Absorption of radiation by canopies and stand growth

Abstract

Growth analysis – old and new When a canopy of leaves is sunlit, photosynthesis proceeds at a rate which depends on how photons are distributed over individual elements of the foliage and on the relationship between photosynthetic rate and irradiance for each foliage element. In principle, therefore, photosynthesis by a canopy, expressed per unit of ground area rather than per unit leaf area, can be estimated from a statistical description of irradiance as a function of leaf disposition. In many models of productivity, this is a central and complex component. In practice, however, modelling can often be greatly simplified with little sacrifice of precision by exploiting the observation that, at least during vegetative growth, uniform stands produce dry matter at a rate which is almost proportional to the amount of radiant energy intercepted by the canopy. In this chapter we consider the theoretical basis of this relationship, its experimental verification, and its usefulness for exploring the dependence of growth on environmental variables in general. Traditional growth analysis is based on the observation that, when single plants are exposed to a more or less constant environment, their rate of growth is approximately proportional to their weight and to their leaf area until a significant fraction of older foliage is shaded by younger foliage. Consequently, relative growth rate (RGR) and net assimilation rate (NAR) are conservative indices of growth initially.