fire occurrence and behaviour (Agee 1993). Warm
temperatures, low relative humidity, and strong winds decrease fuel moisture, whereas
precipitation increases it. Combined, these weather attributes affect fuel combustibility,
probability of ignition, and rate of fire spread (Agee 1993; Taylor et al. 1996). Climate, the
statistics of weather events at a given place over a given period of time, influences fire regimes
(Swetnam and Betancourt 1990; Swetnam 1993). Spatial variation in climate determines the
location and extent of droughts and wetter than normal conditions (Swetnam and Betancourt
1990; Watson and Luckman 2001a, 2001b, 2002, 2004, 2005; Mote et al. 1999; Pohl et al. 2002;
8
Westerling et al. 2006; Swetnam and Anderson 2008). Inter-annual to inter-decadal scale
variation in climate determines the frequency of droughts conducive to fire or wet intervals
hindering fire (Hessburg et al. 2005; Schoennagel et al. 2005).
Dry conditions conducive to fire in western North America have been associated with the
El NiñoSouthern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic
Multidecadal Oscillation (AMO) (Kitzberger et al. 2007; Swetnam and Anderson 2008). These
oscillations exhibit complex teleconnections and interactions driving regional climate patterns.
ENSO results from oceanic-atmospheric interactions originating in the tropical Pacific and
Indian Oceans with teleconnections throughout the Pacific basin (Cook 1992; Diaz and Kiladis
1992; Allen 2000). ENSO is manifest through fluctuations between opposite phases called La
Niña and El Niño, reoccurring intermittently over periods of 2-7 years (Cook 1992; Diaz and
Kiladis 1992; Allen 2000). During La Niña phases, high sea level pressure (SLP) dominates the
eastern tropical Pacific Ocean strengthening the easterly tradewinds across the surface of the
tropical Pacific so that warm surface waters accumulate in the western Pacific and cool surface
waters occupy the eastern Pacific (Sarachik and Cane 2010). The conditions are reversed during
El Niño phases. The SLP differential weakens or reverses and warm surface waters spread across
the equatorial Pacific Ocean and accumulate along the coasts of North and South America
(Sarachik and Cane 2010). The variation in coastal sea surface temperature (SST) affects the
strength and latitudinal position of the northern subtropical high pressure ridge located between
ca. 30°N (border between the northern Sonora region, Mexico and southern California, United
States of America (USA)) and ca. 50°N (border between northern Washington State, USA and
the southern portion of the province of British Columbia (BC), Canada) depending on the ENSO
9
phases and season. In combination, the SSTs and subtropical ridge influence the temperature and
moisture of air masses over western North America (Mote et al. 1999; Kitzberger et al. 2001;
Sarachik and Cane 2010). In particular, at different locations of western North America ENSO
tends to influence winter-spring precipitation with decreased (increased) rates which can
facilitate (hinder) fire activity during the growing season (Mote et al. 1999; Kitzberger et al.
2001; Westerling and Swetnam 2003). During El Niños, the subtropical ridge is located further
north towards 50°N due to warmer surface coastal waters. During winter and the subsequent
spring, dry air masses persist in the Pacific and inland northwest North America including
southern BC (Mote et al. 1999; Pohl et al. 2002; Westerling and Swetnam 2003; Heyerdahl et al.
2008). The location of the ridge and air masses redirect wet air masses into the southwest USA
(Cole et al. 2002; Westerling and Swetnam 2003) and onto the coasts of northern BC and Alaska
(Minobe 1997; Gershunov et al. 1999; Mantua and Hare 2002; Hartmann and Wendler 2005).
The conditions are different during La Niña phases with the subtropical ridge located
further south towards 30°N due to cooler coastal surface waters. During winter and the
subsequent spring, dry air masses persist in the southwest USA (Cole et al. 2002; Westerling and
Swetnam 2003). The southern location of the ridge deflects storms northward such that cool, wet
air masses flow into the Pacific and inland northwest North America (Mote et al. 1999; Pohl et
al. 2002; Westerling and Swetnam 2003; Daniels et al. 2007; Heyerdahl et al. 2008) and onto the
coasts of northern BC and Alaska (Mantua and Hare 2002; Hartmann and Wendler 2005). The
strength and latitudinal position of the northern subtropical high pressure ridge also depends on
seasonality. During the summer, the subtropical ridge shifts towards 50°N such that the Pacific
and inland northwest North America are dry and warm whereas wet air masses are deflected
10
northward towards the coasts of northern BC and Alaska (Sarachik and Cane 2010). The
condition of the air masses over a given location influence fire activity and consequently high
fire activity tends to occur in the southwest during La Niña phases and in the Pacific and inland
northwest during El Niño phases (Morgan et al. 2001).
The PDO is defined as the leading mode of Pacific Ocean SST variability north of 20°N
(Mantua et al. 1997; Minobe 1997, 2000) and is strongly correlated to winter SLP in the North
Pacific related to the intensity of the Aleutian Low located ca. 60°N (Beamish et al. 1997; Wyatt
et al. 2011). The PDO varies over periods of 20 to 40 years, oscillating between positive, warm
and negative, cool phases (Gedalof and Smith 2001; Mantua and Hare 2002), which drive multidecadal climatic regimes in western North America (Cole et al. 2002; Smith et al. 2004;
Heyerdahl et al. 2008; Trouet et al. 2009). During the positive phase of the PDO, the Aleutian
Low is intensified, drawing warm, wet air masses towards North America (Mote et al. 1999;
Mantua and Hare 2002; Pohl et al. 2002; Hartmann and Wendler 2005); however, the
distribution of moisture depends on the position of the subtropical high pressure ridge and the jet
stream. An intensified Aleutian Low diverts the regional jet stream northward and southward of
the subtropical high, forcing storms to bypass the Pacific and inland northwest North America,
whereas a weakened and displaced Aleutian Low directs rainstorms to the Pacific and inland
northwest North America (Mote et al. 1999; Pohl et al. 2002). The combined effect of western
North American coastal SSTs, the Aleutian Low, the subtropical ridge and the regional jet stream
creates wet/dry dipole occurring between 30 and 50°N over western North America (Swetnam
and Anderson 2008; Taylor et al. 2008).
11
The AMO is a pattern of multidecadal variability in the North Atlantic Ocean SST
(averaged across 0-60°N and 75-7.5°W) influenced by the Atlantic Meridional Overturning
Circulation (Kerr 2000; Sutton and Hodson 2003; Wyatt et al. 2011). The AMO varies between
warm (positive) and cool (negative) phases every 50-100 years and causes temperature and
precipitation deviations in western USA over decades (Swetnam and Betancourt 1998; Enfield et
al. 2001; Gray et al. 2004; McCabe et al. 2004, 2008; Sutton and Hodson 2005; Brown 2006;
Knight et al. 2006; Wang et al. 2010; Wyatt et al. 2011). During the positive phase of the AMO,
warm, dry air masses tend to occur over the southwest USA between March and November
(Knight et al. 2006; Wang et al. 2010) and wet air masses occur over the northwest USA except
for the summer season (Wang et al. 2010).
1.4.3
Teleconnection interactions
The teleconnections interact and influence fire activity in different parts of western North
America. El Niño influences on regional precipitation and temperatures are enhanced during
positive PDO phases but moderated during negative PDO phases (Gershunov and Barnett 1998;
Cole et al. 2002; Westerling and Swetnam 2003; Heyerdahl et al. 2008). Conversely, La Niña
influences are enhanced during negative PDO phases but moderated during positive PDO phases.
Wang et al. (2010) found negative PDO/La Niña and positive AMO phases resulted in regional
drought conditions in southwest USA whereas in northwest USA, regional drought conditions
were related to positive PDO/El Niño and negative AMO phases (Hidalgo 2004; McCabe et al.
2004; Sutton and Hodson 2005). The resulting regional droughts in different parts of western
North America which were related to the interactions between the ENSO, PDO and AMO, were
also associated with widespread wildfires (Kitzberger et al. 2007; Trouet et al. 2010).
12
1.4.4
Relative importance of bottom-up and top-down controls
The relative importance of bottom-up and top-down controls on fire regimes varies by
type of ecosystem and by location (Schoennagel et al. 2004; Meyn et al. 2007). In dry, lowerelevation forests such as ponderosa pine (Pinus ponderosa Douglas ex C. Lawson) woodlands or
higher-elevation open pine forests in the southwest USA, bottom-up controls tend to be most
limiting (Baisan and Swetnam 1990; Grissino-Mayer and Swetnam 2000; Heyerdahl et al. 2006).
Fuels are sufficiently desiccated by the regular periods of warm, dry weather that climate is less
of a fire regime control. Instead, fine fuel and ladder fuel accumulation limits fire spread (Fulé et
al. 1997; Hessburg et al. 2005; Heyerdahl et al. 2006) and short periods of above-average
moisture conditions can enhance fuel production (Baisan and Swetnam 1990; Grissino-Mayer
and Swetnam 2000).
In cool, humid and dense higher-elevation subalpine forests and boreal forests, fire
occurrence is most limited by top-down regional climate (Payette 1992; Turner and Romme
1994; Agee 1997; Buechling and Baker 2004; Schoennagel et al. 2004; Podur and Martell 2009).
A significant drying period is necessary for fuel combustion (Agee 1993). In long periods
between infrequent fires fuels accumulate (Veblen 2000; Schoennagel et al. 2004). Fires burn
during the infrequent droughts and the effects are severe (Sibold and Veblen 2000; Buechling
and Baker 2004). In subalpine and boreal forests, climate predominantly influences fire severity
and spread through fuel desiccation and wind, not fuel abundance (Schoennagel et al. 2004;
Podur and Martell 2009). Mixed-severity fire regimes include low- to moderate-severity, standmaintaining fires and high-severity, stand-replacing fires at stand and landscape scales because
13
of complex interactions between bottom-up and top-down controls driving fire at the stand level
(Agee 1998; Lertzman and Fall 1998; Taylor 2000; Heyerdahl et al. 2001, 2012; Buechling and
Baker 2004; Schoennagel et al. 2004; Arno and Fiedler 2005; Baker et al. 2007; Amoroso et al.
2011; Halofsky et al. 2011; Perry et al. 2011).
1.5
Fire and Forest Dynamics
Fires of different frequency and severity strongly influence forest development and
dynamics (Taylor and Skinner 1998, 2003; Fulé et al. 2003; Van Wagner et al. 2006; Amoroso et
al. 2011; Perry et al. 2011; Halofsky et al. 2011). Low-severity fires cause mortality of
understory plants but mature, fire-tolerant thick-barked trees survive (Swetnam and Baisan 1996;
Kimmins 2004; Hessburg et al. 2007). Following low-severity fires in which <20% of trees die,
growing space and resources remain limited for subsequent tree establishment (Schoennagel et
al. 2004; Heyerdahl et al. 2006). Following repeated low-severity fires, the resulting forests are
heterogeneous, composed of patches of regeneration and uneven-aged canopy trees with few
subcanopy trees recruiting to the canopy over any given decade (Agee 1993; Schoennagel et al.
2004; Hessburg et al. 2007; Heyerdahl et al. 2012).
In contrast, following a high-severity fire in which ≥80% of trees die and mineral soil is
exposed, the open growing conditions are conducive to regeneration of exposure-requiring and
shade-intolerant species (Turner et al. 1997; Schoennagel et al. 2004, Turner 2010), as well as
some species that tolerate but do not require shade (Antos and Parish 2002). Following canopy
closure and the stem-exclusion stage of stand development (Oliver and Larson 1996), subcanopy
conditions favour the regeneration of shade-tolerant species, which are often thin-barked and
14
fire-intolerant. In mature stands, canopy gaps form as trees die which release the growth of
subcanopy shade-tolerant trees and facilitate establishment of new trees (Oliver and Larson
1996). The resulting increase in structural diversity also increases ladder fuels (Oliver and
Larson 1996; Franklin et al. 2002; Turner 2010). Over long periods between fires, these
processes result in structurally complex, uneven-aged forests of predominantly shade-tolerant but
thin-barked fire-intolerant trees (Agee 1993; Schoennagel et al. 2004; Van Wagner et al. 2006;
Turner 2010).
Mixed-severity fire regimes do not fit traditional successional trajectories and stand
development stages due to the variation in fire effects within and among fires (Fulé et al. 2003;
Halofsky et al. 2011; Perry et al. 2011; Marcoux et al. 2013). Since individual fires may be of
low, moderate or high severity, variable amounts of subcanopy and canopy trees are killed. As a
result, changes in stand structure and composition, patterns of post-fire recruitment and age
cohort structure are also variable (Agee 1998; Hessburg et al. 2007; Amoroso et al. 2011;
Marcoux et al. 2013). Thus, mixed-severity fire regimes result in forests with more complex
species, structures and dynamics (Hessburg et al. 2007; Halofsky et al. 2011; Heyerdahl et al.
2012; Marcoux et al. 2013).
1.6
Human Impacts on Fire Regimes
Human impacts on fire regimes vary among forest types, given differences in
accessibility, land-use and the relative importance of fuels versus climate for determining fire
frequency and severity (Schoennagel et al. 2004). Low-elevation forests are more accessible and
land-use practices such as fire ignition, logging, livestock grazing, and fire suppression have
15
strongly influenced fire regimes (Barrett and Arno 1982; Veblen et al. 2000; Taylor and Skinner
2003; Schoennagel et al. 2004; Jensen and McPherson 2008). In North America, frequent
ignitions by First Nations people (also known as Native Americans in the USA) added to fire
occurrence caused by lightning (Barrett and Arno 1982; Dey and Guyette 2000; Turner et al.
2003; Murphy et al. 2007; Miller 2010). First Nations used fire as a management tool to facilitate
the growth of food plants, improve forage for grazers, drive animals when hunting, clear
vegetation along travel corridors and to reduce wildfire hazards around communities (Lewis
1985; Bahre 1991; Agee 1993; Kay 1995; Murphy et al. 2007; Miller 2010). As Europeans
arrived and settled throughout North America, First Nations were displaced to reserves (Neu and
Graham 2006) and their traditional use of fire declined (Malone et al. 1991). As more Europeans
arrived and settled, they used fire to clear land for housing, pastures and mineral exploration and
to burn logging slash (Barret and Arno 1982; Wadleigh and Jenkins 1996). Logging practices
and areas cleared for mines, towns and livestock grazing changed forest structure, composition,
and fuels and as a result indirectly reduced and excluded fire (Covington and Moore 1994a,
1994b; Veblen et al. 2000). As the interface between European settlements and the forest
increased in North America, laws to ban burning were implemented to protect settler values
(Cohen and Miller 1978; Rhemtulla et al. 2002). Prohibition of fire use, followed by placement
of towers and roads into landscapes, firefighter training, improved vehicle access and, most
recently, detection of cloud-to-ground lightning have increased the capacity to detect and rapidly
suppress fires (Tande 1979; Murphy 1985, 2007; Murphy et al. 2007; Fulé et al. 1997; Brown et
al. 1999; Taylor 2000; Veblen et al. 2000; Arienti et al. 2006). The resulting changes in fire
frequency and associated changes to fire severity have been most prevalent in low-elevation
forests, although evidence suggests mid-elevation montane forests were also subject to these
16
changes in fire regimes (Tande 1977, 1979; Rhemtulla et al. 2002; Schoennagel et al. 2004;
DaSilva 2009; Nesbitt 2010; Amoroso et al. 2011; Greene 2011; Marcoux et al. 2013)
In contrast to low-elevation dry forests, fire frequency and severity have changed less
during the 20th century in high elevation subalpine forests and boreal forests where fires
historically were less frequent (Turner et al. 2003; Schoennagel et al. 2004). Over the past
century, relatively few people lived in these forests and land-use impacts are minor compared to
impacts in low elevation and montane forests (Romme and Despain 1989; Masters 1990;
Johnson 1992). Moreover, since fuel combustibility controlled by pronounced droughts are more
limiting for fire than fuel availability, the fire regime is less altered by land use change (Romme
and Despain 1989; Johnson and Wowchuk 1993). These interpretations assume most historic
ignitions were due to lightning, and that fire suppression has had no significant effects on the fire
regime (Romme and Despain 1989; Nash and Johnson 1996; Johnson et al. 2001). However,
human ignitions by First Nations peoples had impacts on fire regimes in subalpine forests and
even in northern Canadian boreal forests (Kipfmueller and Baker 2000; Miller 2010). In
addition, significant fire suppression impacts were identified on fire regimes in subalpine forests
and relatively remote northern Canadian boreal forests (Kipfmueller and Baker 2000; Cumming
2005; Podur and Martell 2009).
