Scaling xylem sap flux and soil water balance and calculating variance: A method for partitioning water flux in forests

Abstract

To partition evapotranspiration between canopy and subcanopy components in a 12-m-tall Pinus taeda forest and to assess certain aspects of environmental regulation of canopy transpiration, we quantified water flux in a forest using three approaches: 1) measuring water flux in xylem of trees, and scaling to stand transpiration of canopy trees (E(C)); 2) measuring soil water content and saturated conductivity, and modeling drainage to estimate total evapotranspiration (E(T)) during rainless days based on a local water balance (LWB); and 3) using an eddy correlation approach to estimate total E(T). We calculated variances for each estimate, and proposed an approach to test for differences between estimates of E(C) and E(T). Diurnal 'patterns' in water uptake were similar using direct measurements in stem xylem and LWB. However, LWB was found to be inappropriate for estimating 'absolute' E(T) diurnally when changes in soil moisture between consecutive measurements were small. Eddy correlation estimates of E(T) are of a higher temporal resolution than xylem flux measurements made in branches. Diurnal flux patterns in branches are more similar to the pattern generated by eddy correlation than those in stems. However, differences between the patterns indicate that patchiness in branch transpiration may preclude using branch xylem flux measurements to estimate canopy conductance. In one stand, daily E(C) accounted for ca 70 % of total E(T) estimated by either LWB (in a separate study) or the eddy correlation approach; the difference between E(T) and E(C) was significant based on variances calculated to account for spatial variation in each. Regardless of the vapor pressure deficit, E(C) decreased linearly with soil moisture from 2.5 to 1.5 mm d-1 over a 9-d drying cycle, as soil moisture in the rooting zone (ca 0.35 m depth) declined by 23 mm.