DENSITY-DEPENDENT DYNAMICS IN SIZE-STRUCTURED PLANT-POPULATIONS

Year-to-year changes in numbers of individuals in a plant community occur in response to a confluence of factors-nutrient and water availability, climatic conditions, and the type and status of individual plants. Since effects of these factors are conditioned by plant density, we wish to examine the dynamical changes in the composition of a plant community that can be explained, or at least simulated, by a model driven by density alone. We develop a model of plants classified by size or life-state in which each density vector is generated from that of the preceding time period by an appropriate matrix multiplication. Matrix models have been generally discussed by Caswell. In our formulation, matrix entries representing survival and reproduction rates are determined in each time period as implicit or explicit functions of density. This temporal plasticity in life history "parameters" produces an interaction between individual plant histories and community development. We develop a joint analysis of how plants may allocate energy among reproduction, maintenance, and growth so as to maximize "fitness", and of the long-term dynamics of communities that contain such plants. Density affects plant access to water and to required nutrients, as well as to energy. We shall, however, focus on the latter. In this paper, we emphasize intra- or interspecific competition among perennials, though we sketch out an approach for annuals. Our discussion concerns the model itself, together with some indication, both theoretical and via computer simulations, of the possible dynamical behaviors of the system. (For instance, a species of pioneer type in which all individuals are of the same size will tend to maximize fitness at high density by devoting all energy to maintenance, and none to reproduction. At low density the optimal allocation typically is reversed, and involves either reproduction only, or a mixture of reproduction and maintenance.) Subsequent papers will enlarge on the implications of the model, and will deal more fully with populations of annuals. © 1992 Academic Press Limited.