Large variation in the radial patterns of sap flow among urban trees

Quantifying sap flow patterns along the sapwood radius is necessary for accurate up-scaling and estimation of tree and stand transpiration based on sap flow measurements. However, little information on radial patterns of sap flow exists in urban environments where precise estimates of tree water uptake and transpiration are important to quantify ecosystem services and plan tree management. Because of the heterogeneous conditions in urban environment, upscaling approaches specific to urban environments and species may be required to estimate whole-tree transpiration. We measured sap flow at four sapwood depths for Acer platanoides, Acer saccharinum and Tilia cordata, in two contrasting urban environments (site types): parks and streets, in Montreal, Canada. We then estimated the potential importance of tree-to-tree and temporal variation of radial patterns by comparing generalized additive models (GAM) with or without tree ID or a temporal effect, and using the models to upscale sap flow measurements conducted at the sapwood surface to tree-level daily water use estimates.The radial pattern of sap flow diverged between park and street trees, with a slightly more monotone pattern in park than street trees in the two Acer species. However, tree-to-tree variation was large even within a given species and site type, and partly associated with tree size. The radial pattern of sap flow varied in time in five of the twelve trees that were measured twice over the growing season. Sap flow in deeper layers relative to the sapwood surface correlated with vapor pressure deficit, solar radiation and the sap flow density at the sapwood surface. When comparing the two sources of uncertainty (tree-to-tree and temporal variation) in upscaling daily water use, a larger uncertainty was associated with tree-to-tree variation than with temporal variation. Thus, understanding the inter-tree variability in radial patterns of sap flow patterns should be prioritized in future studies.