Tree transpiration varies spatially in response to atmospheric but not edaphic conditions

Measuring transpiration simultaneously in time and space can establish a better understanding of how to mechanistically scale spatiotemporal values. This study tested the following predictions to falsify a tree hydraulic hypothesis of spatial variation in transpiration: (i) stands with larger trees will a have longer range and greater sill and nugget at a given vapour pressure deficit (D); (ii) the range, sill and nugget will decline faster with increasing D with larger trees; and (iii) soil moisture, texture and/or N levels will be correlated with transpiration. We used cyclic sampling to efficiently collect spatial sap flux data from 144 trees in two forested stands in northern Wisconsin: an Aspen-dominated stand with small trees and a Maple-Pine-dominated stand with larger trees. In the Maple stand, the range of spatial autocorrelation in half-hourly transpiration dropped from 80 to 20 m with increased D, whereas in the Aspen stand the range dropped from 55 to 35 m with a similar increase in D. Differences in the range of spatial autocorrelation at a given D were driven by sapwood area, which is a function of tree size. These results show that species and tree size as well as individual tree hydraulics drive spatial variability in transpiration with little additional variation explained by the measured edaphic conditions. Scaling from individual tree transpiration to the landscape in time and space should incorporate atmospheric drivers in time and investigate other potential drivers of tree size in space such as light competition.