INFLUENCE OF INTENSIVE
MANAGEMENT ON CANOPY
TRANSPIRATION IN LOBLOLLY PINE
Thomas A. Stokes, Lisa Samuelson, Greg Somers, and Tom
Cooksey
School of Forestry, Auburn University
Southlands Experiment Forest, International Paper Company
Growth
14
Control
Irrigation
Fertigation
12
10
8
DBH (cm)
Height (m)
6
4
2
0
1998
LAI
Objectives
• 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)
Hypothesis
• Critical transpiration will increase with nutrient
and water availability.
• Loblolly pine operates close to critical
transpiration rate.
Study 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
Treatments
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.
Methods: Canopy Level
Measurements
• 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.
Methods: 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 Li6400 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.
Sap Flow
6
Control
Irrigation
Fertigation
Sap Flow (l h -1)
5
4
3
2
1
0
0
20
40
60
Hours
80
100
120
Water use (kg tree -1 season-1)
Seasonal Water Use
8000
Control
Irrigation
Fertigation
7000
6000
5000
4000
3000
2000
1000
0
Jun-Aug 99
Sept-Nov 99
Dec 99-Feb 00 Mar-May 00
Month
Average Daily Canopy Transpiration
Rate
Control
Irrigation
Fertigation
2.5
2
a
1.5
a
a
1
ab
b
b
0.5
Month
-9
9
D
ec
9
N
ov
-9
O
ct
-9
9
9
Se
p9
A
ug
-9
9
9
Ju
l-9
9
0
Ju
n9
EC (mmol m-2 s-1)
3
Predawn Xylem Pressure Potentials
-0.1
XPP (MPa)
-0.3
-0.5
-0.7
-0.9
Control
Irrigation
Fertigation
-1.1
-1.3
-1.5
Jun-99
Jul-99
Aug-99
Month
Sep-99
Critical Transpiration
Stomatal Control of Water Loss
150
Control
Irrigation
Fertigation
gs (mmol m-2 s-1)
130
110
90
Ecrit
70
50
0
0.25
0.5
0.75
1
1.25
EC (mmol m-2 s-1)
1.5
1.75
2
Critical Transpiration
Stomatal 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
Critical Transpiration
Stomatal Control of Water Loss
150
130
gs (mmol m-2 s-1)
Control
Irrigation
Fertigation
136.8 a
119.6 ab
110.1 b
110
90
0.70
1.09
0.85
70
P=0.3751
50
0
0.25
0.5
0.75
1
1.25
EC (mmol m-2 s-1)
1.5
1.75
2
Critical Transpiration Rate
Stomatal Control of Water Loss
2
Control
Irrigation
Fertigation
EC (mmol m-2 s-1)
1.75
1.5
1.25
1
0.75
ECrit
0.5
0.25
0
-2
-1.75
-1.5
-1.25
-1
XPP (MPa)
-0.75
-0.5
-0.25
0
Critical Transpiration Rate
Stomatal 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
Critical Transpiration Rate
Stomatal Control of Water Loss
2
Control
Irrigation
Fertigation
EC (mmol m-2 s-1)
1.75
1.5
1.25
1
1.13
1.08
0.75
0.76
-1.61 b
0.5
-1.28 ab
-1.20 a
0.25
0
-2
-1.75
-1.5
-1.25
-1
XPP (MPa)
-0.75
-0.5
-0.25
0
Average Daily Canopy Transpiration
Rate
Control
Irrigation
Fertigation
2.5
2
a
1.5
a
a
1
ab
b
b
0.5
Month
-9
9
D
ec
9
N
ov
-9
O
ct
-9
9
9
Se
p9
A
ug
-9
9
9
Ju
l-9
9
0
Ju
n9
EC (mmol m-2 s-1)
3
Conclusions
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.