Low and Reverse
Sap Flow
Michael Forster
ICT International
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A Brief History of HRM…
– 1998 to 2000, limitations with
existing methods
–Low and reverse flow
– Dr Stephen Burgess,
University of Western Australia
– Seminal publication:
Burgess et al. (2001) Tree Physiology 21: 589-598.
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Why Measure Low Flow?
• Small plants – e.g. grasses, herbs, shrubs
• Understorey vegetation
• Nighttime or nocturnal water use
• All of the above can contribute significantly to overall water budget
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Low Flow – Good Techniques
UGT Lysimeters
Can be used on all plants
Heat Ratio Method
Trees and shrubs only
Heat Field Deformation
Trees only
(But watch this space!)
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How Important is Low Flow?
• UGT Lysimeters can detect dew: important in desert regions
• Shrubs can account for 30 to 70% of total vegetation water use
• Nocturnal transpiration can be upwards to 80% of maximum
daytime transpiration (Rosado et al 2012)
• Nocturnal sap flow, as a percent of total daily flow, is as high as 69%
(Feild and Holbrook 2001)
• Nocturnal sap flow, as a percent of total daily flow, on average is 12%
(Forster submitted)
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Nocturnal Sap Flow - Seasons
%Qn = nocturnal sap flow as a proportion of total daily sap flow
Seasons
Spring
Summer
Autumn
Winter
%Qn
10.25
8.78
11.66
7.56
Dry Season
Wet Season
18.89
12.77
Source: Forster, MA (submitted). Tree Physiology.
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Nocturnal Sap Flow - Biome
%Qn = nocturnal sap flow as a proportion of total daily sap flow
Biome
Equatorial
Tropical
Continental
Mediterranean
Warm Temperate
Nemoral
%Qn
29.10
16.54
15.25
10.56
7.60
10.53
Source: Forster, MA (submitted). Tree Physiology.
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Nocturnal Sap Flow - Genus
%Qn = nocturnal sap flow as a proportion of total daily sap flow
Genus
Acacia
Acer
Betula
Eucalyptus
Picea
Quercus
%Qn
14.96
4.11
12.11
10.27
14.60
6.31
Source: Forster, MA (submitted). Tree Physiology.
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What Drives Nocturnal Sap Flow?
• Stem Refilling/Recharge
• The refilling of low or empty xylem vessels
• Stomatal Conductance (Transpiration)
• Water vapour exiting stomatal pores
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How to Measure Stem Refilling
SFM1 Sap Flow Meter
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81 Sensors in a Tree!
• Dr Sebastian Pfautsch, University of
Western Sydney
• HRM sap flow sensors at various
heights over a summer period
• Check out the You Tube Video:
https://www.youtube.com/watch?v=-FSV4SaLm-I
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Stem Refilling & Sap Flow
SFM1 at Crown Height
SFM1 at Breast Height
Sap Velocity (cm/hr)
24
19
Stem Refilling
14
9
4
-1
Midday
Midnight
Midday
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How to Measure Stem Refilling
DBL60 Dendrometer
SFM1 Sap Flow Meter
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Dendrometer V Sap Flow
DBL60 Dendrometer
Tree Circumference (mm)
SFM1 Sap Flow Meter
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Stem Refilling & Dendrometers
DBL60 Dendrometer
Tree Circumference (mm)
SFM1 Sap Flow Meter
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How to Measure Nocturnal Transpiration
UGT Lysimeters
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How to Measure Nocturnal Transpiration
• Decagon’s SC-1 Leaf Porometer
• Easy and quick during the day, a little
harder during the night (high humidity)
• Spot, isolated measurement. Can data
be scaled to entire plant canopy?
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How to Measure Nocturnal Transpiration
SFM1 Sap Flow Meter
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Nocturnal Transpiration & SFM1
SFM1 at Crown Height
SFM1 at Breast Height
Sap Velocity (cm/hr)
24
19
Nocturnal Transpiration
14
9
4
-1
Midday
Midnight
Midday
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How to Measure Nocturnal Transpiration
-1
Diurnal JS,max (cm3 cm-2 hr-1)
(a)
Nocturnal JS,max (cm cm hr )
Linear correlation is needed between sap flow and VPD
140
-2
120
Sugar Gum
(Eucalyptus cladocalyx)
3
100
80
Yellow Box
(Eucalyptus melliodora)
60
20
Blue-leaved
Mallee
(Eucalyptus polybractea)
0
0
1
2
3
4
5
6
Diurnal VPDmax (kPa)
7
8
(b)
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Nocturnal VPDmax (kPa)
Positive correlation between
transpiration and VPD
Source: Doronila & Forster (in press). Int. J. Phyt. Remed.
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Refilling or Transpiration?
• Sensor Height (%) = height of sensor
Sensor Height (%)
100
B
on tree as a proportion of tree size
80
60
• Data from summer period only from a
40
worldwide data set
20
0
0
5
10
15
20
25
30
35
• Positive correlation (r = 0.62) suggests
nocturnal sap flow is more related to
%Qn
Source: Forster, MA (submitted). Tree Physiology.
transpiration than stem refilling
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Refilling or Transpiration?
Silver Gum
(Eucalyptus crenulata)
Soil-plant-atmosphere
continuum experiment
Well watered and
drought treatments
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Refilling or Transpiration?
