Adult Recruitment as a Function of Juvenile Growth Rate in Size-Structured Plant Populations

Abstract

Summary. The recruitment of adults in size-structured plant populations will be sensitive to changes in juvenile growth rates when: 1) mortality is high, 2) growth rates are low, 3) the difference between seedling and adult size is large and 4) selfthinning among juvenile plants is negligible. This sensitivity can be quantified by a single parameter in a simple, general expression relating recruitment to growth. The model is demonstrated for the prediction of young spruce tree densities on small plots, and the calculation of the parameter is shown for two published demographic studies. The model can also predict features of population response to an environmental gradient, such as long tails or abrupt cut-offs, even when these features are not found in the physiological response to the gradient. The rate of escape from the vulnerability of small size is a critical factor in the life cycle of many plant species. Recruitment into the adult population may depend on many factors such as seed rain, germination rates, local environmental heterogeneity and episodic herbivory; but, in populations where the mortality rate is concentrated in the early stages of growth, adult recruitment may be highly sensitive to differences in growth rate. Some forest dynamic simulation models (Pastor and Post 1985) assume a linear relationship between growing conditions in the understory and recruitment into the adult tree population, but the form of this relationship, while plausible, is arbitrary. The purpose of this paper is to derive a simple relationship between the growth and mortality rates of juvenile plants and the rate of recruitment into the adult population. I apply the model to a field study of young spruce tree densities, and two published demographic studies of long-lived perennials.