PNNL

Abstract

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 contribution of leaf hydraulic resistance (Rleaf) to total soil-to-leaf hydraulic resistance (Rtotal) is thought to be high relative to leaf pathlength (fRleaf = Rleaf/Rtotal), but this has rarely been tested. We compiled a multi-biome dataset of fRleaf using new and previously published measurements of pressure differences within trees in situ. Across 80 samples, fRleaf averaged 0.51 (95% CI = 0.46, 0.57) and it declined with tree height. We also used the allometric relationship between soil-to-leaf hydraulic conductance and laboratory-based measurements of leaf conductance to compute the average fRleaf among 19 tree samples, which was 0.40 (95% CI = 0.29, 0.56). The in-situ technique produces a more accurate descriptor of fRleaf because it accounts for dynamic leaf hydraulic conductance. Both approaches demonstrate the outsized role of leaves in controlling tree hydrodynamics. Higher fRleaf likely protects stems more from hydraulic conductance loss, so the decline in fRleaf with tree height would make large trees more vulnerable to drought stress and could contribute to their observed disproportionate drought mortality.