Although fire exclusion and suppression have had effects on all types of fire regimes, the
impacts have been greatest at valley-bottom and in montane forests historically characterized by
low- and mixed-severity fire regimes, respectively (Schoennagel et al. 2004; Beaty and Taylor
2008; Perry et al. 2011). Suppression efforts are most effective in putting out low- and moderate17
severity fires (Arienti et al. 2006) as opposed to high-severity fires which burn under extreme
weather conditions and are difficult to suppress (Johnson et al. 2001). In absence of low- and
moderate-severity fires as a result of fire exclusion and suppression, many low-elevation forests
and montane forests have changed in composition, structure and fuels (Rhemtulla et al. 2002;
Amoroso et al. 2011; Heyerdahl et al. 2012). In forests with low-severity fires regimes, fire
exclusion and suppression facilitates tree infilling and the development of ladder fuels which
increase the risk of high-severity fires (Swetnam and Baisan 1996; Swetnam et al. 1999;
Schoennagel et al. 2004). In mixed-severity fire regimes, absence of low- and moderate-severity
fires may homogenize the landscape with multi-cohort, mixed-species stands replaced by single
cohorts of post-fire colonizers following high-severity fires (Agee 1998; Margolis and Balmat
2009; Bekker and Taylor 2010; Perry et al. 2011).
1.7
Climate Change Impacts on Fire Regimes
Climatic trends observed during the 20th century and projected climate change impacts
include increased frequency, duration, and severity of droughts in western North America (Dale
2001; Gedalof et al. 2005; IPCC 2007a, 2007b, 2013; Seager et al. 2007; Allen et al. 2009). Due
to the established link between climatic variability and fire (Agee 1993; Grissino-Mayer and
Swetnam 2000; Flannigan et al. 2008), projected changes in climate may lead to increased
occurrence of large and higher-severity wildfires in North America (Flannigan et al. 2003;
McKenzie et al. 2004; Westerling et al. 2006). Understanding spatial and temporal variation in
climatic effects on fire regimes is therefore important for understanding potential effects of
global warming (IPCC 2007a, 2007b, 2013).
18
The effects of climate change will differ among fire regimes; however, over western
USA, increased wildfire activity was associated with increased spring and summer temperatures
and earlier spring snowmelt (Westerling et al. 2006). In low-severity fire regimes such as in the
southwestern USA, increased wildfire activity has been linked to a trend of warming
temperatures and increasing variability in moisture conditions, with wetter conditions promoting
biomass growth and drier conditions promoting drought and burning (Baisan and Swetnam 1990;
Grissino-Mayer and Swetnam 2000). Swetnam et al. (1999) expressed the need to better
understand how global climate change may alter tree species distribution and ensuing fuel
conditions, thus potentially shifting historically low-severity fire regimes towards higher-severity
fire regimes. However, Grissino-Mayer and Swetnam (2000) argued that altered precipitation
and temperature regimes would be unlikely to produce simple linear responses in fire regimes. In
high-severity fire regimes, such as in the Canadian boreal forests, Flannigan et al. (2003)
indicated fire regimes would respond rapidly to changes in climate. High-severity fire regimes in
the subalpine and boreal forests may sustain more frequent fires with increasing fire season
length and increased cloud-to-ground lightning with a corresponding increase in ignitions (Price
and Rind 1994). Mixed-severity fire regimes in mid-elevation montane forests have been found
at the juxtaposition between low- and high-severity fire regimes (DaSilva 2009; Nesbitt 2010;
Amoroso et al. 2011; Greene 2011; Marcoux et al. 2013). Moreover, in some of these forests,
recent wildfires have been of high-severity and burned during extreme fire weather (Agee 1993,
1998; Gedalof et al. 2005). A consequence of climate change for some forests historically
characterized by mixed-severity fire regimes is a shift towards high-severity stand-replacing fires
(Fulé et al. 2003; Schoennagel et al. 2004; Bekker and Taylor 2010). Thus, potential changes in
19
the global climate may alter not only the frequency of high-severity stand-replacing fires but also
their proportion on North American landscapes.
1.8
Fire Regime Research in Jasper National Park, Alberta, Canada
There is growing evidence in the Canadian Cordillera of mixed-severity fires in forests at
montane elevations (Cochrane 2007; DaSilva 2009; Nesbitt 2010; Amoroso et al. 2011; Greene
2011; Kubian 2013; Marcoux et al. 2013). In Jasper National Park (JNP), Alberta, Canada, fire
records have been developed from a network of sites surrounding the Jasper townsite (Tande
1977, 1979) and at two sites north of the townsite (Rogeau 1999) (Fig. 1.1). Tande (1977, 1979)
and Rogeau (1999) analysed cross-sections from living and dead fire-scarred trees and years of
origin from increment cores systematically collected from field sites. The years fire scars formed
were used to establish a fire chronology and to estimate the year stands originated. The stand
origin data were mapped to infer the extent of past fires.
Tande (1979) developed a fire chronology for a 310-year period (1665-1975) in which he
reported 72 fires indicated by fire scars collected from a 43,200 ha study area. Based on this firescar record, he indicated there was one fire per year somewhere in the study area between 1894
and 1908. For the entire study area, fires occurred at one to nine year intervals from 1837 to 1971
and intervals were more variable ranging from 1 to 36 years from 1665 to 1834 (Tande 1979).
Major fires, defined as those which burned at least 500 ha, occurred 24 times at intervals varying
from 1 to 27 years and accounted for most of the area burned from 1665 to 1975 (Tande 1979).
A strong decline in burned area after 1908 coincided with active fire suppression, which started
in 1913 (Tande 1977, 1979; Murphy et al. 2007).
20
Rogeau (1999) developed a fire chronology for a 514-year period (1485-1999) which
showed 15 fires indicated by fire scars and tree ages in a 3,300 ha study area. Fire-scar dates
combined with tree ages suggested fires burned somewhere in the study area at one to seven year
intervals from 1889 to 1915 and intervals were generally longer from 1798 to 1889, varying from
1 to 27 years (Rogeau 1999). Major fires initiating stands at more than ten sites occurred at
longer intervals varying from 6 to 48 years (Rogeau 1999).
Tande’s (1977, 1979) and Rogeau’s (1999) studies include trees with multiple scars
offering evidence of historic low- to moderate-severity fires; however, several limitations for
quantifying mixed-severity fire regimes result from the stand-origin mapping approach they
applied. First, Tande (1977, 1979) and Rogeau (1999) provide composite fire records for their
study areas from which they calculated metrics of fire frequency and size, but their records do
not indicate fire severity. The frequency values, including the means and ranges, apply to areas
equal to the size of their study areas and are difficult to translate to different spatial scales. In
addition, the fire-scar dates were not crossdated therefore they were not reliably annuallyresolved. The two datasets reported consecutive fire years (e.g., fire intervals of one year), but
did not account for false, incomplete and missing rings commonly associated with fire scars so
actual fire return intervals may be longer than reported. This potential source of error affects the
reconstructions of fire size, calculations of fire return intervals and fire frequency and limits the
translation of the outcomes to guide stand-level management and treatments.
21
1.9
Research Objectives
My research contributes to the understanding of mixed-severity fire regimes in mixed-
conifer forests of the Canadian Cordillera. My objectives were: I) develop crossdated, annuallyresolved fire-history records and determine stand age-structure for a spatially-explicit network of
sites in the montane forests of Jasper National Park to estimate past fire frequency and severity;
II) quantify effects of historic fires on contemporary forest dynamics inferred from forest
structure and composition; and III) quantify effects of inter-annual to decadal variation in climate
on historic fire occurrence. In Chapter 2, I test the hypotheses that: I) historical fires included
high- and moderate-severity fires which comprised a mixed-severity regime; and II) fire
suppression has reduced compositional and structural diversity of forests. I focused on
understanding historic changes of stand dynamics and the fire regime in a 2,000 ha study area.
To reconstruct fire history and forest dynamics and to detect fires of a range of severities at
individual sites, I sampled 29 sites for tree composition, age- and size-structure and fire-scar data
(Sherriff and Veblen 2006; Heyerdahl et al. 2012).
In Chapter 3, I test the hypothesis that historic fire occurrence depended on inter-annual
to decadal variation in climate, but that climatic variation alone does not explain the recent
absence of fire from the montane forests of JNP. I quantified the effects of drought, the El NiñoSouthern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal
Oscillation (AMO) on fire occurrence using crossdated fire records at the landscape-level in a
3,300 ha study area distributed east (1,900 ha) and west (1,400 ha) of the Athabasca River valley.
I used the fire-scar data from the 29 sites in the second chapter, as well as reanalysed and
crossdated fire scars and tree ages from 143 additional sites established and sampled by the
22
Foothills Research Institute between 1997 and 2000 (Rogeau 1999). I focused on the temporal
variation of fire occurrence in the study area since past research on Tande’s fire-scar record
indicated ENSO and the PDO had a significant influence on precipitation and fire regimes in
JNP (Schoennagel et al. 2005).
In the final chapter, I discuss the implications of my findings for ecosystem management
and conservation guided by natural range of variability and make recommendations for
restoration of fire regimes to increase forest resilience to severe fires given the anticipated effects
of climate change in Jasper National Park. Finally, I identify key avenues of future research.
Jasper
National
Park
Rivers & lakes
Jasper National
Park boundary
Town of
Jasper
At
ha
Montane ecoregion
ba
sc
a
Fire history study areas
R
.
Tande (1977, 1979)
Rogeau (1999)
0
25 km
Grid North
Figure 1.1 Fire history study areas in Jasper National Park, Alberta, Canada.
23
Chapter 2: Historical Fire Frequency and Severity in Jasper National Park,
Alberta, Canada
2.1
Introduction
Fire is a driver of forest dynamics influencing stand-level composition and structure and
landscape-level heterogeneity and diversity (Turner 2010). Accurate knowledge of historic fire
severity and frequency is critical for understanding current forest conditions and successional
trajectories, particularly where historic disturbance regimes are used to guide ecosystem-based
management and conservation practices (Hessburg et al. 2007; Scholl and Taylor 2010; Turner
2010; Amoroso et al. 2011; Marcoux et al. 2013). Similarly, understanding disturbance regimes
and their drivers is critical for anticipating the effects of climate change on forest dynamics (Dale
et al. 2001; Millar et al. 2007).
In the Canadian Cordillera, the traditional interpretation of disturbance regimes is that
infrequent high-severity fires dominate (Johnson et al. 1998, 2001). Under the influence of highseverity fires, forests tend to be even-aged with low structural diversity since each fire event kills
>80% of trees and establishment occurs in pulses accounting for 71-100% of trees in a stand
(Sherriff and Veblen 2006). When the interval between high-severity fires is short relative to the
lifespan of the component tree species, structurally-complex and uneven-aged old stands are rare
(Agee 1993; Turner et al. 1998; Turner 2010). The landscape is comprised of a mosaic of mostly
even-aged stands that differ in age and stage of development depending on the time since each
24
last burned. Shade-intolerant pioneer species are more common that shade-tolerant, latesuccessional species.
Recent research in the Canadian Cordillera (Cochrane 2007; DaSilva 2009; Nesbitt 2010;
Greene 2011; Marcoux et al. 2013) and Alberta Foothills (Amoroso et al. 2011) provides
evidence that historic fire regimes included low-, moderate-, and high-severity fires and are
better described as mixed-severity than high-severity fire regimes. A mixed-severity fire regime
would explain the observed diversity in forest composition and structures, particularly in
montane forests (Cochrane 2007; Nesbitt 2010; Marcoux 2013; Marcoux et al. 2013). Low- and
moderate-severity fires result in low tree mortality and, conversely, survival of 40-100% and 2070% of trees within a stand, respectively (Sherriff and Veblen 2006). High levels of canopy tree
survival limit opportunities for tree establishment so new cohorts account for less than 20% and
20-70% of trees, respectively (Sherriff and Veblen 2006). Moreover, variable effects within
individual fires lead to patches of low, moderate and high tree survivorship inside the fire
perimeter (Lentile et al. 2006). Fires with a range of severities can burn successively at one
location or through time over a landscape resulting in compositionally and structurally diverse
stands (Lertzman and Fall 1998; Heyerdahl et al. 2012). Given these complexities, forests with
mixed-severity fire regimes do not fit traditional successional and stand development trajectories
(Fulé et al. 2003; Halofsky et al. 2011). Rather, stands tend to develop uneven-age structures
with distinct single to multiple cohorts, composed of diverse species with variable adaptations to
fire and shade (Hessburg et al. 2007; Marcoux et al. 2013). Landscapes are heterogeneous with
high variation in patch sizes and spatial patterns (Hessburg et al. 2007; Amoroso et al. 2011;
Halofsky et al. 2011; Perry et al. 2011; Marcoux et al. 2013).
25
Human impacts on high- versus mixed-severity fire regimes have differed during the 20th
century (Schoennagel et al. 2004). In general, high-severity fires are infrequent enough that fire
suppression during the 20th century has had modest effects on fire frequency and forest
composition, structure and fuel complexes (Romme and Despain 1989; Nash and Johnson 1996;
Johnson et al. 2001; Fauria and Johnson 2008). However, land-use practices through fire
exclusion and suppression have had significant impacts on some forests with mixed-severity fire
regimes (Schoennagel et al. 2004; Beaty and Taylor 2008). Specifically, exclusion and
suppression of low-to-moderate-severity surface fires alters forest composition, structure, fuels
and successional trajectories (Agee 1993; Schoennagel et al. 2004; Amoroso et al. 2011).
Sustained exclusion and suppression of fire contributes to homogenization of forests as shadetolerant but fire-intolerant species establish, replacing fire-tolerant species within stands,
increasing forest densities and adding ladder fuels (Beaty and Taylor 2008). These changes in
composition and structure could make stands less resilient to lower-severity fires and more
conducive to high-severity fires (Littell et al. 2010). Following higher-severity fires, multicohort, mixed-species stands are replaced by single cohorts of post-fire colonizers (Agee 1998;
Perry et al. 2011), making the forests more consistent with those associated with a high-severity
fire regime. In parallel, recent wildfires in montane forests have been of high-severity and burned
during extreme fire weather (Filmon 2004; Gedalof et al. 2005), which could be the result of
climate change (Dale et al. 2001; IPCC 2007a, 2007b, 2013).
Fire history research in the montane forest of Jasper National Park (JNP), Canada
provides preliminary evidence of a mixed-severity fire regime. Tande (1977, 1979) and Rogeau
26
(1999) used increment cores to estimate stand-origin dates and counted rings on fire-scar
samples. Unfortunately, the fire scars were not crossdated to account for false and missing rings
so these fire dates are not accurate at an annual resolution. Nevertheless, the data were used to
infer the extent of past fires and estimate fire return intervals for their respective study areas.
Both studies reported frequent fires during the 1800s and early 1900s but few fires burned after
1913 (Tande 1977, 1979; Rogeau 1999) following active fire suppression (Murphy et al. 2007).
Rhemtulla et al. (2002) used repeat photography to document a shift towards late-successional
vegetation types and increased crown closure in coniferous stands in the Park. The changes were
consistent with observations in other montane forests from which low-to-moderate-severity fires
have been excluded and suppressed. Combined, these studies suggest a change in the fire regime
and forests of JNP during the 20th century, potentially jeopardizing their resilience to fire and
climate change.
Accurate estimates of historic fire frequency and severity are needed to fully understand
the magnitude and effects of 20th century changes to fire regimes and forest dynamics in JNP. To
address this knowledge gap, my research was designed to answer three questions: How frequent
and severe were historic fires? Has the historic fire regime changed during the 20th century? How
has fire history affected forest composition and structure? I focused on understanding historic
changes to the fire regime and stand dynamics in a 2,000 ha study area. I sampled 29 sites for
tree composition and size-structure and reconstructed historic fire frequency and severity by
developing crossdated fire-scar chronologies and high-resolution age structures (Sherriff and
Veblen 2006; Heyerdahl et al. 2012). I used these data to test the following hypotheses: I)
27
historical fires in the montane forests of JNP comprised a mixed-severity regime and II) fire
exclusion and suppression have reduced compositional and structural diversity.
