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INFLUENCE OF INTENSIVE INFLUENCE OF INTENSIVE MANAGEMENT ON CANOPY MANAGEMENT ON CANOPY TRANSPIRATION IN LOBLOLLY PINETRANSPIRATION IN LOBLOLLY PINE
Thomas A. Stokes, Lisa Samuelson, Greg Somers, and Tom Cooksey
School of Forestry, Auburn UniversitySouthlands Experiment Forest, International Paper Company
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GrowthGrowth
0
2
4
6
8
10
12
14
1998
ControlIrrigationFertigation
DBH (cm) Height (m) LAI
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ObjectivesObjectives
• Quantify stand and tree water use• Determine how resource availability mediates canopy
physiological response to environmental stress• Examine the influence of resource availability on critical
transpiration (Ecrit)
– Ecrit is the rate at which transpiration begins to level off or even decline to reduce water loss (adapted from Kolb and Sperry 1999)
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HypothesisHypothesis
• Critical transpiration will increase with nutrient and water availability.
• Loblolly pine operates close to critical transpiration rate.
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Study SiteStudy Site
• 15-ha plantation in Bainbridge, GA• 44 x 44m plot size• 2.5 x 3.7m spacing / 1080 trees ha-1
• drip irrigation system• randomized complete block design
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TreatmentsTreatments
Control: complete weed control
Irrigation: drip irrigation
1998: January through October, of 51,000-75,000 l plot-1 month-1.
Fertigation: fertilizer solution
1998: 112 kg N ha-1 yr-1, 28 kg P ha-1 yr-1, and 90 kg K ha-1 yr-1.
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Methods: Canopy Level Methods: Canopy Level MeasurementsMeasurements
• Sap flow measurements recorded hourly along with VPD, PAR, and air temperature from June 1999 to May 2000.
• 30 mm thermal dissipation probes were installed the north and south aspect of each tree used for leaf level measurements.
• Dendrometer bands were placed on each tree to obtain current sapwood area measurements for each month.
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Methods: Leaf Level MeasurementsMethods: Leaf Level Measurements
• Leaf gas exchange and XPP measured at 0900, 1100, 1300 and 1500 with predawn XPP June through September 1999.
• Leaf gas exchange measurements were made with a Li-6400 on four fascicals per tree, two tree per treatment plot and replicated on two blocks per measurement time.
• XPP was measured with a PMS pressure chamber on one fascical per measurement time on the same trees as gas exchange measurements.
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Sap FlowSap Flow
0
1
2
3
4
5
6
0 20 40 60 80 100 120
Hours
Sap
Flow
(l h
-1)
ControlIrrigationFertigation
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Seasonal Water UseSeasonal Water Use
0
1000
2000
3000
4000
5000
6000
7000
8000
Wat
er u
se (k
g tr
ee -1
sea
son-1
)
Jun-Aug 99 Sept-Nov 99 Dec 99-Feb 00 Mar-May 00
Month
ControlIrrigationFertigation
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Average Daily Canopy Transpiration Average Daily Canopy Transpiration RateRate
0
0.5
1
1.5
2
2.5
3
EC (m
mol
m-2
s-1
)
Month
ControlIrrigationFertigation
aba
b
aa
b
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Predawn Xylem Pressure PotentialsPredawn Xylem Pressure Potentials
-1.5
-1.3-1.1-0.9-0.7-0.5-0.3-0.1
XPP
(MPa
)
Jun-99 Jul-99 Aug-99 Sep-99Month
ControlIrrigationFertigation
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Critical TranspirationCritical TranspirationStomatal Control of Water LossStomatal Control of Water Loss
50
70
90
110
130
150
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
EC (mmol m-2 s-1)
g s (m
mol
m-2
s-1
)
ControlIrrigationFertigation
Ecrit
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Critical TranspirationCritical TranspirationStomatal Control of Water LossStomatal Control of Water Loss
• Obtain a quadratic function of stomatal conductance over time.– gsmax = B0 + B1T + B2T2
• Take the derivative of the function in respect to time to determine the time at which the slope = 0 which corresponds to the time of gsmax.– T = -B1/(2*B2)
• To determine the transpiration rate at the time of gsmax simply enter the time into the linear equation for transpiration over time.– E @ gsmax = D0 + D1T
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Critical TranspirationCritical TranspirationStomatal Control of Water LossStomatal Control of Water Loss
50
70
90
110
130
150
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
EC (mmol m-2 s-1)
g s (m
mol
m-2
s-1
)
ControlIrrigationFertigation
a
abb
1.090.850.70
136.8
119.6110.1
P=0.3751
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Critical Transpiration RateCritical Transpiration RateStomatal Control of Water LossStomatal Control of Water Loss
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
-2 -1.75 -1.5 -1.25 -1 -0.75 -0.5 -0.25 0
XPP (MPa)
EC (m
mol
m-2
s-1
)
ControlIrrigationFertigation
ECrit
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Critical Transpiration RateCritical Transpiration RateStomatal Control of Water LossStomatal Control of Water Loss
• Obtain a linear and quadratic function for transpiration over XPP.– E = B0 + B1XPP– E = D0 + D1XPP + D2XPP2
• Take the derivative in respect to XPP to determine when the relationship between transpiration and XPP deviates from linear.– XPP = (B1 – D1)/(2 * D2)
• Solve for critical transpiration by entering the XPP value into the quadratic function.– Ecrit = D0 + D1XPP + D2XPP2
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Critical Transpiration RateCritical Transpiration RateStomatal Control of Water LossStomatal Control of Water Loss
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
-2 -1.75 -1.5 -1.25 -1 -0.75 -0.5 -0.25 0
XPP (MPa)
EC (m
mol
m-2
s-1
)
ControlIrrigationFertigation
aabb-1.61 -1.28 -1.20
1.131.08
0.76
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Average Daily Canopy Transpiration Average Daily Canopy Transpiration RateRate
0
0.5
1
1.5
2
2.5
3
EC (m
mol
m-2
s-1
)
Month
ControlIrrigationFertigation
aba
b
aa
b
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ConclusionsConclusions
Ecrit (mmol m-2 s-1)
gs vs E 1.0 (0.35)
E vs XPP 0.9 (0.26)
Pataki et al 1998 1.09
• Ecrit appears stable with varying resource availability and degree of canopy development.• Trees operate close to Ecrit.