Time constant for water transport in loblolly pine trees estimated from time series of evaporative demand and stem sapflow |
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Authors: | N Phillips Abhijit Nagchaudhuri R Oren Gabriel Katul |
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Institution: | (1) Nicholas School of the Environment, Duke University, Durham, NC 27708, USA, US;(2) School of Engineering, Turabo University, Gurabo, Puerto Rico, PR;(3) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA, US |
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Abstract: | The use of stem sap flow data to estimate diurnal whole-tree transpiration 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-year-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 and solar radiation measurements. Water uptake through stems
was measured using a constant-heat sapflow probe. Canopy transpiration was correlated to stem uptake using a resistance-capacitance
equation that incorporates a time constant parameter. A least-squares auto-regression determined the parameters of the resistance-capacitance
equation. The time 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) min. A direct-cross correlation analysis between canopy transpiration and sap flow time series showed
maximum correlation at an approximately 30 min lag. Residuals (model-predicted minus actual stem flow data) increased with
increasing soil moisture depletion. While the time constants did not vary significantly within the range of tree sizes studied,
hydraulic resistance and capacitance terms were individually dependent on stem cross-sectional area: capacitance increased
and resistance decreased with stem volume. This result may indicate an inverse adjustment of resistance and capacitance to
maintain a similar time constant over the range of tree sizes studied. |
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Keywords: | Tree transpiration Hydraulic resistance Hydraulic capacitance |
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