2.2
Study Area
This research was conducted in Jasper National Park (JNP), Alberta, (Fig. 2.1) in the
Main and Front Ranges of the Canadian Rocky Mountains (Tande 1979). Late Paleozoic
limestone and Mesozoic shale underlie peaks and valleys respectively (Gadd 1986), whereas
surface materials are till and glaciofluvial deposits comprised of sandstones and quartzites with
slate and limestone (Stringer and LaRoi 1970). Gentle slopes and steep cliffs and ridges of
exposed bedrock create a complex topography (Tande 1979). Soils are predominantly brunisols
formed on glacial till, loess and outwash deposits (Tande 1977; Sanborn et al. 2011). Climate is
subarctic (Longley 1970; Kottek et al. 2006). The instrumental records for 1981-2010 from the
Jasper climate station (52°53' N, 118°04' W, 1,062 m.a.s.l.) (Environment Canada 2014) show
mean annual precipitation was 392.6 mm, 22% of which fell as snow from September through
June. Mean annual temperature was 3.6C, with mean monthly temperatures of 15.0 and -9.1C
for July and December, respectively (Environment Canada 2014).
Three ecoregions are represented in JNP: the montane (1,000-1,350 m.a.s.l.), the
subalpine (between 1,350 m.a.s.l. and tree line) and the alpine (above tree line) (Rhemtulla et al.
2002). This research was conducted in the montane ecoregion along the Athabasca River valley
in JNP (Fig. 2.1). Forests in the montane ecoregion are dominated by lodgepole pine (Pinus
contorta Douglas ex Loudon) and include Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco],
white spruce [Picea glauca (Moench) Voss], Engelmann-white spruce natural hybrids (Picea
28
engelmannii Parry x Picea glauca; hereafter “Picea”), and trembling aspen (Populus tremuloides
Michx.) interspersed with grasslands (Holland and Coen 1982; Rajora and Dancik 1999). Forests
in the subalpine ecoregion are dominated by Picea and subalpine fir [Abies lasiocarpa (Hooker)
Nuttall] (Holland and Coen 1982; Rajora and Dancik 1999; Peet 2000).
2.3
2.3.1
Materials and Methods
Site selection
The study area is located 15 km north of the Jasper townsite (52°50' N, 118°05' W) on
both sides of the Athabasca River valley (Fig. 2.1). I reconstructed fire history and stand
dynamics at 29 sites originally sampled by Rogeau (1999). To create a stand-origin map for a
3,300 ha area, Rogeau (1999) sampled 180 sites along 36 transects. For this research, I selected
eight widely distributed transects in the area; five transects were south or west and three transects
were east of the Athabasca River. Sites were located approximately 350 m apart along transects
and extended upslope from the montane into the subalpine ecoregion. To avoid bias, I selected
sites independently of the fire evidence reported by Rogeau (1999). Of the 29 selected sites, 21
included fire-scarred trees and 8 sites had no scars (Rogeau 1999). Of the 151 not resampled
sites, 106 included fire-scars, charred bark, charred logs/stumps and/or soil charcoal and the
other 45 sites had no fire evidence.
2.3.2
Fire scars
From the centre-point of each site, I searched a one hectare circular plot (radius = 56.4 m)
for fire-scarred trees, snags, stumps and logs (hereafter “scarred trees”). A full or partial crosssection (Cochrane and Daniels 2008) was sampled from up to six scarred trees per site by
29
selecting the large trees and those with the most visible scar-lobes to maximize the fire record.
This sampling design ensured sample areas were consistent from site to site and were directly
comparable to other fire histories conducted in western Canada (Cochrane 2007; DaSilva 2009;
Nesbitt 2010; Greene 2011; Marcoux 2013). When possible, fire-scar samples collected by
Rogeau (1999) were included.
Fragile fire-scar samples were mounted on wooden supports and all samples were sanded
following standard protocols (Stokes and Smiley 1996). I scanned high-resolution (1,200 or
2,400 dots per inch) digital images along radii from the bark to the inner-most ring including the
tips of fire scars. Ring widths were measured using the program CooRecorder (Larsson 2011a). I
visually crossdated and statistically verified the ring dates using the programs COFECHA and
CDendro (Grissino-Mayer 2001a; Larsson 2011b) to determine the years of the inner-most and
outer-most rings and individual fire scars (hereafter “fire-scar year”) (Grissino-Mayer 2001a).
2.3.3
Forest composition and structure
I sampled forest composition and structure at the centre of each site using a modified n-
tree design (Jonsson et al. 1992; Lessard et al. 2002; Heyerdahl et al. 2006). For stands with one
canopy layer, I sampled the ten canopy trees or snags with diameter at breast height (dbh) ≥5 cm
closest to the centre-point. For stands with multiple canopy layers, ten canopy and ten subcanopy
trees or snags with dbh ≥5 cm were sampled. The distance and bearing from plot centre to the
distal canopy and subcanopy tree were measured (Heyerdahl et al. 2006). For each tree, I
recorded species, dbh (in cm), status (live or dead) and estimated age using increment cores.
Cores were taken ca. 30 cm from the ground and aimed to include the pith. To account for
30
sapling regeneration, I searched a 100 m2 circular plot (radius = 5.64 m) from the centre-point of
each site for saplings. I counted the number of live saplings which were ≥1.37 m in height and
<5 cm in dbh and I recorded the species.
Like the fire-scar samples, cores were mounted on wooden supports, sanded following
standard protocols (Stokes and Smiley 1996) and ring widths were measured using the program
CooRecorder (Larsson 2011a). I visually crossdated and statistically verified the cores using the
programs COFECHA and CDendro (Grissino-Mayer 2001a; Larsson 2011b) to determine the
years of the inner and outer rings (Grissino-Mayer 2001a). Crossdating was performed using
existing published Pseudotsuga menziesii chronologies that spanned from 1630 to 1965
(Ferguson and Parker 1965) and new chronologies for Pseudotsuga menziesii and Picea that I
developed for each site and the region. Tree age estimates included corrections for cores that
missed the pith (Duncan 1989) and the number of years for trees to grow to coring height
(Powell et al. 2009; Daniels unpublished data). The total age correction averaged 12 years and
80% of ages had a correction ≤15 years. Therefore, the age structure of each site was represented
using histograms with 15-year establishment classes.
2.3.4
Cohorts and veteran trees
Even-aged cohorts were defined as 30-year periods when 25% of trees per site
established (after Heyerdahl et al. 2012). To identify even-aged cohorts, I assessed the number of
trees per 30-year period, starting from the pith date of the oldest tree, shifting one year at a time,
and ending with the pith date of the youngest tree. Post-fire cohorts were the subset of even-aged
cohorts that (a) coincided with a fire scar at the same site or (b) included the oldest trees at the
31
site. The fire year assigned to each post-fire cohort was (a) the fire-scar year or (b) the year
preceding the establishment of the oldest tree in the cohort. Any tree that survived fire (e.g.,
established prior to a fire scar or post-fire cohort) was considered a veteran tree. At each site, I
calculated the percentage of trees that were veterans, which I used to classify the severity of the
last fire at the site as low (81-100%), moderate (20-80%) or high (0-19%) (adapted from Agee
1993; Sherriff and Veblen 2006).
2.3.5
Fire frequency and severity
Composite fire chronologies were developed for each site using fire scars and post-fire
cohorts. For each site-level fire chronology, I determined the length of the fire record, the
number of fires, the minimum, mean and maximum intervals between fires, and time since last
fire (TSLF). TSLF was calculated as the number of years from the most recent fire year (scar or
initiation of a post-fire cohort) to the year 2012.
I used the presence of fire scars, even-aged cohorts and veteran trees to classify the
severity of historic fires through time at individual sites (Sherriff and Veblen 2006; Heyerdahl et
al. 2012). Sites were classified as having evidence of a mixed-severity fire history based on the
following criteria: (1) ≥2 fire years represented by scars, indicating low- to moderate-severity
fires, and ≥1 even-aged cohort indicating moderate- to high-severity fire, or (2) a single fire-scar
year preceded by veteran trees and ≥1 cohort. Sites were classified as having evidence of only
high-severity fires based on the following criteria: (1) the oldest trees formed a cohort and there
were no fire scars, or (2) the oldest trees formed a cohort and all fire scars coincided with it.
32
2.3.6
Fire history and stand dynamics
I determined the lag between fire and tree establishment by identifying the fire-scar year
or post-fire cohort preceding the establishment of subject trees (after Amoroso et al. 2011;
Marcoux 2013). Lags were calculated as: (1) the difference between the fire-scar year and tree
pith date indicating predominantly less severe fires causing scars or (2) the difference between
the pith date of the oldest tree in the post-fire cohort and the subject tree, indicating that highseverity fires initiated the cohort. Frequency histograms of lag times relative to fire scars and
post-fire cohorts were compared among species.
To further evaluate the effect of fire at the tree scale, I calculated average, initial and
relative growth rates of trees (Amoroso et al. 2011). The average growth rate was the average
ring width over the lifespan of the tree (radius inside bark divided by age in years), the initial
growth rate was the average width of the ten rings closest to the pith, and the relative growth rate
was the ratio of the initial growth rate divided by the average growth rate (Amoroso et al. 2011).
I compared initial and relative growth rates among species and among less severe fires causing
scars and high-severity fires initiating cohorts only.
To distinguish the influences of high- versus mixed-severity fires on stand dynamics, I
calculated age- and size-structure diversity indices. The evenness index was the number of 15year age or 5-cm size classes occupied by at least one tree (Marcoux et al. submitted). Low
values denoted a narrow range and relatively even-aged (sized) stands; high values denoted a
broader range and uneven-aged (sized) stands. The continuity index was the number of vacant
15-year age or 5-cm size classes within the range of occupied classes (Marcoux et al. submitted).
33
Age- and size-structures with no or few vacant classes were relatively continuous and a greater
number of vacant bins denoted discontinuous stands. As well, I calculated the Shannon diversity
index (Shannon and Weaver 1949) from the relative proportion of trees in each age or size class
per stand. I compared evenness, continuity and diversity of the age and size structures between
fire-severity classes using box plots and Mann-Whitney rank sum tests.
2.3.7
Successional trajectories
To assess the influence of fire on successional trajectories, I compared forest composition
and structure between canopy and subcanopy strata of sites with evidence of mixed- versus highseverity fires. Relative density and basal area per hectare were calculated for living trees and
snags of each species and canopy stratum at each site. Populus tremuloides and balsam poplar
(Populus balsamifera L.) occurred infrequently and at low abundances and were grouped
(hereafter “deciduous trees”). For each site, importance values (IV) (Curtis and McIntosh 1951)
for conifer species and deciduous trees were calculated as the sum of relative density and relative
basal area of live trees and snags in the canopy and living trees in the subcanopy. This allowed a
comparison between current and future canopy trees, assuming current subcanopy trees are
potential future canopy trees. Multivariate discriminant analysis (MDA) (SAS 9.3) was used to
compare IVs among four groups of trees: (1) the canopy layer of sites with mixed-severity fire
history, (2) the subcanopy layer of sites with mixed-severity fire history, (3) the canopy layer of
sites with high-severity fire history, and (4) the subcanopy layer of sites with high-severity fire
history. Mahalanobis distances were used to assess similarity (low values) and differences (large
values) among groups (Mahalanobis 1930; Dillon and Goldstein 1984).
34
2.4
2.4.1
Results
Forest composition and structure
The forest at most sites was composed of mixed species and included two canopy strata
which varied in density and basal area among sites (Table 2.1; Fig. 2.2). For sites with two
canopy strata, the dominant canopy trees based on tree density were Picea (n = 15 sites), Pinus
contorta (n = 8) and Pseudotsuga menziesii (n = 4), whereas Picea dominated the subcanopy.
Two sites had a canopy dominated by Pinus contorta (≥90% of sampled canopy trees) but no
subcanopy strata. Populus tremuloides was present at seven sites and balsam poplar was at one
site.
At 20 sites, Picea saplings accounted for ≥50% of the regeneration. Of these 20 sites, 13
included Picea only. Four sites had only Pseudotsuga menziesii and one site had only Pinus
contorta sapling regeneration. Sapling regeneration was absent at four sites.
2.4.2
Fire history
Among the 29 sites, 20 had fire-scarred trees (Table 2.2) from which 64 fire-scarred
cross-sections yielded 88 fire scars. Fires indicated by scars occurred in 13 separate years
ranging from 1646 to 1905. Of the 560 sampled trees, I successfully aged cores from 458 live
trees and 70 snags (n = 528 total). The oldest trees were Picea, which established in the 1500s
(age >400 years) (Fig. 2.3); however, 70% of all trees established between 1820 and 1920.
Most trees (93%) established in even-aged cohorts dominated by Picea or Pinus contorta
(Fig. 2.3). All sites had one or two cohorts, for a total of 34 cohorts, 33 of which likely originated
35
after fire. Eighteen cohorts coincided with a fire scar at the same site and 15 cohorts included the
oldest trees at the site. Of the 15 cohorts including the oldest trees at the site, 10 cohorts included
shade-intolerant Pinus contorta and Populus tremuloides, whereas the other 5 cohorts included
trees with wide tree-ring patterns suggesting open-grown conditions. One cohort did not coincide
with a fire scar and did not include the oldest trees at the site suggesting its establishment may
have been stand dynamics or a disturbance other than fire.
Veteran trees were identified at 15 sites, within which they accounted for 5% to 95%
percent of the sampled trees (Table 2.2; Fig 2.3). Based on the percentage of veteran trees, the
severity of the last fire was classified as low at 2 sites, moderate at 9 sites, and high at 18 sites.
2.4.3
Fire frequency and severity
Fire histories for individual sites ranged from 107 to 457 years based on the age of the
oldest tree that could have recorded fire at each site (Fig. 2.3 and 2.4; Table 2.2). Nine sites had
cohorts only, whereas 20 sites had cohorts and fire scars, with up to five fire-scar years per site.
At the 11 sites with multiple fires, fire intervals ranged from 27 to 165 years. Time since last fire
(TSLF) ranged from 107 to 193 years. The composite fire record (29 sites combined) included 13
fire years indicated by scars and 8 years indicated by the cohorts independent of fire scars.
Widespread fires in 1772, 1827, 1889 and 1905 scarred trees and/or generated cohorts at five,
nine, eight and seven sites, respectively. No fire scars were recorded after 1905 (Fig. 2.4).
At 18 of 29 sites, there was evidence of mixed-severity fires through time and the
remaining 11 sites included evidence of high-severity fires only (Table 2.2). Of the sites with
36
mixed-severity fires, 11 had ≥2 fire-scar years, whereas 7 sites had one fire-scar year with
veteran trees. Of the sites with multiple fire-scar years, two sites had high density of veteran
canopy trees (>90%) and low post-fire regeneration suggesting the most recent fire was of low
severity. In contrast, the age structure of four sites included a single cohort and no veterans,
suggesting the last fire was high in severity. The other 12 mixed-severity sites included variable
numbers of veterans and post-fire regeneration suggesting the recent fires sustained were of
moderate (9 sites) or high-severity (3 sites). One to five fires were recorded per site and time
since last fire (TSLF) averaged 129 ±31 years (mean ± standard deviation; range = 107 to 185
years).
Of the sites with high-severity fires, nine sites had one cohort which included the oldest
trees at the site but had no scars. Two sites had one fire-scar year coinciding with a cohort which
included the oldest trees at the site. TSLF was longer at the high-severity fire history sites (143
±31 years; range = 107 to 193 years) than at the mixed-severity fire history sites.
2.4.4
Fire history and stand dynamics
All three conifers established after both low- and moderate-severity fires (hereafter
“lower-severity fires”) indicated by scars and high-severity fires that initiated cohorts (Fig. 2.5).
Of the Pinus contorta, 73% established after fires causing scars. More than half (63%) of the
estimated establishment dates for Pinus contorta were within 15 years and 88% within 30 years
of fire, regardless of severity. Of the Pseudotsuga menziesii, 55% established after fires causing
scars; however, only 50% of establishment dates were within 30 years of a fire scar and lags
were up to 108 years. Pseudotsuga menziesii had shorter lags after more severe fires, with 86%
37
of trees establishing within 30 years of the oldest tree in post-fire cohorts and a maximum lag of
36 years. Picea regenerated in similar proportions after fire causing scars or initiating cohorts,
with >75% of establishment dates within 30 years of fire but Picea had the longest lags of up to
117 years.
