The leaf mass/number trade‐off of Kleiman and Aarssen implies constancy of leaf biomass, its density and carbon uptake in forest stands: scaling up from shoot to stand level

Abstract

Summary 1 Kleiman & Aarssen (2007) propose that the regression slope of the mean individual leaf mass across species vs. the number of leaves does not differ significantly from –1.0, based on log-transformed experimental data for tree shoots (i.e. isometric trade-off). A quantitative model is set out here that explains the mechanism of isometric trade-off of leaf mass/number across species. 2 From Kleiman and Aarssen's result for the leaf mass/number trade-off in trees, the constancy of leaf biomass density per unit volume of shoot is derived theoretically. This constancy may scale up to the tree crown level, assuming proportionality of total shoot volume and crown volume, and also up to the canopy at stand level, based on the observed proportionality between crown depth and tree height. 3 On the basis of the constancy of leaf biomass density, the constancy of leaf biomass in a fully closed forest stand is analysed, since tree height tends to a limit as tree age increases due to hydraulic constraints. The resulting constancy of leaf biomass is consistent with previous reports for actual forest stands. 4 The allometric scaling theory of Enquist and colleagues suggests that metabolic rates of an entire organism scale as the 3/4 power of mass, so that scale-up is important: indeed, scaling up may be related to tree physiology and ecosystem carbon balance. The present scaling model from shoot to forest stand level is consistent with the work of Enquist and colleagues. The scaled-up result of leaf biomass constancy indicates that carbon uptake by forest stands may be almost constant if the mean leaf photosynthetic rate remains constant after closure of the forest canopy. 5 Synthesis. By analytically explaining the mechanism of the leaf mass/number trade-off at shoot level proposed by Kleiman and Aarssen, it is predicted that leaf biomass and carbon uptake are constant in fully closed forest stands, by scaling up the constancy of leaf biomass density from shoot to canopy level. The present analysis provides the theoretical basis for leaf biomass constancy in forest stands.