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 University

Southlands Experiment Forest, International Paper Company

GrowthGrowth

0

2

4

6

8

10

12

14

1998

ControlIrrigationFertigation

DBH (cm) Height (m) LAI

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)

HypothesisHypothesis

• Critical transpiration will increase with nutrient and water availability.

• Loblolly pine operates close to critical transpiration rate.

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

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.

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.

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.

Sap FlowSap Flow

0

1

2

3

4

5

6

0 20 40 60 80 100 120

Hours

Sap

Flo

w (l

h-1

)

ControlIrrigationFertigation

Seasonal Water UseSeasonal Water Use

0

1000

2000

3000

4000

5000

6000

7000

8000

Wat

er u

se (

kg

tree

-1 s

easo

n-1

)

Jun-Aug 99 Sept-Nov 99 Dec 99-Feb 00 Mar-May 00

Month

ControlIrrigationFertigation

Average Daily Canopy Transpiration Average Daily Canopy Transpiration RateRate

0

0.5

1

1.5

2

2.5

3

EC (

mm

ol m

-2 s

-1)

Month

ControlIrrigationFertigation

aba

b

aa

b

Predawn Xylem Pressure PotentialsPredawn Xylem Pressure Potentials

-1.5

-1.3

-1.1

-0.9

-0.7

-0.5

-0.3

-0.1

XP

P (

MP

a)

Jun-99 Jul-99 Aug-99 Sep-99

Month

ControlIrrigationFertigation

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 (

mm

ol m

-2 s

-1)

ControlIrrigationFertigation

Ecrit

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

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 (

mm

ol m

-2 s

-1)

ControlIrrigationFertigation

a

abb

1.090.850.70

136.8

119.6110.1

P=0.3751

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 (

mm

ol m

-2 s

-1)

ControlIrrigationFertigation

ECrit

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

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 (

mm

ol m

-2 s

-1)

ControlIrrigationFertigation

aabb-1.61 -1.28 -1.20

1.131.08

0.76

Average Daily Canopy Transpiration Average Daily Canopy Transpiration RateRate

0

0.5

1

1.5

2

2.5

3

EC (

mm

ol m

-2 s

-1)

Month

ControlIrrigationFertigation

aba

b

aa

b

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.