Initial growth rates for Pinus contorta were fastest (1.16 ±0.67 mm.year-1; mean ±
standard deviation), followed by Pseudotsuga menziesii (0.84 ±0.74 mm.year-1) and Picea (0.73
±0.55 mm.year-1). Initial growth rates for all species were faster at sites with high-severity fires
that initiated as single cohorts compared to sites with lower-severity fires indicated by scars
(Mann-Whitney U = 22167, p ≤0.001). After high-severity fires, Pinus contorta had wide rings
closest to the pith (1.40 ±0.83 mm.year-1) and initial growth rates were twice as fast as the
average growth rate over their lifespan (relative growth = 2.17 ±1.12). After lower-severity fires
the rings closest to the pith were not as wide (1.08 ±0.59 mm.year-1; Mann-Whitney U = 1420.5,
p = 0.019) and relative growth was not as rapid (1.62 ±0.70; Mann-Whitney U = 1364, p =
0.009). Pseudotsuga menziesii and Picea had wider rings closest to the pith (1.09 ±0.79 and 0.90
±0.64 mm.year-1, respectively) after high-severity fires compared to lower-severity fires (0.57
±0.57 and 0.62 ±0.43 mm.year-1, respectively) (Mann-Whitney U = 327.5, p ≤0.001 and MannWhitney U = 20628, p ≤0.001, respectively). Moreover, Pseudotsuga menziesii and Picea
relative growth was more rapid after high-severity fires (1.15 ±0.54 and 0.99 ±0.57,
respectively), compared to lower-severity fires (0.76 ±0.37 and 0.89 ±0.51, respectively), with a
statistically significant difference for Pseudotsuga menziesii (Mann-Whitney U = 422, p ≤0.009).
38
Age-structure continuity was the strongest indicator of mixed-severity fire history,
whereas size-structure diversity indices were weak indicators of fire history class (Fig. 2.6).
There were several indications age structure was more complex at sites with mixed- versus highseverity fire history, although only continuity was statistically significant. The age structures of
mixed-severity sites were more discontinuous than the high-severity sites (Mann-Whitney U =
50, p = 0.03). Age structures tended to be more uneven and diverse at the mixed-severity sites;
however, large variation overlapped with values from the high-severity sites. Size structure was a
weaker indicator of fire severity. Although medians indicated discontinuous, uneven and diverse
sizes at sites with mixed- versus high-severity fire histories, differences were not significant due
to large variation within and between fire-history classes.
2.4.5
Successional trajectories
For all sites combined, Picea was most abundant in the canopy, followed by Pinus
contorta and Pseudotsuga menziesii, regardless of fire history class (Fig. 2.7). The mean
importance values of canopy trees were more even at mixed- than high-severity fire history sites,
although the mean importance value of Pseudotsuga menziesii was greater at the mixed-severity
sites. Picea dominated the subcanopies, with its greatest mean importance value at high-severity
sites. Among fire-history classes and canopy layers, the canopies of mixed- and high-severity
sites were most similar (Fig. 2.8). The greatest differences were between the canopy and
subcanopy layers at high-severity sites and between the canopy at mixed-severity sites and the
subcanopy at high-severity sites.
39
2.5
Discussion
2.5.1
Frequency and severity of historic fires
In the montane forests of Jasper National Park historical fires burned at a wide range of
frequencies and severities at site and study-area scales. Fire scars caused by lower-severity
surface fires were found at 20 of 29 sites. Fire scars were most common on Pinus contorta,
followed by Pseudotsuga menziesii. Most Pinus contorta had only one scar likely caused by
lower-severity fires, as high-severity fires would have killed the trees given their relatively thinbark (Agee 1993). Multiple fire scars on thick-barked Pseudotsuga menziesii indicated two to
five surface fires through time at 11 sites. At these sites, fire-to-fire intervals were variable (27165 years) but most were 40-60 years. These intervals were similar to those in the nearby
Foothills ecoregion (Amoroso et al. 2011) and mixed-conifer forests in southeastern British
Columbia (Cochrane 2007; DaSilva 2009; Nesbitt 2010; Greene 2012; Marcoux et al. 2013), but
longer than fire return intervals reported for the mixed-conifer forests in drier climates further
south in western North America (Fulé et al. 2003; Beaty and Taylor 2008; Bekker and Taylor
2010).
The multiple techniques I used to interpret fire history provided evidence that low- to
high-severity fires historically affected individual sites and the fire regime at the study-area scale
was of mixed-severity. Fire-scarred trees combined with single or multiple post-fire cohorts were
legacies indicating past fires of a range of severities through time at 18 of 29 sites. At two sites
with ≥2 fire-scar years, high densities of veteran trees and low post-fire recruitment suggested
the most recent fires were low-severity surface fires (Sherriff and Veblen 2006). At four other
sites, the absence of veteran trees and the dominance of a single post-fire cohort suggested the
40
most recent fire was of high severity. However, the fire-scar records at each of these sites
indicated ≥2 distinct surface fires prior to the most recent fire so they were classified as having a
mixed-severity fire history (Heyerdahl et al. 2012). Of the 12 other mixed-severity sites which
included at least one cohort and veterans, 3 had low densities of veteran trees and high post-fire
recruitment suggesting the most recent fire was of high severity. The remaining nine sites had
intermediate densities of veteran canopy trees and intermediate post-fire recruitment suggesting
the most recent fire was of moderate severity. I found evidence of only past high-severity fires at
11 sites supporting my interpretation that the study area historically had a mixed-severity fire
regime in which high-severity fires were common.
2.5.2
Changes to the fire regime during the 20th century
The scars and cohorts I crossdated indicated the historic fire frequency varied at the
study-area scale. Fire frequency was greatest from the 1870s to 1905, potentially caused by
human use of fire by First Nation peoples, European peoples, or peoples of mixed ancestry
during settlement (MacLaren 2007; Murphy et al. 2007). The fire records of 15 sites began prior
to 1800 and included only six fires between 1646 and 1800. Lack of fire scars could indicate few
fires burned or fire evidence may be lacking. Lack of fire evidence could be due to the young age
of trees at many sites which limits the number of recorders and further in the past successive
fires, mechanical damage and wood decomposition remove recorders and evidence of past fires
(Van Pelt and Swetnam 1990; Parsons et al. 2007; Swetnam et al. 2011). I found no fire scars that
formed after 1905 in the study area, although this is the period during which the sample depth
and potential to record fires is greatest. This finding is unprecedented in 250 years.
41
The lack of fire scars during most of the 20th century may be attributed to fire exclusion
and suppression. Jasper became a national park in 1907 and in 1910 local families were
displaced from their homesteads which removed the application of fire as a land management
method (MacLaren 2007; Murphy et al. 2007). Fire protection and suppression were
implemented in the Park from 1913 onwards (Tande 1979; Kay and White 1995; Taylor 1998;
Rhemtulla et al 2002; Van Wagner et al. 2006; MacLaren 2007; Murphy et al. 2007).
Changes in fire frequency in the Canadian boreal forests and some Cordilleran forests
have been attributed to a long-term regional climatic shift to cooler and wetter conditions since
the end of the Little Ice Age (Johnson and Larsen 1991; Flannigan et al. 1998). Although interannual to decadal variations in climate have been associated with fire occurrence in JNP
(Schoennagel et al. 2005), there is little evidence weather and climate have been unsuitable for
fire throughout the 20th century. Tree-ring reconstructions of precipitation in JNP do not support
the argument of a climatic shift to persistent wetter conditions since the turn of the last century
(Luckman 1998; Watson and Luckman 2001, 2004). Moreover, research on the occurrence of
fires of lightning origin, lightning strikes and ignitions do not support changes in lightning fire
occurrence in the Canadian national parks in the Rocky Mountains (White 1985; Wierzchowski
et al. 2002). Since 1880, lightning-ignited fires >40 ha have been recorded in Banff National
Park (BNP), which is 6,641 km2 in size and immediately south of JNP. Three fires ignited
between 1880 and 1940 and four ignited between 1940 and 1980 (White 1985). My post-hoc
analysis of the fire records for JNP (Parks Canada 2013), which is 10,878 km2 in size, shows 101
lightning-ignitions since 1940, only eight of which resulted in fires >40 ha in size. Given the
differences in the size of the two Parks, the numbers of large lightning-ignited fires between
42
BNP and JNP are comparable. At the regional scale, relatively few lightning-ignited fires >40 ha
occurred over the past century. White (1985) determined the lack of fires in BNP was due fire
suppression. Since JNP is an adjacent landscape to BNP and the same fire management policy
was applied to both Parks, the coincidence of the fire exclusion and suppression policy at the
onset of the 20th century was likely an important factor which led to the lack of fires in the study
area.
2.5.3
2.5.3.1
Forest dynamics and influence of the historical fire regime.
Stand Dynamics
At the stand level, historic fires of different severity promoted compositional and
structural diversity; however, differences between stands with mixed- versus high-severity fire
histories in JNP were subtle. On average, importance values of canopy trees were more even
among species and greater for Pseudotsuga menziesii at sites with mixed-severity fire histories.
Presence of veteran trees and discontinuous age structures also distinguished mixed-severity
sites; however, the evidence was revealed by dendrochronological reconstructions. Combined,
these attributes were indicators of mixed-severity fire histories, as documented in other forests of
the Canadian Cordillera (Heyerdahl et al. 2012; Marcoux et al. 2013). Nonetheless, these
attributes were neither exclusive to nor present at all mixed-severity sites.
Similarities in forest composition and structure among sites with mixed- versus highseverity fire histories reflected the overriding effects of high-severity fires that burned ca. 107 to
123 years ago (Table 2.2). The last fire to burn at 18 sites was classified as high severity. Stand
age structures at 24 sites were dominated by post-fire cohorts in which Pinus contorta, Picea and
43
Pseudotsuga menziesii formed the canopy whereas subcanopies were strongly dominated by
Picea.
In JNP, Pinus contorta, Picea and Pseudotsuga menziesii established simultaneously
following historic low- to high-severity fires; however, species-specific growth rates and
adaptations to shade and fire resulted in size stratification among trees and canopy layers. Of the
three species, Pinus contorta is shade-intolerant (Burns and Honkala 1990; Klinka et al. 2002).
Since fire opens its serotinous cones facilitating seedling establishment immediately following
fire (Burns and Honkala 1990; Klinka et al. 2002), Pinus contorta had the shortest regeneration
lags and formed distinct cohorts (Antos and Parish 2002; Amoroso et al. 2011). It had the fastest
initial growth rates, whether it established after lower- or high-severity fires, thus allowing
lodgepole to dominate or co-dominate the canopy at many sites.
Picea is shade-tolerant but does not require shade or protection to establish (Burns and
Honkala 1990; Klinka et al. 2002) and regenerated immediately following fire and after long lags
since fire. Post-fire regeneration lags result from relatively slow seed dispersal by wind over
distances >300m from surrounding forests or island remnants that survive fire (Dobbs 1976;
Burns and Honkala 1990; Greene and Johnson 2000). Among trees establishing after fire, Picea
growth rates were variable; however, its initial growth rates were slow compared to Pinus
contorta and Pseudotsuga menziesii. Some trees grew to large diameters and dominated the
canopy layer, while slow-growing individuals dominated the subcanopy at most study sites.
Given its shade tolerance, Picea can regenerate beneath an existing canopy (Burns and Honkala
1990; Klinka et al 2002) and form secondary cohorts as stands develop. Among my study sites,
44
subcanopy Picea formed a cohort that was younger than the canopy trees only at site 11;
however, at 19 sites, Picea saplings accounted for the majority of the regeneration (93 ±13%).
The regeneration dynamics of Pseudotsuga menziesii were variable among sites. Mature
and thick-barked veterans that resisted and survived surface fires would provide a viable seed
source both immediately following fire and over longer periods (Arno and Fiedler 2005).
Pseudotsuga menziesii germinates well on mineral soil exposed by fire; however, it is
intermediate in shade tolerance and can establish beneath an existing canopy (Burns and Honkala
1990; Klinka et al 2002). Pseudotsuga menziesii had intermediate initial growth rates. Like
Picea, some Pseudotsuga menziesii established immediately following fire and grew rapidly into
the canopy, while others regenerated after longer lags or grew more slowly particularly after
lower-severity fires. Lower-severity fires likely resulted in smaller openings and more
competitive growing conditions due to veteran trees, which would favour regeneration of shadetolerant Pseudotsuga menziesii and Picea more than shade-intolerant Pinus contorta.
Pseudotsuga menziesii establishment peaked 16 to 30 years after high-severity fires. The cause
of the lag could be slow colonization following fire or slow initial growth rates. Slow growing
trees take longer to grow to coring height (30cm) and the age corrections that we applied may
have under-estimated the number of rings formed below coring height (Gutsell and Johnson
2002).
2.5.3.2
Impacts of Fire Exclusion on Successional Trajectories
In some forests with historic mixed-severity fire regimes, within-stand comparisons of
the species composition of the subcanopy relative to the canopy has been used as an indicator of
45
successional trajectories resulting from fire exclusion (Taylor and Solem 2001; Beaty and Taylor
2008). This approach assumes (1) subcanopy trees are younger than canopy trees and represent
future canopy trees and (2) succession will proceed in absence of stand-level disturbances (White
et al. 1985; Oliver and Larson 1996). This approach has been successfully applied in forests
where fire exclusion has promoted stands to the mature or old-growth stages of stand
development (Bekker and Taylor 2010; Scholl and Taylor 2010). In these forests, shade-tolerant
trees have regenerated beneath the canopy and are recruiting in gaps as canopy trees die.
Moreover, structures in these forests include dense subcanopies and ladder fuels with
regenerating trees which are often thin-barked and less resistant to lower-severity fires (Fulé et
al. 2003; Perry et al. 2011). Combined, these changes result in forests that are less resistant to
lower-severity fires and more prone to high-severity crown fires (Littell et al. 2010).
In JNP, forest canopies were mixed in composition and subcanopies were dominated by
shade-tolerant Picea, regardless of fire history classes. However, the subcanopy trees were
similar in age as the canopy trees, except at site 11. Trees in both strata had originated after highseverity fires with both composition and structure of the forest canopy reflecting the stemexclusion stage of stand development. The subcanopy trees were slow-growing, shade-tolerant
Picea, rather than a recently-established cohort resulting from understory reinitiation. For the
current subcanopy Picea to form the canopy in future, they will need to outlive current canopy
trees and recruit in gaps formed as canopy trees die.
Based on the average time between fires found here I would have expected up to five
surface fires among the 29 study sites in the absence of 20th Century fire exclusion. My fire-scar
46
record showed 13 fire years between 1646 and 1905, averaging one fire every 20 years. Some of
the historic fires burned and scarred trees at multiple sites. Since 1905, up to five fire events may
have been missed, some of which may have burned multiple sites, had fires continued to burn
rather than being suppressed. Such disturbances would have led to more diverse size and age
structures and possibly species composition among my study sites. Periodic lower-severity fires
would have increased stand-level diversity (Agee 1993) by reducing the number of thin-barked
Picea in the subcanopy and created canopy openings and substrates suitable for Pinus contorta
and Pseudotsuga menziesii regeneration, in addition to Picea (Burns and Honkala 1990;
Amoroso et al. 2011). High-severity fires would have diversified the ages and stages of
development among stands (Agee 1993). Instead, in absence of fire, all stands have developed
similarly. At most sites, the canopy was closed, subcanopy Picea formed ladder fuels and mostly
shade-tolerant regeneration was in the sapling strata.
My reconstructions of stand-level dynamics for individual sites are consistent with the
consequences of fire exclusion documented at the landscape level in JNP (Rhemtulla et al. 2002).
Photographs taken in 1915 of the montane and subalpine ecoregions in JNP (Bridgland 1924)
revealed a landscape with young age structures at the stand-initiation stage (Rhemtulla et al.
2002). In the montane ecoregion, five vegetation types dominated alongside non-vegetative
cover (water, wetland, sand-gravel, rock and anthropogenic): closed-canopy forest (16-70%
crown closure) represented 35% of total cover, open forest (<16% crown closure) 16%, shrub
13%, forb-grassland 11% and juvenile forest (sapling stage) 7% (Rhemtulla et al. 2002). This
variability in vegetation cover type is consistent with my fire records indicating 30 even-aged
cohorts were <20 to ca. 100 years old in 1915 and recent surface fires causing scars were found
47
at 20 sites. In 1997 the same areas in the montane ecoregion were photographed (Rhemtulla et al.
