In most studies of crop canopies or of the foliage of single plants, all leaves are treated as if they have the same properties. This is done so that we may make generalisations about the ways in which plant or crop growth rates may be interpreted as a function of leaf area. There is no gainsaying that this approach with its underlying assumption has been profitable. Concepts such as leaf area index (LAI) and net assimilation rate (NAR) have contributed greatly to our understanding of how a photosynthetic surface contributes to determining the growth rate of plants. However, the assumption is false.

The leaves on a plant or in a crop form a population, an assemblage of things that can be counted, and they are manifestly not all the same. Their heterogeneity derives in part from the fact that they (like a population of rabbits in a field or of blue tits in a woodland) are not of the same age and change their properties as they age. They are also borne in different positions relative to each other and their positions determine which leaves shade which. The positions that they occupy in a canopy are also related to their age – in general, young leaves are found in the fringes of a canopy with older ones in their shade.

Population biologists have much experience of studying the behaviour of age-structured populations and the aim of this chapter is to explore how far the study of populations (demography) may contribute to the study of plant canopies.