The Blood Pressure
of Trees:
PSY1
Stem Psychrometer
Michael Forster
ICT International
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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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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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The Pressure Bomb
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Research Background
• Hawkesbury Forest Experiment
• University of Western Sydney,
Richmond, NSW
• Whole Tree Chambers
• CO2 Experiment
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Research Background
CO2 Experiment:
• Ambient CO2: 380ppm
• Elevated CO2: Am + 300ppm
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Research Background
Sap Flow
CO2
Stomatal
Conductance
(Stomatal closure)
Plant
Water
Potential
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Stomatal Conductance Results
Data from:
LiCor 6400
Graph Source:
David Ellsworth, UWS;
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Sap Flow Results
Graph Source:
Michael Forster, UWS; Derek Eamus, UTS
Data from:
Dynagage
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Water Potential Results
Time of Day
Data from:
Pressure Bomb
17/12/2012
Leaf Water Potential (MPa)
8p
m
7p
m
5p
m
3p
m
1p
m
11
am
9a
m
7a
m
5a
m
0
-0.5
-1
-1.5
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Ambient
-2
Graph Source:
Michael Forster, UWS; Derek Eamus, UTS
-2.5
Elevated
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The Pressure Bomb
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1 Day of Data
Time of Day
Leaf Water Potential (MPa)
8p
m
7p
m
5p
m
3p
m
1p
m
11
am
9a
m
7a
m
5a
m
0
-0.5
-1
-1.5
-2
Ambient
Elevated
-2.5
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PSY1 Stem Psychrometer
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Stem Water Potential (MPa)
17 Days of Data from Psychrometer
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A Brief History…
– First published: 1984
(Dixon & Tyree (1984) Plant, Cell & Environment 7: 693-697)
– Professor Mike Dixon,
University of Guelph, Canada
– Difficult with 1980’s technology
– Few publications
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A Brief History…
Micro-volt changes in temperature
Micro-volt = one millionth of a volt
= Volt x 10-6
PSY1 Stem Psychrometer was
released in 2010
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PSY1 Stem Psychrometer
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The Psychometric Method
Condensation
Dew Point = Condensation
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The Psychometric Method
Temperature
=
Dew Point
=
Relative Humidity=
Water Potential =
25 C
23 C
89 %
-16.0 MPa
Temperature
=
Dew Point
=
Relative Humidity=
Water Potential =
25 C
16 C
57 %
-77.2 MPa
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The Psychometric Method
Ambient Temperature
C-thermocouple Dew Point
Corrections
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Step 1: Dry Bulb Reference
Temperature difference
equals close to
zero
Electronic Dry Bulb Offset
(EDBO)
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Step 2: ΔT: Chamber vs Stem Temperature
ΔT is temperature
difference between
chamber air and
plant stem
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Step 3: Peltier Cooling Pulse
Condense water onto
C-thermocouple for
10 seconds
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Step 4: Wet Bulb Depression
Measure how cold the
C-thermocouple was
in order to condense
water
How cold was
C-thermocouple in order
to change water
from vapor to liquid phase
Dew Point
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Step 5: Ambient Temperature
Measure temperature of
chamber for ambient
temperature
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Review: The Psychometric Method
Ambient Temperature
C-thermocouple Dew Point
Corrections
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Review: The Psychometric Method
STEP 1: Dry Bulb Reference
Correction
STEP 2: ΔT
Correction
STEP 3: Peltier Cooling Pulse
Condensation
STEP 4: Wet Bulb Depression
Dew Point
STEP 5: Ambient Temperature
Ambient Temp
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Does It Work?
From Mike Dixon,
University of Guelph
17/12/2012
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1:1 Relationship
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 PSWC and PSY1
-2.5
Data and information from Mike Dixon, Guelph University
-3.0
Stem Water Potential (MPa, PSY1)
Temperature Correction
0.0
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Exact Relationship
Unpublished Data - Courtesy George Koch Northern Arizona University
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Installation – Important!
Install on XYLEM not phloem
Chamber to be sealed against xylem
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Installation – Important!
Insulate against extreme temperature fluctuations
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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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Tree Transplantations
Images from www.treeworldinfo.com
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Transplantations & Wireless
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Experimental Treatments
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Ivory Silk (Syringa reticulata)
Treatment = YES fungus (mycorrhiza)
Control = NO fungus (mycorrhiza)
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Gravity & Stem Water Potential
Tree Height
Water Potential
40m
-0.4MPa
30m
-0.3MPa
20m
-0.2MPa
10m
-0.1MPa
0m
0.0MPa
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Gravity & Stem Water Potential
Baradur, Tree Top West
Psychrometer at 88m Height
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
0
50
100
150
200
250
300
Daily cycles of water potential measured with ICT
psychrometer on a branch at about 88 meter height in a
91 meter tall Sequoiadendron giganteum
At Whitaker’sForest, California during the period Aug. 13 –
Aug. 17, 2010. Unpublished Data - Courtesy George Koch
North Arizona University
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Gravity & Stem Water Potential
Baradur, Tree Top West
Psychrometer at 88m Height
0.0
-0.2
-0.4
-0.6
-0.8
-0.88MPa
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
0
50
100
150
200
250
300
Difference due to:
- Soil water deficit?
- Stem storage deficit?
- Stem resistance?
(hydraulic conductivity)
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Plant Parasites
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Plant Parasites
Figure from Forster 2012
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Plant Parasites
Figure from Forster 2012
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Plant Parasites
TREE
BRANCH
FUNGAL GALL
DAY TIME:
Positive flow into dry atmosphere
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Plant Parasites
TREE
BRANCH
FUNGAL GALL
NIGHT TIME:
Reverse flow into dry plant
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