On the representativeness of plot size and location for scaling transpiration from trees to a stand

Scaling transpiration from trees to larger areas is a fundamental problem in ecohydrology. For scaling stand transpiration from sap flux sensors we asked if plot representativeness depended on plot size and location, the magnitude of environmental drivers, parameter needs for ecosystem models, and whether the goal was to estimate transpiration per unit ground area (E-C), per unit leaf area (E-L), or canopy stomatal conductance (G(S)). Sap flux data were collected in 108 trees with heat dissipation probes, and biometric properties were measured for 752 trees within a 1.44 ha Populus tremuloides stand along an upland-to-wetland gradient. E-C was estimated for the stand using eight different plot sizes spanning a radius of 2.0-12.0 m. Each estimate of E-C was derived from 200 plots placed randomly throughout the stand. We also derived leaf area index (L), canopy closure (P-CC), and the canopy average reference stomatal conductance (G(Sref)), which are key parameters used in modeling transpiration and evapotranspiration. With increasing plot size, E-C declined monotonically but E-L and G(Sref) were largely invariant. Interplot variance of E-C also declined with increasing plot size, at a rate that was independent of vapor pressure deficit. Plot representativeness was dependent on location within the stand. Scaling to the stand required three plots spanning the upland to wetland, with one to at most 10 trees instrumented for sap flux. Plots that were chosen to accurately reflect the spatial covariation of L, P-CC, and G(Sref) were most representative of the stand.