Not all sapwood is functionally equal: Improving estimates of whole-tree water use from sap flux measurements

We often use sap flux, or water transported by the xylem to estimate whole-tree water use, an important component of understanding ecosystem water use. However, not all sapwood is functionally equal and axial sap flux generally declines radially and inward from the outer cambium to the inner heartwood. This radial variation in sap flux makes estimating whole-tree water use challenging. In fact, a majority of models used to scale up from sap flux measurements assume homogenous flow throughout the xylem, providing inaccurate estimates of whole-tree water use.

Pinus echinata
Pinus echinata.. Image: USDA / Wikipedia.

To address this issue, Berdanier et al. (2016) used a large data set of sap flux measurements and radial profiles by wood type and tree size of diverse species to develop a model that allows upscaling by depth and xylem type including tracheids (e.g. conifers), diffuse-porous (e.g. maple, cherry), and ring-porous xylem (e.g. oak, ash). The best part? The authors provide the Excel and R computer code for the model in Supplementary Data in Tree Physiology Online.

Reference List

Aaron B. Berdanier, Chelcy F. Miniat, James S. Clark, 2016, 'Predictive models for radial sap flux variation in coniferous, diffuse-porous and ring-porous temperate trees', Tree Physiology, p. tpw027