2002). Of the total cover in 1997, 65% was dominated by forests with 51-100% crown closure
indicating a decrease in landscape heterogeneity (Rhemtulla et al. 2002). This landscape-level
change can be explained by the lack of fires in the study area since 1905, which allowed
individual stands throughout the landscape to develop to the stem-exclusion and understory reinitiation stages.
2.6
Conclusion
In the montane ecoregion of Jasper National Park, low- to high-severity fires historically
affected individual sites and the fire regime at the study-area scale was of mixed-severity. Site
level fire histories reconstructed from fire scars and cohorts indicated 18 sites were of mixedseverity whereas 11 were of high-severity. Differences between stands with mixed- versus highseverity fire histories in JNP were subtle. Key differences distinguishing mixed-severity sites
were importance values of canopy trees which were more even among coniferous species and
greater for Pseudotsuga menziesii, the presence of veteran trees, and discontinuous age
structures. Current stand composition and age structures reflected the effects of higher-severity
fires that burned ca. 100 years ago at 24 of 29 sites, followed by an absence of fire since 1905.
Canopy composition varied among sites; however, subcanopies were strongly dominated by
shade-tolerant and fire-intolerant Picea, regardless of fire history severity class. In the absence of
fires of a range of severities during the 20th century, individual stands have developed similarly,
resulting in a decrease of landscape heterogeneity.
48
Table 2.1 Site attributes and species composition of the 29 study sites in the montane forests of Jasper National Park. Density (ha-1)
and basal area (m2 ha-1) of living and dead trees are given for the dominant conifers (Pinus contorta, Pseudotsuga menziesii and
Picea) and deciduous trees (Populus tremuloides and Populus balsamifera combined). Regeneration is the relative abundance of live
conifer saplings.
Transect no. Elevation
Site
I
(m.a.s.l.)
Tree Density (ha-1 )
Slope
Slope
Angle
Aspect
(degrees)
(degrees)
P. contorta
P. menziesii
Density of Regeneration (%)
Live (Dead) Canopy Trees
Live (Dead) Subcanopy Trees
Picea
Deciduous
P. contorta
P. menziesii
-
Live Saplings
Picea
Deciduous
P. contorta
P. menziesii
Picea
100
1
1029
0
315
765
-
-
786 (314)
-
472
-
-
-
2
1024
0
333
-
-
142 (16)
-
191
-
382 (64)
-
-
-
-
3
1024
0
3
29
29
229
-
59
-
118
-
-
-
100
4
1024
0
27
-
197
-
-
-
334
334
-
-
100
-
5
1038
0
143
70
-
56 (14)
-
-
-
793 (227)
113
-
-
100
6
1002
0
90
(62)
-
218
31
(361)
180
1083
-
-
-
-
7
990
0
26
1973
-
-
-
-
-
-
-
-
100
-
8
1031
17
57
852
95
-
-
-
-
-
-
25
-
75
9
1010
3
120
-
-
352
88
-
-
340
113 (113)
-
-
100
10
1063
8
128
97 (16)
-
49
-
736 (589)
-
147
-
-
-
100
11
1116
7
87
527 (226)
-
-
-
91 (274)
91
457
-
-
50
50
12
1040
7
85
-
38 (154)
-
-
168
126 (126)
-
-
33
67
13
1062
8
29
-
38 (154)
-
-
212
106 (35)
-
-
-
100
14
1112
33
96
26
79
26
-
-
457
366
183
-
100
-
15
1039
3
97
-
-
127 (14)
-
-
-
441
-
-
-
-
16
1060
5
122
-
-
180
-
-
-
1273
-
-
-
100
17
1100
9
183
-
74 (37)
258
-
-
-
58 (29)
87 (115)
-
100
-
18
1155
29
123
236
79
79
-
127 (127)
-
509 (382)
127
-
-
-
19
1147
22
358
-
147
-
-
-
577 (577)
-
-
-
100
20
1268
32
332
65
130
455
-
-
-
3108
-
-
-
100
21
1411
21
335
70
52
52
-
4212
-
1805
-
5
-
95
VII 22
1083
19
309
-
18
144 (18)
-
-
-
2981 (331)
-
-
4
96
23
1126
10
340
54
54
272 (163)
-
-
-
274 (640)
-
-
-
100
24
1186
15
230
99 (33)
166
33
-
-
588
392
-
-
-
100
25
1195
10
240
170
-
113
-
(271)
271
406
135 (271)
-
-
100
VIII 26
1153
36
360
106
159
265
-
107
-
429
-
14
29
57
27
1214
38
24
379
126
126
-
185
-
1663
-
-
-
100
28
1425
26
327
23
93
116
-
102
-
508 (406)
-
-
20
80
29
1570
8
18
42 (14)
14
71
-
757 (189)
-
-
-
100
-
-
II
III
IV
V
VI
192 (38)
118 (29)
192 (38)
49
Table 2.1 (continued).
Basal Area (m 2 .ha-1 )
Transect no.
Site
I
II
III
IV
V
VI
VII
Live (Dead) Canopy Trees
P. contorta
P. menziesii
1
22
-
2
-
-
3
1
2
4
-
17
-
5
6
-
4 (2)
Live (Dead) Subcanopy Trees
Picea
Deciduous
P. contorta
P. menziesii
-
-
12 (2)
-
4
-
11 (2)
9
Picea
Deciduous
-
2
-
-
17 (2)
-
1 (0.1)
-
1
-
-
-
7
4
-
-
-
-
8 (2)
2
-
6
(5)
-
10
3
2 (7)
2
14
7
38
-
-
-
-
-
-
-
8
16
7
-
-
-
-
-
-
9
-
-
44
5
-
-
8
12 (3)
10
9 (1)
-
9
-
31 (18)
-
6
-
11
15 (5)
-
-
-
1 (4)
0.3
3
-
12
-
6 (22)
10 (3)
-
-
5
2 (2)
-
13
-
14 (11)
3 (17)
-
-
4
1 (0.4)
-
14
6
14
2
-
-
2
4
5
15
-
-
6 (1)
-
-
-
11
-
16
-
-
12
-
-
-
18
-
17
-
4 (3)
15
-
-
-
1 (1)
2 (1)
18
12
10
5
-
4 (7)
-
8 (7)
3
19
-
11 (3)
5
-
-
-
2 (8)
-
20
2
9
12
-
-
-
33
-
21
2
3
3
-
39
-
6
-
22
-
2
5 (1)
-
-
-
17 (5)
-
23
3
4
32 (8)
-
-
-
6 (13)
-
24
7 (2)
16
4
-
-
13
4
-
25
13
-
15
-
(5)
5
2
3 (5)
VIII 26
3
7
6
-
1
-
5
-
27
9
7
5
-
3
-
11
-
28
1
4
6
-
0.2
-
2 (7)
-
29
3 (1)
1
3
-
8 (1)
-
-
-
50
Table 2.2 Fire record for the 29 study sites in the montane forests of Jasper National Park.
no.
51
Jasper
National
Park
Town of
Town of
Jasper
Jasper
Rivers & lakes
Jasper National
Park boundary
Montane ecoregion
Study area
0
25 km
Grid North
ke
La
53°5’N
er
p
as
00
m
J
13
0
11
0m
19 20
r
c
as
aR
ive
22
b
ha
At
18
17
Ran
ge
1300m
26
14 13 12
11 10 9
7
Jacq
ues
23 24
25
1100m
16 15
8
21
27 28
150
29
0m
1700
6 5
4
3
2
m
1
Co
lin
Ra
ng
0
2
4 km
118°5’W
e
118°0’W
Figure 2.1 Fire history study area in Jasper National Park (top). The 29 fire-history sites in the
Athabasca River valley, 15 km north of Jasper townsite (bottom).
52
10
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
19
20
21
22
23
24
25
26
27
28
29
5
0
10
5
0
10
5
0
Frequency (number of trees)
10
5
0
10
18
5
0
10
5
0
10
5
0
10
5
0
5
15
25
35
45
55
65
5
15
25
35
45
55
65
5
15
25
35
45
55
65
5
15
25
35
45
55
65
Diameter (upper limit of 5-cm bins)
Figure 2.2 Size-structure histograms of trees (dbh ≥5 cm) by species for the 29 fire-history sites
located along eight transects (rows). Histograms include all live and dead trees (n = 20) sampled
per site. Species are Pinus contorta (black), Pseudotsuga menziesii (grey), Picea (white),
Populus tremuloides (diagonal crosshatch) and Populus balsamifera (vertical crosshatch).
53
20
1 (n = 20)
2 (n = 20)
3 (n = 20)
4 (n = 19)
5 (n = 17)
6 (n = 19)
7 (n = 9)
8 (n = 10)
9 (n = 14)
10 (n = 18)
11 (n = 20)
12 (n = 17)
13 (n = 19)
14 (n = 18)
15 (n = 18)
16 (n = 20)
17 (n = 20)
19 (n = 17)
20 (n = 20)
21 (n = 20)
22 (n = 19)
23 (n = 20)
24 (n = 19)
25 (n = 17)
26 (n = 20)
27 (n = 20)
28 (n = 19)
29 (n = 19)
10
0
20
10
0
20
10
0
Frequency (number of trees)
20
10
0
20
18 (n = 20)
10
0
20
10
0
20
10
0
20
10
0
1550
1700
1850
2000 1550
1700
1850
2000 1550
1700
1850
2000 1550
1700
1850
2000
Year of establishment (upper limit of 15-year bins in calendar years)
Figure 2.3 Age-structure histograms of trees (dbh ≥5 cm) by species for the 29 fire-history sites
located along eight transects (rows). Histograms include all live and dead trees sampled per site.
Fire-scar years are represented by black triangles and the beginning of an even-aged cohort by
white triangles. Species are Pinus contorta (black), Pseudotsuga menziesii (grey), Picea (white)
and Populus tremuloides (diagonal crosshatch).
54
Figure 2.4 Fire history from 1550 to 2012 at all 29 sites. Horizontal lines represent the site-level composite tree-ring records (top
to bottom: sites 1 to 29). The length of each line represents the period of record, starting from the pith of the oldest tree to 2012.
Fire evidence includes crossdated, annually-resolved fire scars (black triangles) and the year establishment of the oldest tree in
post-fire cohorts (white triangles). For sites lacking fire scars, dashed lines indicate the length of the record. The bottom graph
shows the number of sites with fire scars (black line) and the number of sites recording a fire (grey bars) over time.
55
Pseudotsuga menziesii (n = 44)
80
40
40
0
0
80
Picea (n = 135)
80
106-120
91-105
76-90
61-75
0
46-60
0
31-45
40
1-15
40
Pseudotsuga menziesii (n = 35)
Picea (n = 133)
106-120
80
91-105
0
76-90
0
61-75
40
46-60
40
Pinus contorta (n = 38)
31-45
80
16-30
Pinus contorta (n = 101)
16-30
Relative frequency
80
(b) Post-fire cohorts
1-15
(a) Fire scars
Lag in establishment (years)
Figure 2.5 Species-specific lags in tree establishment following (a) predominantly less severe
fires causing scars (left column) and (b) high-severity fires initiating cohorts only (right column).
56
10
10
5
5
0
0
12
12
10
10
8
8
6
6
4
4
2
2
0
0
1.4
1.4
1.2
1.2
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
Mixed
High
Continuous
15
Uneven
15
Even
20
Diverse
20
Discontinuous
Size Structure
25
Uniform
Shannon diversity index
Evenness index
Continuity index
Age Structure
25
Mixed
High
Figure 2.6 Comparison of age- and size-structure continuity (top), evenness (middle) and
diversity (bottom) between fire severity classes (Marcoux et al. 2013). In each box plot, the black
horizontal line represents the median and box boundaries are the 25th and 75th percentiles; and
bars are the 10th and 90th percentiles.
57
Mean importance value
of canopy trees
200
Mean importance value
of subcanopy trees
Mixed-severity (n = 18)
200
High-severity (n = 11)
150
100
50
0
150
100
Deciduous trees
Picea
P. menziesii
P. contorta
Deciduous trees
Picea
P. menziesii
0
P. contorta
50
Figure 2.7 Mean and standard errors of importance values by coniferous species and deciduous
trees according to fire history class (columns) and canopy layer (rows).
58
S
hi ubc
gh a
-s no
ev py
er &
ity
fir
e
Su
hi bc
gh a
-s no
ev py
er &
ity
fir
e
Su
hi bc
gh a
-s no
ev py
er &
ity
fir
e
C
m ano
ix p
ed y
-s &
ev
er
ity
fir
e
C
hi ano
gh p
-s y &
ev
er
ity
fir
e
C
hi ano
gh p
-s y &
ev
er
ity
fir
e
1.68
0.47
fir
e
C
m ano
ix p
ed y
-s &
ev
er
ity
C
hi ano
gh p
-s y &
ev
er
ity
m bc
ix a
ed no
-s py
ev &
er
ity
fir
Su
e
m bc
ix a
ed no
-s py
ev &
er
ity
fir
Su
e
m bc
ix a
ed no
-s py
ev &
er
ity
fir
C
e
m ano
ix p
ed y
-s &
ev
er
ity
fir
e
Su
1.38
0.41
0.26
0.08
Figure 2.8 Mahalanobis distances between canopy and fire history classes. Shorter (longer)
distances imply greater similarity (difference) between classes.
59
fir
e
Chapter 3: Temporal Climate-Fire Relations
3.1
Introduction
Fire regimes in western North America are anticipated, based on climate-change
projections, to include wildfires of increasing number, size and severity, with measureable
effects already observed in some subalpine and boreal forests (Flannigan et al. 2003; McKenzie
et al. 2004; Westerling et al. 2006). Apart from climate, fire regimes can also be influenced by
land use and fire management (Agee 1993); however, these effects vary among forests
(Schoennagel et al. 2004; Meyn et al. 2007). Understanding the relative importance of climate
and humans and their interacting effects on fire regimes is important for developing effective
management plans in response to climate change.
At regional scales, weather and climate are drivers of fire occurrence and regimes,
respectively (Cyr et al. 2007). Weather influences fire occurrence and behaviour (Agee 1993).
Low relative humidity, warm temperatures and strong winds decrease fuel moisture, whereas
precipitation increases it (Agee 1993; Fauria et al. 2011). Combined, these weather attributes
affect fuel combustibility, probability of ignition, and fire spread, intensity and severity (Taylor
et al. 1996; Fauria et al. 2011). Climate influences fire at a range of temporal scales (Falk et al.
2007). Within years, seasonal variation in temperature and precipitation determine the timing and
length of the fire season (Agee 1993; Meyn et al. 2007). At inter-annual to multi-decadal scales,
variation in climate determines the frequency of droughts conducive to fire or of wet intervals
hindering fire (Johnson et al. 1999; Bergeron et al. 2000; Grissino-Mayer and Swetnam 2000;
Heyerdahl et al. 2002; Hessburg et al. 2005; Schoennagel et al. 2005; Taylor and Beaty 2005).
60
Long-term climatic variation can influence fire frequency within fire regimes (Veblen et al.
2000; Dale et al. 2001; Morgan et al. 2001; Whitlock et al. 2003).
The effects of climatic variation on fire regimes vary among forest types (Schoennagel et
al. 2004; Meyn et al. 2007). In warm dry climates, such as in ponderosa pine (Pinus ponderosa
Douglas ex C. Lawson) forests in the southwest United States of America (USA), historic fires
burned frequently (Swetnam and Betancourt 1990; Swetnam 1993). In these forests, fuels are
sufficiently desiccated by warm, dry weather each fire season; however, fuel availability is
limiting (Schoennagel et al. 2004; Meyn et al. 2007). Fine and ladder fuel accumulation limits
fire spread (Fulé et al. 1997; Hessburg et al. 2005; Heyerdahl et al. 2006) and short periods of
above-average moisture conditions can enhance fuel production and thereby remove the
limitation in fuel availability (Baisan and Swetnam 1990; Grissino-Mayer and Swetnam 2000).
In contrast, subalpine and boreal forests grow in cool, humid climates in which fires burn less
frequently and are limited by fuel combustibility (Payette 1992; Turner and Romme 1994; Agee
1997; Buechling and Baker 2004; Schoennagel et al. 2004; Podur and Martell 2009).
