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.