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 DBH (cm)Height (m)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 (E crit ) –E crit 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 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 Leaf gas exchange measurements were made with a Li 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

Seasonal Water Use

Average Daily Canopy Transpiration Rate ab a b a a b

Predawn Xylem Pressure Potentials

Critical Transpiration Stomatal Control of Water Loss E crit

Critical Transpiration Stomatal Control of Water Loss Obtain a quadratic function of stomatal conductance over time. –g smax = B 0 + B 1 T + B 2 T 2 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 g smax. –T = -B 1 /(2*B 2 ) To determine the transpiration rate at the time of g smax simply enter the time into the linear equation for transpiration over time. g smax = D 0 + D 1 T

Critical Transpiration Stomatal Control of Water Loss a ab b P=0.3751

Critical Transpiration Rate Stomatal Control of Water Loss Critical Transpiration Rate Stomatal Control of Water Loss E Crit

Critical Transpiration Rate Stomatal Control of Water Loss Obtain a linear and quadratic function for transpiration over XPP. –E = B 0 + B 1 XPP –E = D 0 + D 1 XPP + D 2 XPP 2 Take the derivative in respect to XPP to determine when the relationship between transpiration and XPP deviates from linear. –XPP = (B 1 – D 1 )/(2 * D 2 ) Solve for critical transpiration by entering the XPP value into the quadratic function. –E crit = D 0 + D 1 XPP + D 2 XPP 2

Critical Transpiration Rate Stomatal Control of Water Loss Critical Transpiration Rate Stomatal Control of Water Loss aabb

Average Daily Canopy Transpiration Rate ab a b a a b

Conclusions E crit (mmol m -2 s -1 ) g s vs E1.0 (0.35) E vs XPP0.9 (0.26) Pataki et al E crit appears stable with varying resource availability and degree of canopy development. Trees operate close to E crit.