A significant drying period is necessary for fuel combustion (Agee 1993; Fauria et al.
2011). In the long periods between fires, fuels accumulate (Veblen 2000; Schoennagel et al.
2004) then burn during unusual droughts (Johnson and Wowchuck 1993; Bessie and Johnson
1995; Sibold and Veblen 2000; Stocks et al. 2002; Buechling and Baker 2004). In montane
mixed-conifer forests, topographic variation creates complex moisture gradients (Agee 1998).
Fuel availability and combustibility are less limiting than in ponderosa pine forests in the
61
southwest USA and subalpine or boreal forests, respectively. As a result, fires burn with a mix of
frequencies (Taylor and Skinner 1998; Hessburg et al. 2005; Marcoux et al. 2013).
Wildfire ignitions are predominantly from lightning or humans (Agee 1993; Fauria et al.
2011). Historically, in North America, First Nation peoples used fire as a management tool to
grow food, facilitate foraging and hunting, clear vegetation for travel and to reduce wildfire
hazards around communities (Lewis 1985; Bahre 1991; Agee 1993; Kay 1995; Murphy et al.
2007; Miller 2010). Contemporary fire records in Canada indicate about 55% of fires are ignited
by people either intentionally (e.g., prescribed burns or arson) or accidentally (Government of
Canada 2013).
However, the levels of secondary human impacts on fire such as fire exclusion and
suppression have been debated for different forested landscapes in western North America
(Schoennagel et al. 2004). Fire exclusion is the indirect reduction in fire due to land use and
cover changes (Covington and Moore 1994a, 1994b; Veblen et al. 2000), whereas fire
suppression is the direct reduction in fire due to preventative measures, initial attack and the
management of large fires (Martell 2001; Arienti et al. 2006). Fire exclusion and suppression can
alter fire regimes by reducing fire frequency which potentially affects severity (Agee 1993;
Schoennagel et al. 2004). In dry forests with historically short fire return intervals, impacts of
fire suppression can be measured within decades (Swetnam et al. 1999). In the absence of
frequent surface fires, forest structure and fuels change relatively quickly (Schoennagel et al.
2004). Conversely, in mesic forests with historically long fire intervals, impacts of fire
62
suppression require longer periods to have measurable effects (Turner et al. 2003; Schoennagel
et al. 2004).
In the Canadian Cordillera, boreal Canada and elsewhere, the decrease in fire frequency
during the 20th century has been attributed to fire exclusion and suppression and the fire regime
is considered outside its historical range and variability (Swetnam et al. 1999; Kipfmueller and
Baker 2000; Taylor and Skinner 2003; Veblen 2003; Amoroso et al. 2011). However, debates
have been raised about the pervasiveness of the reduced fire frequency and its impact on forest
structure and composition (Veblen 2003; Schoennagel et al. 2004). Reduced fire frequency in
boreal forests has been attributed to cooler and wetter climate since the end of the Little Ice Age
(Johnson and Larsen 1991; Johnson 1992; Bergeron and Archambault 1993). These studies
assume fire suppression has had no significant effects on the fire regime (Romme and Despain
1989; Nash and Johnson 1996; Johnson et al. 1998, 2001). However, recent compelling evidence
shows fire suppression has significantly impacted fire regimes in some subalpine and boreal
forests (Cumming 2000; Kipfmueller and Baker 2000; Podur and Martell 2009).
Jasper National Park (JNP) is located at the transition between Cordilleran and Boreal
ecozones in the Rocky Mountains of Alberta, Canada (Peet 2000). Within the Park, steep
environmental gradients creates shifts from grasslands to subalpine forests over short distances
(Holland and Coen 1982). Fire regimes also vary over short distances with a mixed-severity fire
regime in montane forests (Chapter 2) and high-severity fires in the subalpine forests (Peet
2000). Documented changes to the fire regime in the 20th century include decreased fire
occurrence (Chapter 2), increased forest cover (Rhemtulla et al. 2002) and shifts to closed63
canopy late-successional vegetation cover at stand (Chapter 2) and landscape scales (Rhemtulla
et al. 2002). Whether these changes were caused by climatic change, human impacts or both,
remains undetermined. There is abundant evidence people of different cultures have historically
influenced the fire regime in the montane forests of JNP, including active fire suppression after
1913 (Tande 1979; Kay and White 1995; Taylor 1998; Rhemtulla et al 2002; Van Wagner et al.
2006; MacLaren 2007; Murphy et al. 2007). Past research determined the El Niño-Southern
Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) had a significant influence on
precipitation and historic fire regimes in JNP (Schoennagel et al. 2005). However, the climatefire analysis was based on a fire record that was not crossdated (Tande 1977, 1979) and errors
have been identified in it (L. Daniels, personal communication, July 2012).
In this study I examined the role of climatic variation as a driver of the fire regime in
montane forests of JNP. I tested two inter-related hypotheses. (1) Historic fire occurrence
depended on inter-annual to decadal variation in climate. (2) Climatic variation explains the
absence of fire within my study area in JNP during the 20th century. To represent the landscape, I
crossdated fire scars and tree ages at 172 sites in a 3,300 ha study area distributed east (1,900 ha)
and west (1,400 ha) of the Athabasca River valley. To allow comparison with other studies in
montane forests of western North America, I quantified the effects of drought, the ENSO, PDO
and Atlantic Multidecadal Oscillation (AMO) on fire occurrence using crossdated fire records. I
combined these results with the well-documented land-use history of JNP to understand the
relative importance of climate and human impacts on the historic and contemporary fire regime.
My research contributes to knowledge of the drivers of historical fire regimes, which can be used
64
to enhance scientifically-based conservation and fire management in JNP, as well as sustainable
forest management practices in similar forests surrounding the Park.
3.2
3.2.1
Materials and Methods
Research design
This research builds on a study conducted from 1997 to 2000 as collaboration between
the Foothills Research Institute and Jasper National Park (Rogeau 1999; D. Andison, personal
communication October 2011). The original goal of the study was to construct a time-since-fire
map depicting forest patches of different ages that established after different fires through time
(Johnson and Van Wagner 1985). Sampling was carried out in discrete patches of forests
identified on air photos (Heinselman 1973; Johnson et al. 1990) and at forest edges between
forest patches where fire-scars may occur (Johnson and Gutsell 1994). Patches of forest were
sampled along 37 transects. Transects were distributed on both sides of the Athabasca River, 24
west of the Athabasca River and north of the Snaring River and 13 east of the Athabasca River
and north of Colin Range. On each transect two to nine sites were sampled. Transects started at
the valley bottom in the montane forest and extended upslope into the subalpine forests. Distance
between sites averaged 340 m (range = 50 to 1035 m).
3.2.2
Field sampling
At each of the 172 sites, a centre-point tree was selected around which a circular plot
with a 20 m radius was searched for fire-scarred trees, snags, stumps and logs (hereafter “scarred
trees”). At each site, a full cross-section was sampled from up to six scarred trees (n = 104). Up
to eight live trees representing different coniferous species, size and height classes were sampled
65
within 20 m of each centre-point tree (n = 825). For each sampled tree, the distance from the
centre-point, species, diameter at breast height (dbh) and canopy layer was recorded. The age of
trees was estimated using increment cores taken ca. 30 cm from the ground and aimed to include
the pith. At a subset of 29 sites, I conducted detailed fire history reconstruction through time
using the methods in Chapter 2.
3.2.3
Laboratory analysis
Fire-scar and core samples were mounted on wooden supports and all samples were
sanded following standard protocols (Stokes and Smiley 1996). High-resolution (1200 or 2400
dots per inch) digital images were taken from the bark to the inner-most ring and, in the case of
fire scars, along the radii that included the tips of fire scars. Using Coo-recorder (Larsson 2011a),
I measured ring widths and using the programs CDendro (Larsson 2011b) and COFECHA
(Grissino-Mayer 2001a), I visually crossdated and statistically verified the ring dates to
determine the years of the inner-most and outer-most rings and individual fire scars (hereafter
“fire-scar year”) (Grissino-Mayer 2001a). For 361 cores that missed the pith, I applied a
correction to estimate the number of missed rings (Duncan 1989). I applied species-specific ageheight corrections (Powell et al. 2009; Daniels unpublished data) to estimate the number of years
for the trees to grow to sampling height.
3.2.4
Fire history
I developed composite fire chronologies for individual sites that included fire-scar years.
The length of each chronology was determined by the age of the oldest tree at the site, even if it
was not scarred. Since the study area is dominated by fire-susceptible Picea and Pinus contorta
66
(Chapter 2), the oldest trees represented the period during which fires were likely to be recorded
had the site burned. For 45 sites with fire scars, I determined the number of fires, mean,
minimum and maximum fire intervals and time since last fire (TSLF).
Site-level fire chronologies were combined into composite fire chronologies for two
locations west and east of the Athabasca River and for all sites in the study area combined. For
each composite fire chronology, I determined the length of the fire record, the number of fires,
mean, minimum and maximum fire intervals and TSLF. Using the composite fire record for the
entire study area, years of widespread fire were years in which fire was recorded at >5 sites.
3.2.5
Local Pseudotsuga menziesii chronology as a proxy of drought
Pseudotsuga menziesii is sensitive to drought and has been used to reconstruct
precipitation in Jasper National Park (Watson and Luckman 2001, 2004, 2005). I acquired the
original ring-width data for Pseudotsuga menziesii for the Pyramid and Patricia lake sites which
are closest to my study area. The data included 36 series sampled by Ferguson and Parker (1965)
and 43 series sampled by Watson and Luckman (2001). I combined the series with the crossdated
ring-width series from 308 Pseudotsuga menziesii I sampled, to develop a regional chronology
using the program COFECHA (Grissino-Mayer 2001a). I selected a subset of 81 cores that were
highly correlated (inter-series correlations ≥0.663) and had no flagged segments.
I used ClimateWNA (Wang et al. 2012), a climate mapping system based on regression
modelling, to derive climate records from 1901 to 2009 for the midpoint between Pyramid and
Patricia lakes (interpolated location point: 52°54’ N, -118°05’ W, 1,182 m.a.s.l.). The climate
67
parameters were monthly total precipitation and monthly maximum and mean temperatures.
Using the monthly climate records, I calculated monthly heat-moisture indices (after Wang et al.
2006; Chavardès et al. 2012):
Heat-moisture indexmonthly = (Tmean + 10) / (P / 100)
where, Tmean is the monthly mean temperature (°C) and P is monthly total precipitation (mm).
Low moisture availability resulting from low precipitation and/or warm temperatures is indicated
by high indices.
I conducted climate-growth analyses to verify Pseudotsuga menziesii radial growth is a
suitable proxy for growing season drought in the study area. I used the program ARSTAN to
detrend individual ring-width series and to derive a standard and residual chronology (Cook
1985; Cook and Holmes 1986). To account for the non-climatic, age-related trend in ring widths,
a single detrending was applied by fitting a negative exponential curve or linear regression
through each ring-width series. I compared climate-growth relationships between the resulting
residual chronology and the modelled climate data using the program Dendroclim2002 (Biondi
and Waikul 2004). Pearson’s correlation coefficients were calculated between ring-widths and
each of the monthly climate variables (monthly total precipitation, maximum and mean
temperature, and heat-moisture indices) (Biondi and Waikul 2004). To assess direct and lagged
climate influences on tree growth, correlation coefficients were calculated for the 18-month
window from the April prior to ring formation through September of the year the ring was
formed (e.g., summer, fall and winter prior to ring formation plus spring and summer of the year
of ring formation). Statistical significance was determined by bootstrapping (Biondi and Waikul
2004).
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3.2.6
Climate-fire relations
I used superposed epoch analysis (SEA) to determine the influence of inter-annual
climatic variation on fire occurrence between 1646 and 1915 (Grissino-Mayer 2001b). SEA was
used to test whether the 18 fire years I documented in Jasper National Park (JNP) occurred
during anomalous climate conditions. To assess statistical significance, a bootstrapped Monte
Carlo simulation randomly selects 1,000 years and calculates conditions associated with those
years to define 95% and 99% confidence intervals (Grissino-Mayer 2001b). To represent
climate, I used the residual Pseudotsuga menziesii chronology as a proxy of drought conditions
(Watson and Luckman 2001, 2004, 2005), as well as tree-ring reconstructions of the El NiñoSouthern Oscillation (ENSO) (Cook et al. 2008), Pacific Decadal Oscillation (PDO) (Gedalof
and Smith 2001) and Atlantic Mutidecadal Oscillation (AMO) (Gray et al. 2004), the
hypothesized drivers of drought in JNP (Schoennagel et al. 2005). Specifically, I tested whether
mean values of climate indices were significantly different during fire years or during the four
years preceding and two years following fires. I tested lagged effects to assess whether patterns
of climate prior to a fire year exert any influence on a given fire (Grissino-Mayer 2001b).
Climate does not influence a fire in years following that year; however, analyses which include
the years following fire may reveal important multi-year climatic patterns associated with fire,
such as ENSO phases (Grissino-Mayer 2001b).
To test potential drivers of drought I stratified multiple years and/or decades as (a) El
Niño or non-El Niño phases (Garcia-Herrera et al. 2008; Yu and Kim 2013), (b) positive or
negative PDO phases (Gedalof and Smith 2001) and (c) positive, neutral or negative AMO
69
phases (Gray et al. 2004). To assess the effects of prolonged drought, I stratified multiple years
and decades as dry or wet phases based on the low-frequency analyses by Watson and Luckman
(2004). I assessed their effects on fire occurrence between 1646 and 1915 using contingency
tables. Each year was stratified as a fire or non-fire year based on my composite fire scar record
for all sites. I calculated the proportion of fires occurring during different climatic phases
separately. I also tested if potential drivers of drought interacted to amplify (same sign) or
dampen (opposite sign) their effects on fire occurrence via two-way combinations (Schoennagel
et al. 2005). Statistical significance was determined using χ2 goodness-of-fit tests (α = 0.05). The
number of fires was too small to test three-way interactions among potential drivers.
3.2.7
Twentieth century climatic conditions and fire occurrence
I determined if climatic conditions had been conducive to fire during the 20th century.
First, I assessed climatic conditions associated with the historic fire record from 1646 to 2009. I
used Pseudotsuga menziesii ring-widths as an indicator of drought because high temperatures,
low precipitation and high heat-moisture indices during the growing season result in narrow
rings. I used analysis of variance (ANOVA; SAS 9.3) and post hoc Tukey tests to compare mean
ring-width indices during the 18 fire years versus periods of dry or wet climate from 1646 to
1915 as defined by Watson and Luckman (2004). To compare historic conditions with 20th
century climate and to assess variation during the 20th century, I used ANOVA and post hoc
Tukey tests to compare mean ring-width indices, annual (previous September to current August)
maximum temperature, precipitation and heat-moisture indices during three periods from 1916 to
2009. Watson and Luckman (2004) reported a dry period, 1916 to 1947, followed by a wet
70
period, 1948 to 1982. I included an unclassified period from 1983 to 2009, which exceeded
Watson and Luckman’s (2004) analysis.
3.3
3.3.1
Results
Fire history
The 104 fire-scarred cross-sections recovered from 45 of 172 sites yielded 138 fire scars
(Fig. 3.1 and 3.2). Among all sites, I found fires burned in 18 years from 1646 to 1915. Fire
records for the 115 sites west of the Athabasca River averaged 181 ±79 years (range = 78 to 416
years) based on the age of the oldest tree at each site (Table 3.1). Twenty-five sites (22%) had
fire scars, with up to five fire-scar years per site. At the ten sites with multiple fires, mean fire
intervals ranged from 31 to 62 years. Time since last fire (TSLF) averaged 141 ±36 years (range
= 107 to 273 years). The composite fire record included 11 fire-scar years at intervals of 19 to
121 years (Fig. 3.3a).
Fire records for the 57 sites east of the Athabasca River averaged 192 ±105 years (range
= 86 to 533 years) based on the age of the oldest tree at each site (Table 3.2). Twenty sites (35%)
had fire-scars, with up to three fire-scar years per site. At the eight sites with multiple fires, mean
fire intervals ranged from 11 to 164 years. Time since last fire (TSLF) averaged 109 ±28 years
(range = 83 to 193 years). The composite fire record included ten fire-scar years at intervals of
15 to 132 years (Fig. 3.3b).
