TIME CONSTANT FOR WATER TRANSPORT IN LOBLOLLY-PINE TREES ESTIMATED FROM TIME-SERIES OF EVAPORATIVE DEMAND AND STEM SAPFLOW

The use of stem sap flow data to estimate diurnal whole-tree transpira tion and canopy stomatal conductance depends critically upon knowledge of the time lag between transpiration and water flux through the stem . In this study, the time constant for water movement in stems of 12-y ear-old Pinus taeda L. individuals was estimated from analysis of time series data of stem water flux and canopy transpiration computed from mean daytime canopy conductance, and diurnal vapor pressure deficit a nd solar radiation measurements. Water uptake through stems was measur ed using a constant-heat sapflow probe. Canopy transpiration was corre lated to stem uptake using a resistance-capacitance equation that inco rporates a time constant parameter. A least-squares auto-regression de termined the parameters of the resistance-capacitance equation. The ti me constants for ten loblolly pine trees averaged 48.0 (SE = 2.0) min and the time lag for the diurnal frequency averaged 47.0 (SE = 2.0) mi n. A direct-cross correlation analysis between canopy transpiration an d sap flow time series showed maximum correlation at an approximately 30 min lag. Residuals (model-predicted minus actual stem flow data) in creased with increasing soil moisture depletion. While the time consta nts did not vary significantly within the range of tree sizes studied, hydraulic resistance and capacitance terms were individually dependen t on stem cross-sectional area: capacitance increased and resistance d ecreased with stem volume. This result may indicate an inverse adjustm ent of resistance and capacitance to maintain a similar time constant over the range of tree sizes studied.