Scaling of leaf area with biomass in trees reconsidered: constant metabolically active sapwood volume per unit leaf area with height growth

Abstract

¾-power scaling between metabolic rate and body mass is regarded as near-universal across organisms. However, there are compelling reasons to question ¾-power scaling in woody plants, where metabolic rate≈leaf area. This leaf area must provide carbon to the metabolically active sapwood volume (V ). V is necessarily a much smaller volume than total wood volume, meaning that scaling of total leaf area LA with V should be >¾. Within populations of a species, variants in which V increases per unit leaf area with height growth (e.g. ¾ scaling) would have proportionally less carbon for growth and reproduction as they grow taller. Therefore, selection should favor individuals in which, as they grow taller, leaf area scales isometrically with V . Using tetrazolium staining, we measured total V and total leaf area across 22 individuals of Ricinus communis and confirmed that leaf area scales isometrically with V , and that V is much smaller than total sapwood volume. With the potential of the LA -V relationship to shape factors as diverse as the crown area-stem diameter relationship, conduit diameter scaling, reproductive output, and drought-induced mortality, our work suggests that the notion that sapwood increases per unit leaf area with height growth requires revision.

Xylem structure and hydraulic function in roots and stems of chaparral shrub species from high and low elevation in the Sierra Nevada, California

“…These values were used to calculate the branch Huber value as the stem area divided by the total branch LA. Since these branches were relatively distal and young, we assumed that all of the sapwood within the branches was metabolically active (Olson et al, 2023).…”