Leaves as bottlenecks: The contribution of tree leaves to hydraulic resistance within the soil-plant-atmosphere continuum.

Within vascular plants, the partitioning of hydraulic resistance along the soil-to-leaf continuum affects transpiration and its response to environmental conditions. In trees, the fractional contribution of leaf hydraulic resistance (R ) to total soil-to-leaf hydraulic resistance (R ), or fR (=R /R ), is thought to be large, but this has not been tested comprehensively. We compiled a multibiome data set of fR using new and previously published measurements of pressure differences within trees in situ. Across 80 samples, fR averaged 0.51 (95% confidence interval [CI] = 0.46-0.57) and it declined with tree height. We also used the allometric relationship between field-based measurements of soil-to-leaf hydraulic conductance and laboratory-based measurements of leaf hydraulic conductance to compute the average fR for 19 tree samples, which was 0.40 (95% CI = 0.29-0.56). The in situ technique produces a more accurate descriptor of fR because it accounts for dynamic leaf hydraulic conductance. Both approaches demonstrate the outsized role of leaves in controlling tree hydrodynamics. A larger fR may help stems from loss of hydraulic conductance. Thus, the decline in fR with tree height would contribute to greater drought vulnerability in taller trees and potentially to their observed disproportionate drought mortality.