For the entire study area, the return intervals for the 18 fires averaged 55 ±34 years
(range = 15 to 132 years). Widespread fires in 1772, 1827, 1889 and 1905 scarred trees at 6, 10,
71
12 and 15 sites, respectively (Fig. 3.3c). A 54 year gap with no fire scars occurred between 1773
and 1826, inclusive, and no fire scars were recorded after 1915.
3.3.2
Pseudotsuga menziesii tree-rings as a proxy of drought
The Pseudotsuga menziesii residual chronology was a strong indicator of drought (Fig.
3.4). The correlations between radial growth and monthly precipitation between 1902 and 2009
were predominantly positive. Correlations with previous year summer precipitation were
consistently positive, with significant correlations for August and September. Correlations with
current year precipitation were positive from May to September, except July, with significant
correlations in May and June.
Correlations between radial growth and monthly maximum temperatures, mean
temperatures and heat-moisture indices were predominantly negative (Fig. 3.4; mean temperature
not shown). Correlations were significant for previous July to September heat-moisture indices
and previous September maximum temperature. Correlations with current year May to
September temperatures and heat-moisture indices were negative, except July heat-moisture
indices. Correlations were significant in May and June for maximum temperature and heatmoisture indices.
3.3.3
Climate-fire relations
Most fires burned during years when the ring-widths were narrower than one standard
deviation below average (39%) (Fig. 3.5). Widespread fires only burned when ring-widths were
narrower than average. Fires were associated with narrow ring-width indices in the residual
72
Pseudotsuga menziesii chronology (α = 0.01), indicating drought conditions during fire years
(Fig. 3.6). Ring-width indices during antecedent or succeeding years were not significant.
Annual values of the El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO)
or the Atlantic Multidecadal Oscillation (AMO) indices did not have significant relationships
with fire occurrence. The 18 fires between 1646 and 1915 burned during dry multiyear periods (p
= 0.041) (Fig. 3.7a) and showed a weak association with El Niño phases during negative phases
of the PDO (Fig. 3.7b,c,e) but no association with AMO phases (Fig. 3.7d,f,g). Overall, the
power of the statistical tests was limited by the small number of fires over 270 years.
3.3.4
Fire occurrence and drought
Mean ring-width indices were smallest, indicating drought, during the 18 fire years
between 1646 and 1915 (Fig. 3.8). Relative to the fire years, the mean indices during dry periods
and the unclassified period were not significantly different but the mean indices were
significantly larger during wet periods. Variation among years within periods was large,
indicating individual dry/wet years even during wet/dry periods. As a result of this variation, the
mean ring-width indices during the dry and unclassified periods were not significantly different
from the wet period during the 20th century.
The instrumental climatic records varied significantly between dry and wet periods
during the 20th century and were consistent with the above interpretations using ring-widths as a
proxy for drought (Fig. 3.9). Maximum temperature was significantly greater and annual
precipitation and moisture availability were significantly lower during the dry periods relative to
the wet periods. The unclassified period was intermediate; maximum temperature did not differ
73
significantly from the dry period but it was wetter. As a result, moisture availability did not differ
significantly from the other two periods.
3.4
3.4.1
Discussion
Climatic variation and historic fires in Jasper National Park
Historic fires burned during significant drought years; however, relations with the El
Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) were less clear. Since
my fire record included only 18 fires, the low sample size limited the statistical power to discern
patterns between fire occurrence and inter-annual to decadal climate influences. Nevertheless,
my findings that fires were weakly associated with El Niño events are consistent with other
climate-fire studies in Jasper National Park (JNP), western Canada and the Pacific Northwest of
the USA (Westerling and Swetnam 2003; Schoennagel et al. 2005; Heyerdahl et al. 2008).
However, the observed association with the negative phase of the PDO contrasts other studies
(Westerling and Swetnam 2003; Hessl et al. 2004; Gedalof et al. 2005; Kitzberger et al. 2007;
Heyerdahl et al. 2008; Da Silva 2009). Below, I compare my findings with other studies in
western North America to understand the observed spatial and temporal variation in climate-fire
interactions.
The influences of ENSO vary with latitude and affect fire regimes differently depending
on location (Westerling and Swetnam 2003; Schoennagel et al. 2005; Kitzberger et al. 2007). At
latitudes north of ~45°N, historic fires commonly burned during droughts associated with El
Niño phases (Westerling and Swetnam 2003; Trouet et al. 2006). In the Pacific Northwest of the
USA and western Canada, El Niño phases are associated with shallow snow packs, early melt,
74
longer growing and fire seasons, and higher temperatures (Heyerdahl et al. 2002; Pohl et al.
2002; Hessl et al. 2003; Westerling and Swetnam 2003; Kitzberger et al. 2007; Heyerdahl et al.
2008). In my study, seven out of eight fires burned after 1863 during well-documented El Niño
phases (Mote et al. 1999; Garcia-Herrera et al. 2006; Li et al. 2011; Yu and Kim 2013). Prior to
1863, no fires burned during El Niño phases; however, few studies report El Niño data at an
annual resolution prior to 1870. It is possible that ENSO-fire relations changed through time.
Johnson and Larsen (1991) suggest climate has been cooler and wetter since the end of the Little
Ice Age. Short term droughts related to El Niño phases might only have become more important
recently. To test this hypothesis more fire data and reliable pre-1870 ENSO records are needed.
I found fires were associated with the negative phase of the PDO, which contrasts with
other studies in the western Cordillera (Hessl et al. 2004; Gedalof et al. 2005; Kitzberger et al.
2007; Da Silva 2009), including Schoennagel et al.’s (2005) analysis of fires around Jasper
townsite. Half of the fires I documented, including two widespread fires in 1889 and 1905,
burned during the long negative PDO phase from 1840 to 1922. Of the remaining nine fires that
formed scars prior to 1840, five burned during positive PDO phases including widespread fires
in 1772 and 1827. Schoennagel et al. (2005) used 72 fire-scar dates reported by Tande (1977,
1979) for the area around Jasper townsite. Those fire-scar dates are based on ring counts and
subsequent crossdating of samples which show several errors (L. Daniels, personal
communication July 2012). When I compared my fire-scars dates with Tande’s (1977, 1979)
records, I found that eight dates, all of which fell between the mid-19th and early 20th centuries,
matched: 1915, 1905, 1901, 1896, 1889, 1878, 1863 and 1846. Six fire-scar dates differed by ≤3
years: 1890, 1831, 1772, 1740, 1724, 1711 and 1677. Only one of these, 1890, was after the mid75
19th century. All of the rest were between the late 17th and early 19th centuries. Two fire dates,
1827 and 1706, differed by five or six years whereas my oldest fire-scar date, 1646, differed
from Tande’s (1977, 1979) oldest fire-scar date, 1665, by 19 years. It is likely some of these
discrepancies could be due to crossdating issues.
Another difference is Schoennagel et al.’s (2005) study was based on D’Arrigo et al.’s
(2001) tree-ring reconstruction of the PDO which spans from 1700 to 1979. Apart from the
shorter time span of the reconstruction which did not capture two of my fire years (1646 and
1677), post-hoc analyses showed other differences between reconstructions. The correlation
between D’Arrigo et al.’s (2001) and Gedalof and Smith’s (2001) PDO reconstruction for the
1700 to 1979 interval was relatively low (0.374). Moreover, the result from a superposed epoch
analysis using D’Arrigo et al.’s (2001) reconstruction showed a non-significant trend between
the positive phase of the PDO and fire occurrence consistent with Schoennagel et al.’s (2005)
study.
Fauria and Johnson (2008) associate the occurrence of widespread fires on the west/east
side of the Canadian Rockies with negative/positive PDO phases and place JNP at the boundary
between different PDO phase influences. However, Fauria and Johnson (2006, 2008) use records
of fire extents spanning from 1959-1999 and 1918-2005, respectively, when a strong fire
suppression effect has been identified in many regions of North America (White 1985; Fulé et al.
1997; Brown et al. 1999; Taylor 2000; Veblen et al. 2000; Taylor and Skinner 2003; Hessl et al.
2004; Schoennagel et al. 2004; Arienti et al. 2006), therefore it is not clear if their findings are
applicable to understand historic climate-fire interactions. My findings suggested JNP is located
76
in a regional climate that differs from study areas further south in the Rocky Mountains and east
in the boreal. “Dipoles” in teleconnections are well documented further south (Westerling and
Swetnam 2003; Schoennagel et al. 2005; Trouet et al. 2006; Kitzberger et al. 2007; Heyerdahl et
al. 2008; Pederson et al. 2011). To effectively discern effects of teleconnections on the historic
fire regime I would need more fire-scar data.
3.4.2
Multi-decadal variation in fire frequency
The multi-decadal variation in my fire scar record for the montane forests in JNP, were
similar to long-term patterns in other fire reconstructions in western North America. I found: (1)
a declining number of fire scars prior to 1772; (2) no fire scars between 1773 and 1826; (3) many
fire scars between 1827 and 1915; and (4) no fire scars after 1915. Much of this variation is
consistent with well-documented climate variation at the continental to hemispheric spatial
scales and with land use changes (Meyn et al. 2007). Both likely explained the variation in fire
occurrence in JNP.
Between 1646 and 1772, seven fires burned with one fire every 18 years in the study area,
on average. Fires were recorded during this period even though only 39 sites had trees that
established prior to 1773 and 9 sites included recorder trees. Fire was likely more frequent than
indicated by the fire-scar record due to “disappearing evidence”, an inherent limitation of firescar studies (Swetnam et al. 1999). Successive fires, mechanical damage and wood
decomposition effectively remove recorders of fire thus decreasing the chances of finding older
fire-scar dates (Van Pelt and Swetnam 1990; Parsons et al. 2007; Swetnam et al. 2011).
77
From 1773 to 1826, no fire scars were recorded in my study area. The lack of fire during
this period is consistent with the well-documented “fire gap” throughout western North America
(Veblen et al. 2000; Heyerdahl et al. 2002; Brown 2006; Sibold and Veblen 2006; Trouet et al.
2006; Kitzberger et al. 2007) and in northern Patagonia in South America (Kitzberger et al.
2001; Veblen and Kitzberger 2002; Kitzberger and Veblen 2003). The potential cause of the fire
gap has been attributed to broad-scale climatic variation given the common patterns in North and
South America (Kitzberger et al. 2001, 2007). Specifically, the fire gap has been linked to the
coldest phase of the Atlantic Multidecadal Oscillation (AMO) since the mid-1600s (Kitzberger et
al. 2007); however, I did not find a statistical association between the AMO and fire occurrence
in JNP. Alternately, widespread changes in land use throughout North and South America were
concurrent with this period (Meyn et al. 2007). Written and oral records indicate smallpox
epidemics in the 1780s and 90s affected First Nation communities in JNP and both east and west
of the Rocky Mountains (Johnson and Larsen 1991; Wikeem and Ross 2002; Murphy et al. 2007;
Payne 2007). In parallel, tuberculosis, influenza, measles and typhoid fever also led to First
Nation peoples’ sickness and death in the 18th and 19th centuries across North America (Durie
2003; Waldram et al. 2006). Depopulation would result in a strong decrease in fire ignitions
since archaeological and historic records show evidence of First Nation use of fire in the region
(Anderson and Reeves 1975; Francis 1997; Hudecek-Cuffe 2000; Murphy 2007; Murphy et al.
2007).
From 1827 to 1915, fires were most frequent and included three widespread fires. High
fire frequency in the late 19th and early 20th centuries is a common pattern in fire history studies
throughout North America (Veblen et al. 2000; Taylor and Skinner 2003; Beaty and Taylor
78
2008) and South America (Veblen et al. 1999), which reflects climate conditions favourable to
fire, increased use of fire by settlers and better conservation of fire-scar evidence. During this
period, seven out of eight fires in JNP burned during El Niño phases. The weak relationship with
the negative phase of the PDO particularly over the prolonged phase between 1840 and 1922
contrasted with other climate-fire studies conducted in the western Cordillera. Other climate-fire
analyses found fires tended to be associated with positive PDO phases (Hessl et al. 2004;
Schoennagel et al. 2005; Heyerdahl et al. 2008; Da Silva 2009). Of all these studies, only Hessl
et al.’s (2004) used Gedalof and Smith’s (2001) PDO reconstruction; however, they did not use
the identified multi-decadal phases. Rather Hessl et al. (2004) compared the PDO indices on an
annual basis against the fire dates whereas I used the multi-decadal positive and negative phases
identified by Gedalof and Smith (2001). Other climate-fire analyses in the western Cordillera
used D’Arrigo et al.’s (2001) or MacDonald and Case’s (2005) PDO reconstructions. Since treering based reconstructions of climate patterns have relatively low correlations especially prior to
the 20th century differences between climate-fire interaction analyses are likely (Watson and
Luckman 2005). The lack of a relationship with the AMO contrasted with Kitzberger et al.’s
(2007) study which found positive AMO phases were related to higher-than-expected fire
synchrony across western North America. This period coincided with the presence of increasing
European explorers and peoples of mixed-ancestry in Jasper, as well as the establishment of
several posts and homesteads within and near the study area, and the development of the Grand
Trunk Pacific railway along the Athabasca River (Murphy 2007; Murphy et al. 2007; Payne
2007; Taylor 2007). Settlement has been linked to fire activity both in North and South America
(Barrett and Arno 1982; Veblen et al. 1999; Veblen et al. 2000; Taylor and Skinner 2003;
Schoennagel et al. 2004; Beaty and Taylor 2008; Jensen and McPherson 2008).
79
No fire scars were recorded after 1915 in the study area, although this is the period during
which the sample depth and potential to record fires was greatest. Starting in 1919, I found live
trees at 100% of the 172 sites. Since thin-barked Picea and Pinus contorta were the dominant
species, chances were high that lower-severity fires would have scarred these trees. The lack of
fire scars at all 172 sites for at least 82 consecutive years (1916-1997) is unprecedented in the
533-year fire record for the study area. Climatic variation alone could not explain the absence of
fire scars since 1915. Tree-ring proxy indicators of drought and instrumental climate data
indicated variable climatic conditions during the 20th century including relatively dry periods
from 1916 to 1947 and 1983 to 2009 that were similar to the drought conditions in which historic
fires burned. Climate during these periods appeared favourable for fire; however, the lack of
scars implies either fire ignition or spread were limiting.
Evidence of lightning and human ignitions exists in the Canadian Rocky Mountain
National Parks (Tyrrell 1916; Wierzchowski et al. 2002; Murphy 2007; Murphy et al. 2007;
Parks Canada 2013). However, changes over time in rates of lightning strikes and ignitions are
not evident (White 1985; Wierzchowski et al. 2002); however, there is strong evidence that
human ignitions have changed substantively over time exists. Written, oral and archaeological
records document historic and prehistoric First Nation use of fire within and near JNP over
hundreds to thousands of years (Anderson and Reeves 1975; Francis 1997; Hudecek-Cuffe 2000;
Murphy 2007; Murphy et al. 2007). Written and oral records convey how families living in and
nearby the study area between 1895 and 1910 lit fires in the spring to create forage for local
wildlife and facilitate hunting (Murphy et al. 2007). However, after 1913, prohibition of fire use
80
and the displacement of local people excluded fire from the landscape (Murphy 2007; Murphy et
al. 2007).
Fire suppression has directly affected fire regimes in the national parks in western North
America during the 20th century (Barrett and Arno 1982; Arno 1985; Gruell 1985; Veblen et al.
2000; Taylor and Skinner 2003; Schoennagel et al. 2004; Jensen and McPherson 2008).
Following the implementation of a fire protection and suppression policy in 1913 within JNP
(Murphy et al. 2007), park managers were mandated to suppress fires before they spread in size
or to an intensity that was difficult to control. Over time, enhanced fire-fighting training, access
and equipment have increased the capacity to detect and rapidly suppress fires (Tande 1979;
Murphy 1985, Fulé et al. 1997; Brown et al. 1999; Taylor 2000; Veblen et al. 2000; Arienti et al.
2006). For example fire records from Parks Canada (2013) include 101 lightning-ignitions since
1940, eight of which resulted in fires >40 ha in size. Only six wildfires were ignited by lightning
in my study area and all were between 2004 and 2009; however, all were suppressed before they
exceeded one ha in size.
