Forest canopy hydraulic properties and catchment water balance: observations and modeling

We present observations and simulations examining plant-water relations in a forested catchment characterized by strong topographic control over surface hydrology and stand structure. The system is dominated by xeric and mesic ecotypes of Quercus rubra L. Soil depths are typically very thin and the severity of seasonal water stress in these trees is determined largely by position along the local topographic gradient. Water balance related ecotypic differences in Q. rubra were measured during the 2000 growing season at an upland and lowland site. Significant site differences in stomatal conductance, sap flux density, and leaf to sapwood area ratios were observed. However, mid-day leaf water potentials and the leaf area-specific canopy hydraulic conductance were not significantly different despite a 20% increase in soil moisture content, an average 10 m increase in tree height, and a higher leaf area index at the lowland site. These results suggest a close coordination of tree morphology, stand structure, and the hydraulic conductance of the combined soil-root-leaf pathway that governs leaf-level water vapor exchange rates similarly across the topographic gradient. To place these stand-level observations in the context of catchment-scale water balance we linked the SPA canopy model with a 1-D soil column model and TOPMODEL hydrologic formulations. The SPA model was used to represent the canopy because of its specific inclusion of hydraulic constraints on transpiration and leaf water potential. The combined model is spatially explicit in the distribution of ecotype morphology, and calculates transpiration rates for TOPMODEL-derived saturated and unsaturated area fractions within the watershed separately. Model predictions of stomatal conductance, upland latent energy flux, stream discharge, and soil moisture content compared favorably with observations. Sensitivity analyses of canopy model parameters indicate stream discharge in this system is most sensitive to changes in the maximum leaf area index, the minimum leaf water potential, and belowground resistance. Discharge was least sensitive to changes in stem hydraulic conductivity and capacitance. Model results and observations are discussed with respect to adaptations to water stress, hydraulic controls on canopy water use. and ecosystem water use. (C) 2002 Elsevier Science B.V. All rights reserved.