A MODEL FOR PLANT AND CROP GROWTH, ALLOWING FOR COMPETITION FOR LIGHT BY THE USE OF POTENTIAL AND RESTRICTED PROJECTED CROWN ZONE AREAS

Relations for competition for light are developed and used in a plant growth model applicable to the isolated plant, to plants in even-aged monoculture and to plants in mixed-aged monoculture. In an isolated plant, it is assumed that a leaf area, proportional to the plant mass, is contained within a crown whose projected zone area is proportional to plant mass to the 2/3 power. Self-shading progressively reduces the specific growth rate. If light were the sole limiting resource and were constant, one can derive a growth equation, [[inline]] which, integrated, gives [[inline]]. It approximates, initially, to a particular case of the Richards (1959) empirical growth equation. In even-aged evenly-spaced monocrops competing only for light, it is assumed that the zone areas merge at canopy closure, and growth then follows the expolinear equation of Goudriaan and Monteith (1990), giving a continuous function based on ground cover. For mixed-aged monocrops, we assume a phase of canopy closure that affects the younger plants earlier than the older ones. Under varying environmental conditions in the field, plant growth may be affected by other factors in addition, e.g. temperature. In the growth conductance model of Aikman and Scaife (1993), the shading expressions are applied to the light-dependence. Data from two sowings of cabbage and carrot in even-aged and mixed-aged monocrops were used to test the model. The parameter values derived from the even-aged monocultures predict the growth rates in the mixed-aged monocultures better than models which assume uniform canopies. © 1994 Annals of Botany Company.