Leaf economics and plant hydraulics drive leaf : wood area ratios.

Biomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value H _v (sapwood area/leaf area ratio) is central to plant water balance and drought responses. However, its coordination with key plant functional traits is poorly understood, and prevents developing trait-based prediction models. Based on theoretical arguments, we hypothesise that global patterns in H _v of terminal woody branches can be predicted from variables related to plant trait spectra, that is plant hydraulics and size and leaf economics. Using a global compilation of 1135 species-averaged H _v , we show that H _v varies over three orders of magnitude. Higher H _v are seen in short small-leaved low-specific leaf area (SLA) shrubs with low K _s in arid relative to tall large-leaved high-SLA trees with high K _s in moist environments. All traits depend on climate but climatic correlations are stronger for explanatory traits than H _v . Negative isometry is found between H _v and K _s , suggesting a compensation to maintain hydraulic supply to leaves across species. This work identifies the major global drivers of branch sapwood/leaf area ratios. Our approach based on widely available traits facilitates the development of accurate models of above-ground biomass allocation and helps predict vegetation responses to drought.
(© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)