Why size matters: the interactive influences of tree diameter distribution and sap flow parameters on upscaled transpiration

In stands with a broad range of diameters, a small number of very large trees can disproportionately influence stand basal area and transpiration (Et). Sap flow-based Et estimates may be particularly sensitive to large trees due to nonlinear relationships between tree-level water use (Q) and tree diameter at breast height (DBH). Because Q is typically predicted on the basis of DBH and sap flow rates measured in a subset of trees and then summed to obtain Et, we assessed the relative importance of DBH and sap flow variables (sap velocity, V-s, and sapwood depth, R-s) in determining the magnitude of Et and its dependence on large trees in a tropical montane forest ecosystem. Specifically, we developed a data-driven simulation framework to vary the relationship between DBH and V-s and stand DBH distribution and then calculate Q, E-t and the proportion of E-t contributed by the largest tree in each stand. Our results demonstrate that variation in how R-s is determined in the largest trees can alter estimates up to 26% of E-t while variation in how V-s is determined can vary results by up to 132%. Taken together, these results highlight a great need to expand our understanding of water transport in large trees as this hinders our ability to predict water fluxes accurately from stand to catchment scales.