Drought Stressed Trees
Well Watered Trees (Control Treatment)
Drought Ended
50
45
%Qn
40
35
Drought Started
30
25
20
15
10
5
0
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A Brief History of HRM…
– 1998 to 2000, limitations with
existing methods
–Low and reverse flow
– Dr Stephen Burgess,
University of Western Australia
– Seminal publication:
Burgess et al. (2001) Tree Physiology 21: 589-598.
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24
Hydraulic Redistribution – Reverse Flow
Only HRM and HFD can measure reverse sap flow
HRM on
Trunk and
Root
HFD on
Trunk
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Hydraulic Redistribution – Reverse Flow
Only HRM and HFD can measure reverse sap flow
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From: Caldwell, et al. 1998
Hydraulic Redistribution – Case Study
•
Oliveira et al. (2005) Oecologia 145: 354-363.
•
Floresta Nacional do Tapajos
•
Seasonally dry rainforest
•
Trees grow substantially during dry season
•
Hydraulic redistribution?
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Hydraulic Redistribution
• Dry season hydraulic movement:
• Upward movement from depth into
dry shallow root layers
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Hydraulic Redistribution
• Dry to Wet season transition
hydraulic movement:
• Downward movement from shallow
layers into drier deeper layers
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Hydraulic Redistribution
Dry Season
Dry-Wet Season
Transition
Note
low flow
values
Values less
than zero
indicate
reverse flow
reverse
flow
Figure from Oliveira et al..2005
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Water Potential & Trees
Nadezhdina et al. (2009) New Phytologist 184: 932-943.
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Water Potential & Trees
Nadezhdina et al. (2009) New Phytologist 184: 932-943.
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Water Potential Gradients
• Water Potential:
– Free energy associated with water
– Free energy is the potential to do work
– Water moves from higher to lower potentials
– Water moves from mountains to ocean
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Water Potential Gradients
• Water Potential:
– Plant water potential is a measure of how well-hydrated a
plant is.
– A plant that is more hydrated is healthier, grows faster, has
higher reproductive output (yield), and can cope with
diseases, pruning, disturbance etc, better
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Water Potential Gradients
• Water Potential Units of Measurement:
J/kg
MPa
Bars
RH
-1
-0.001
-0.01
0.999993
-10
-0.01
-0.1
0.999926
-33
-0.033
-0.33
0.999756
-100
-0.1
-1
0.999261
-1000
-1
-10
0.992638
-1500
-1.5
-15
0.988977
-10000
-10
-100
0.928772
Air Dry
-100000
-100
-1000
0.477632
Oven Dry
-1000000
-1000
-10000
0.000618
High Potential
Field Capacity
Wilting Point
Low Potential
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Soil-Plant-Atmosphere Continuum
Low Potential
Atmosphere = -100 MPa
Leaf air = -7.0 MPa
Leaf cell = -1.0 MPa
Trunk = -0.8 MPa
Soil = -0.33 MPa
High Potential
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Water Potential & Trees
Hydraulic Redistribution
Low Potential
Low Potential
High Potential
High Potential
From: Caldwell, et al. 1998
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The Pressure Bomb (or PWSC)
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PSY1 Stem Psychrometer
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Stress and Recovery
Well-hydrated
The plant is becoming less hydrated
Unpublished Data - Courtesy of Alec Downey and Alvaro Arias, species: coffee, location: Costa Rica
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PSY1 versus PSWC
Stem Water Potential (MPa, PSWC)
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-0.5
-1.0
-1.5
-2.0
1:1 Relationship
Between PSY1 and PWSC
Data and information from Mike Dixon, Guelph University
-2.5
Stem Water Potential (MPa, PSY1)
0.0
-3.0
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PSY1 versus PSWC
Data kindly provided by Huade Guan, Flinders University and Cicheng Zhang from Hunan Normal University
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PSWC and Pre-Dawn Water Potential
• Traditionally, researchers measure pre-
dawn leaf water potential as a measure
of soil water potential
• Soil, stem and leaves, are all thought to
be in equilibrium at pre-dawn
• Research has shown this assumption
can be incorrect (Kavanagh et al 2007)
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Testing Soil and Stem Water Potential
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Testing Soil and Stem Water Potential
Soil Water Potential
Stem Water Potential
0
-0.5
-1
Rainfall Event
-1.5
Drought Started
-2
-2.5
-3
Drought Ended
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Pre-Dawn Water Potential
Soil Water Potential
Stem Water Potential
0
-0.5
-1
-1.5
-2
-2.5
-3
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Pre-Dawn Water Potential
• Pre-dawn plant water potential does not necessarily match the soil
• Why? Because of nocturnal sap flow and hydraulic redistribution!
• If you want to know plant water potential – measure the plant directly!
• If you want to know soil water potential – measure the soil directly!
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Sap Flow Training Workshop
Where: ICT International, Armidale, NSW
When: January 6th to 8th, 2014, (with attendees expected to arrive on
January 5th)
Cost: $550 (including GST)
Who Should Attend: The workshop is targeted at beginners or those with
limited knowledge on how to use ICT International’s sap flow equipment.
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ICT International Pty Ltd
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[email protected]
Phone: +61 2 6772 6770
INTERNATIONAL
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