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Ecological Restoration Institute
Fact Sheet: Testing alternative ponderosa pine restoration treatments
April 2015
Forest Structure and Fuels Dynamics Following Ponderosa Pine
Treatments, White Mountains, Arizona, USA
By John Paul Roccaforte, Senior Research Specialist
Introduction
Southwestern ponderosa pine forest ecosystems have become uncharacteristically dense as a result of intensive livestock grazing, logging, and fire exclusion, which have contributed to a buildup of fuels and increased vulnerability to high-severity, landscape-scale crown fires and epidemic pathogenic outbreaks. The
two largest wildfires in Arizona recorded history – the 2002 Rodeo-Chediski fire and the 2011 Wallow fire
(combined area exceeded 1 million acres) – occurred in the White Mountains of east-central Arizona. In addition to the loss of critical wildlife habitat and old/large trees, and the possibility that previously forested
areas may experience vegetation type conversion,
these “mega-fires” have cost an estimated $465 million to date. This number includes fire suppression,
loss of property, and rehabilitation costs.
To test alternative approaches for restoring degraded
ponderosa pine forests and reducing the potential of
high-severity fires, we implemented an experiment in
the White Mountains of east-central Arizona. The objectives of this study were to (1) quantify site-specific
reference conditions using dendroecological reconstruction methods, (2) analyze effects of elevation on
historical and contemporary changes in forest structure and canopy fuels, (3) compare effects of alternative restoration treatments (thin plus burn and burnonly) on stand structure, surface fuels, and canopy
fuels, and (4) evaluate effectiveness of treatments for
restoring attributes to near historical reference conditions.
Of the treatments tested in this study, the full treatment (thin plus
burn) was the only one that rapidly shifted forest structure and
canopy fuels values to near or within the historical reference conditions. Photo by John Paul Roccaforte
Research Findings

Reconstructed forest structure and fuels were low prior to fire exclusion. The r econstr ucted
(1880) estimate of basal area (BA) averaged 40 ft²/ac and tree density averaged 35 trees/ac with the
highest density values occurring at the two upper elevation sites; 1880 mean canopy fuel load (CFL)
and mean canopy bulk density (CBD) values were the lowest observed in any time period in this
study.

Forest structure and fuels values increased substantially by 2002. Pr e-treatment (2002) BA averaged 126 ft²/ac and tree density averaged 376 trees/ac representing three- and ten-fold increases, respectively, compared to 1880 values; mean CFL increased by 230% and mean CBD increased by
292% between 1880 and 2002.
The Ecological Restoration Institute is dedicated to the restoration of fire-adapted forests and woodlands. ERI provides services that support the social
and economic vitality of communities that depend on forests and the natural resources and ecosystem services they provide. Our efforts focus on science
-based research of ecological and socio-economic issues related to restoration as well as support for on-the-ground treatments, outreach and education.
Ecological Restoration Institute, P.O. Box 15017, Flagstaff, AZ 86011, 928.523.7182, FAX 928.523.0296, www.nau.edu/eri
 Full treatment (thinning plus burning) rapidly shifted forest structure and canopy fuels values
to levels near or within reference conditions. Following the thinning plus burning treatment, values for BA, tree density, CFL, CBD, and canopy base height (CBH) approached or were within the
historical reference conditions (Figure 1); diameter distributions following the full treatment closely
resembled reference conditions.
 Post-treatment burn-only values remained outside reference conditions. Although slight r eductions in tree density occurred following the burn-only treatment, values for BA, tree density, CFL, and
CBD remained closer to pre-treatment values than to reference conditions; CBH was the only canopy
fuels variable that approached the historical reference conditions (Figure 1).
 Mortality of trees established before European settlement circa 1880 varied by treatment. Although some presettlement tree mortality occurred in the full (1.5 trees/ac, or 25% of presettlement
trees alive in 2002 died or were cut by 2013) and burn-only (1.0 trees/ac, or 31% died) treatments, 1.1
trees/ac, or 15% died in the control without active treatment. Presettlement tree mortality was higher
in the full and burn-only treatment units compared to the control due to disturbance from the prescribed burning that occurred in both of those areas. Disturbance from mechanical thinning in the full
treatment also likely had impacts. The mortality in the control represents the background mortality that
occured without any treatments.
Figure 1. Tree density (A) and canopy fuel load (B) were reduced more in the full treatment (thin plus burn) compared to the burn-only or
untreated control. These reductions shifted stand conditions closer to the historical reference conditions (gray shaded area determined from
1880 reconstruction) than the burn-only or control. The full and burn-only treatment raised canopy base height (C) similarly to near reference
conditions while crown base heights remained relatively low in the control. Years shown are 2002 (pre-treatment), 2009 (1-year posttreatment), and 2013 (5-year post-treatment).
Conclusions
This study reconstructed reference conditions across an elevation gradient and tested the effectiveness of
full and burn-only treatments to restore forest structure and fuel loads in a ponderosa pine forest in the
White Mountains, Arizona. Of the treatments tested in this experiment, the full treatment (thin plus burn)
was the only one that immediately shifted forest structure, diameter distributions, and canopy fuel loadings
to levels near or within the historical reference conditions. Although burn-only treatments may be appropriate in certain situations, if tree density and canopy fuels are not effectively reduced following the initial
burn, subsequent repeated prescribed burns may have limited effectiveness for restoring ponderosa pine forests. While some presettlement tree mortality occurred following the full and burn-only treatments due to
prescribed fire and other disturbance, it is important to keep in mind that without treatment these areas remain susceptible to high-severity crown fires and higher levels of tree mortality. Managers will have to consider tradeoffs between full and burn-only treatments and increasingly make those decisions in the context
of climate-induced, landscape-scale forest fires.
This Fact Sheet summarizes information from the following publication:
Roccaforte, J.P., D.W. Huffman, P.Z. Fulé, W.W. Covington, W.W. Chancellor, M.T. Stoddard, and J.E. Crouse. 2015. Forest structure and
fuels dynamics following ponderosa pine restoration treatments, White Mountains, Arizona, USA. Forest Ecology and Management
337:174-185. http://library.eri.nau.edu/gsdl/collect/erilibra/index/assoc/D2015001.dir/doc.pdf
Contact
John Paul Roccaforte, [email protected]
NAU is an equal opportunity provider.
This research was funded by a grant from the USDA Forest Service.