3.5
Conclusion
The montane forests of Jasper National Park (JNP) provided evidence both climate and
humans have influenced the historic fire regime. In particular, the recent absence of fire was
associated with a period of active fire suppression in JNP. Eighteen fires from 1646 to 1915
burned during droughts, which had a weak association with the El Niño phase, especially since
1863, and the negative phase of the Pacific Decadal Oscillation. To better understand climate
drivers I would need additional data from JNP. Although the climate-fire relations at the inter81
annual level were ambiguous, multi-decadal scale trends were consistent between JNP and many
fire-scar based studies in western North and South America. After accounting for the depleting
fire-scar record, fire frequency varied through time consistent with continental to interhemispheric climate drivers and changes in land use, including settlement by Europeans, loss of
First Nation peoples and their cultural use of fire, and fire suppression policies and practice. JNP
provided a long, well-documented human use of fire in montane forests. The increase in fire
frequency in the 1800s and early into the 20th century corresponded with settlement; however,
fires at that time also consistently burned during El Niño phases. Since 1915, I found a lack of
fire scars despite potential recorder trees at all sites and multi-year/decade periods when climate
was conducive to fire. Park records substantiated the realisation of fire suppression and its
consequent effect on the historic fire regime.
82
Table 3.1 Summary of the fire records for 115 sites west of the Athabasca River.
83
Table 3.1 Continued
84
Table 3.2 Summary of the fire records for 57 sites east of the Athabasca River.
85
Figure 3.1 Fire history sites (n = 172) in the Athabasca River valley in montane and subalpine forests of Jasper National Park. Firescar dates are in red. Year of establishment of the oldest tree per site are in black. Squares are homesteads established from 1895
and 1910. Triangles are suppressed lightning-ignited fires from 2004 to 2009. In the inset map, JH2 = Jasper House II (1826-1884),
LH = Larocque House (1824-1825), HH = Henry House (1811-1830s) and PP = Pyramid and Patricia lakes.
86
Figure 3.2 Fire history records from 1465 to 2012 at 172 sites. Horizontal lines represent the sitelevel composite tree-ring records (top to bottom: sites 1 to 115 west of the Athabasca River left
column; sites 116 to 172 east of the Athabasca River right column). The length of each line
represents the period of record, starting from the pith of the oldest tree to 1997-2000 or 2012
when trees were sampled. Fire evidence includes crossdated, annually-resolved fire scars. For
sites lacking fire scars, dashed lines indicate the length of the record determined by the age of the
oldest living tree.
87
a) West of Athabasca River (n = 115)
20
25
0
b) East of Athabasca River (n = 57)
20
20
10
0
0
c) All study sites (n = 172)
20
45
sites with fire scars
No. of sites recording fire
0
No. of
10
10
0
1600
0
1700
1800
1900
2000
Year
Figure 3.3 Composite fire records from 1600 to 2012 for a) the 115 sites west of the Athabasca
River, b) the 57 sites east of the Athabasca River and c) all 172 sites in the study area. In each
composite fire record, the graph shows the number of sites with fire scars (black line) and the
number of sites recording a fire (grey bars) over time.
88
0.4
Precipitation
0.2
C o rre la ti o n c o e ffi ci e nts
0.0
-0.2
-0.4
0.4
Maximum temperature
0.2
0.0
-0.2
-0.4
0.4
Heat-moisture index
0.2
0.0
-0.2
-0.4
A
M
J
J
A
Summer
(prior)
Mo nth a nd
S
O
N
D
J
F
M
A
M
J
J
A
Winter
(prior)
Summer
(current)
s e a s o n re la ti ve
to ri ng fo rma ti o n
S
Figure 3.4 Climate-growth relations for Pseudotsuga menziesii from 1902 to 2009. Bars are the
correlation function coefficients between the Pseudotsuga menziesii residual chronology and
monthly precipitation, maximum temperature, and heat-moisture indices and the (top to bottom).
Dots are significant correlation function coefficients (α = 0.05).
89
Residual Index
3
2
1
0
-1
-2
-3
ENSO Index
2
1
0
-1
-2
PDO Index
2
1
0
-1
-2
AMO Index
3
2
1
0
-1
-2
-3
1650
1700
1750
1800
1850
1900
1950
Year
Figure 3.5 Fire occurrence (black dots) from 1646 to 1985 relative to inter-annual to multidecadal variation in climate. The residual ring-width chronology represents drought (top),
followed by ENSO, PDO and AMO indices reconstructed from tree rings (Cook et al. 2009;
Gedalof and Smith 2001; Gray et al. 2004, respectively). Within panels (top to bottom), dark
grey (white) areas represent dry (wet) periods (Watson and Luckman 2004), El Niño (non-El
Niño) events (Garcia-Herrera et al. 2008; Yu and Kim 2013), positive (negative) phases of the
PDO (Gedalof and Smith 2001) and AMO (Gray et al. 2004) time series. For the AMO, light
grey areas represent neutral phases (Gray et al. 2004).
90
1
a) Residual chronology
0
-1
-4
Departure (%) from mean
1
-3
b) ENSO
-2
-1
0
1
2
-2
-1
0
1
2
0
-1
1
c) PDO
0
-1
1
d) AMO
0
-1
-4
-3
Years relative to fire (0)
Figure 3.6 Departure (%) from the mean of fire years (year 0) relative to the four years preceding
and two years following fire for the a) residual chronology, b) ENSO, c) PDO and d) AMO
climate patterns from 1642 to 1917. Two dots are significant correlation function coefficients (α
= 0.01) derived from 1,000 Monte Carlo simulations.
91
0.8
χ 2 = 4.156
p = 0.041
a)
Proportion of years
0.6
0.4
0.2
0
Dry
Wet
Multi-year climate
0.8
χ 2 = 2.296
p = 0.130
b)
χ 2 = 1.355
p = 0.244
c)
χ 2 = 0.274
p = 0.872
d)
Proportion of years
0.6
0.4
0.2
0
El Niño
non-El Niño
+PDO
ENSO phases
0.8
e)
-PDO
+AMO
PDO phases
f)
nAMO
-AMO
AMO phases
g)
Proportion of years
0.6
0.4
0.2
0
El Niño
+PDO
-PDO
non-El Niño
+PDO
-PDO
ENSO x PDO phases
El Niño
+AMO nAMO -AMO
non-El Niño
+AMO nAMO -AMO
ENSO x AMO phases
+PDO
-PDO
+AMO nAMO -AMO +AMO nAMO -AMO
PDO x AMO phases
Figure 3.7 Tests of association between fire (black) and non-fire (white) years and climatic
variation. Years from 1645 to 1915 are stratified by a) multi-year periods of dry and wet climate,
b) ENSO (El Niño and non-El Niño), c) positive (+) and negative () PDO, and d) positive (+),
neutral (n) and negative () AMO phases, and e-g) two-way interactions between ENSO, PDO
and AMO. Chi-squared goodness of fit and p-values indicate significant tests ( = 0.05).
92
Annual ring-width indices
2
1
b
b
a
ab
ab
a
0
Dry
Wet
Wet
Unclassified
Fire
Dry
years
periods
periods
period
period
period
1646-1915 1646-1915 1646-1915 1916-1947 1948-1982 1983-2009
Figure 3.8 Annual ring-width indices by fire years and periods from 1646 to 2009. In each box
plot, the black horizontal line represents the median and box boundaries are the 25th and 75th
percentiles; bars are the 10th and 90th percentiles; and different letters denote significant
differences among mean values (α = 0.05).
93
Annual precipitation (mm)
1600
1400
1200
1000
b
c
800
a
600
Annual maximum temperature (°C)
400
12
11
10
9
8
a
a
b
7
6
5
Annual heat-moisture indices
50
40
30
20
a
ab
b
10
0
Dry
Wet
Unclassified
period
period
period
1916-1947 1948-1982 1983-2009
Figure 3.9 Annual precipitation (top), maximum temperature (middle) and heat-moisture indices
(bottom) by period from 1916 to 2009. In each box plot, the black horizontal line represents the
median and box boundaries are the 25th and 75th percentiles; bars are the 10th and 90th
percentiles; and different letters denote significant differences among mean values (α = 0.05).
94
Chapter 4: Conclusions
4.1
Summary and Contribution to Research
My thesis contributes to the understanding of the influence of mixed-severity fire regimes
and their drivers in the montane forests of Jasper National Park (JNP). The traditional paradigm
was that high-severity fire regimes were prevalent in the Canadian Rocky Mountains (Johnson et
al. 1990; Masters 1990); however, my findings at a network of sites estimated the fire regime
was of mixed-severity with a greater component of high-severity rather than low-severity fires
(Chapter 2). This difference in fire regimes is subtle yet the significance for JNP’s forest
management is important. Planning for fire hazard mitigation, the protection of species at risk
and the restoration of the mixed-severity fire regime as a fundamental ecological process within
JNP can benefit from my crossdated, annually resolved fire records. My exploration of climatefire interactions indicated that fires were most likely to burn during drought years (Chapter 3).
Associations with regional-scale climate teleconnections were weak; however, additional
crossdated fire records within Jasper and over a broader area in the Southern Canadian Cordillera
would increase statistical power and improve understanding of climatic drivers. Such knowledge
is needed to support risk and hazard assessments and to plan for future forest and wildfire
management given climate change.
4.1.1
Importance of dendrochronological analyses
Identifying fire-scar years, even-aged and post-fire cohorts, and veteran trees surviving
fires were criteria to classify and differentiate the severity of historic fires through time at
individual sites (Chapter 2; Sherriff and Veblen 2006; Heyerdahl et al. 2012). At sites with
mixed-severity fire histories, age structures were significantly more discontinuous than at high95
severity sites. Mixed-severity sites also had greater diversity in age, size and species than highseverity fire history sites, although differences were not statistically significant. The patterns of
uneven and discontinuous age and size structures and tree species diversity at mixed-severity
sites that I observed were consistent with other research on mixed-severity fire regimes
(Amoroso et al. 2011; Halofsky et al. 2011; Perry et al. 2011; Marcoux et al. 2013). However,
wide variation among sites classified as mixed- versus high-severity fire history class made the
classes difficult to differentiate without detailed information on tree ages and fire-scar dates.
My comparative analyses of canopy and subcanopy strata between fire-history classes
showed composition and size-structure indices were poor indicators of historical fire severity. As
a result, the type of information derived from aerial photographs or remote sensing technologies
and recorded in standard vegetation resource inventories, and the standard measurements of
forest composition and tree size included in field-based timber cruises, will not effectively
distinguish sites of different fire histories. To best understand the historic fire regime, detailed
dendrochronological analyses are essential to reconstruct disturbance-related mechanisms and
processes of change and to differentiate sites with mixed- versus high-severity fire histories.
4.2
4.2.1
Management Implications
Fire regimes and management
In many forests of western North America, tree composition is shifting towards shade-
tolerant but fire-intolerant species due to fire exclusion (Taylor and Solem 2001; Fulé et al. 2003;
Beaty and Taylor 2008; Bekker and Taylor 2010; Perry et al. 2011). In JNP ongoing shifts in
species composition were subtle. Forest composition was strongly influenced by past high96
severity fires that resulted in even-aged cohorts in the stem-exclusion or under re-initiation
stages of stand development. Subcanopy trees were slow-growing poor competitors with limited
potential to recruit to the canopy; however, the sapling stratum of most sites was dominated by
shade-tolerant, fire-intolerant Picea. Over time, the recruitment of suppressed Picea saplings as
canopy gaps form due to tree senescence and/or within stand disturbances would lead to a
decrease in species diversity within and among stands. Subcanopy Picea also creates ladder fuels
and increases the risk of crown fire. Given the widespread increase in closed-canopy forests in
JNP (Rhemtulla et al. 2002), many of which are at similar stages of stand development with
increasing ladder fuels due to fire exclusion during the 20th century (Chapter 2), there is risk that
the landscape-scale fire regime could shift from a mixed-severity fire regime to exclusively a
high-severity, stand-replacing fire regime.
The cumulative effects of homogenization of stand structures, compositional shifts
towards Picea and increasing ladder fuels, increase the chance of large crown fires in the
montane forests. In combination with climate change, the cumulative effects could jeopardize
resilience of montane forests in JNP. In the advent of a wildfire occurring under extreme weather
conditions (Johnson and Wowchuck 1993; Bessie and Johnson 1995; Nash and Johnson 1996;
Westerling et al. 2006), the greatest impacts would be most likely at sites with mixed-severity
fire histories (Schoennagel et al. 2004; Perry et al. 2011). Thus, my results support management
actions that increase forest compositional and structural diversity such as thinning, burning and
modified response to wildfires. Such active management will restore structures and processes
associated with mixed-severity fires and mitigate risk of high-severity fires in locations that
would pose a threat to park residents, visitors and infrastructure (Westhaver et al. 2007).
97
4.2.2
Importance of mixed-severity fires for at risk Rangifer tarandus and Ursus arctos
populations
Wildlife managers in the Canadian Rocky Mountain parks and adjacent areas are
concerned about the fragmentation and quality of habitat for at risk woodland caribou (Rangifer
tarandus caribou Gmelin) and grizzly bear (Ursus arctos horribilis Linnaeus) populations
(O’Neill 2011; Environment Canada 2012). My fire records provided evidence mixed-severity
fires historically characterized portions of the landscape (Chapter 2). Mixed-severity fire regimes
produce a range of effects on vegetation, including enhancing landscape heterogeneity (Lertzman
and Fall 1998; Perry et al. 2011) and providing habitat diversity (Halofsky et al. 2011). Since
woodland caribou and grizzly bear require a sequence of seasonal habitat conditions for their
wellbeing (Terry et al. 2001; Munro et al. 2006; Nielson et al. 2006; Environment Canada 2012;
Stenhouse and Graham 2012), restoring onto portions of the Park’s landscape and surrounding
forests of the Rocky Mountain Foothills a mixed-severity fire regime or a silvicultural treatment
which emulates a range of fire effects similar to a mixed-severity fire regime could provide
and/or enhance habitat quality for at risk populations.
4.3
4.3.1
Future Research
Importance of human versus lightning ignitions
Variation in fire frequency over decades in JNP suggested both humans and climate
influenced the historical fire regime. However, differentiating human versus lightning sources of
ignition for historic fires is very difficult. To better understand the human component I propose
to develop fire records for a paired analysis of fire history at sites of similar biophysical
98
characteristics where one group of sites would be adjacent to archaeological sites whereas
another group of sites would be removed from archaeological sites. Results could be integrated
in a geographic information system to identify whether spatial patterns exist between historic fire
occurrence and archaeological sites. To develop a more complete fire scar record, oral history
and archaeology could provide further evidence of First Nation peoples’ use of fire in the Park.
4.3.2
Monitoring regeneration and fuel
Monitoring the stands at my network of sites would test the hypothesis that the
successional trajectory of increasing Picea at stand to landscape scales. It would also identify
sites in which forest composition and structure present a fuel hazard. The observed regeneration
in the network of sites was predominantly Picea. In the absence of fire, future regeneration is
most likely to continue to be Picea as it is shade tolerant and does not need exposed mineral soil
to regenerate (Burns and Honkala 1990; Klinka et al. 2002). Quantifying and monitoring fuels at
my sites would anticipate local fire effects should a fire burn there.
4.3.3
Climate-fire relations
To better understand climate-fire interactions in JNP would require annually resolved
fire-scar records with sufficient fire dates. Increasing the amount of data would also improve
understanding of historic human ignited fires in the Park. Theresa Dinh, M.Sc. candidate, at the
University of Guelph, Ontario, crossdated 170 fire-scarred samples from Tande’s (1977, 1979)
study. In addition to increasing the number of crossdated fire-scar samples in JNP, a second
approach would be to compare fire records among the Canadian Rocky Mountain parks and in
the Canadian Cordillera. Expanding the current study to include annually-resolved fire-scar
99
records from Kootenay National Park (Kubian 2013), the montane forests of the Southern Rocky
Mountain Trench in British Columbia (BC) (Cochrane 2007; Daniels et al. 2011) and the Joseph
and Gold Creek watersheds near Cranbrook, BC (Da Silva 2009; Marcoux 2013; Marcoux et al.
2013) would allow a regional-scale assessment of the associations between spatio-temporal
climate patterns and fire. Improved knowledge of variation of climate-fire interactions over the
Southern Canadian Cordillera would help to anticipate the conditions conducive to extreme fire
events, especially given projected climate change in the region. It would also support the practice
of ecosystem-based management guided by range of fire-disturbance variation.
100
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