Does disturbance prevent total basal area and biomass in indigenous forests from being at equilibrium with the local environment?

Abstract

Understanding the determinants of the amount of biomass in various forests is presently a global imperative because forests may (e.g. Phillips et al. 2002) or may not be carbon sinks (Clark 2002). Presently, the assumption for modelling and empirical studies is that forest biomass and net primary production (NPP) are in equilibrium. For example, Beerling & Woodward (2001) modelled world-wide plant biomass on the basis of how net primary productivity (NPP) and annual transpiration rates affect tree size. They predicted that the largest stands of organic carbon are in the wet tropics. Similarly, in a recent textbook, Chapin et al. (2002) indicate that amongst forests, tropical forests have the greatest biomass and greatest NPP. In contrast, Midgley (2001) drew attention to the negative correlation between basal area and disturbance rates in some tropical forests. Also, Enquist & Niklas (2001) demonstrated that biomass is not correlated with latitude. Indeed, many indigenous forests with exceptionally large total basal areas and thus total standing stem biomass occur in the cool-temperate areas. For example, the Pacific Northwest redwood and Tasmanian mountain-ash forests have a total basal area of 300+ m² ha⁻¹, which exceeds the mean of many tropical forests (i.e. 35 m² ha⁻¹) (see Midgley 2001 and references therein).