Disturbance and Population Structure on the Shifting Mosaic Landscape

Abstract

A stochastic model of plant population dynamics is developed and analyzed to determine how density and age structure depend on thinning rates and disturbance regimes. Probability distributions of age and density are derived from the distribution of regeneration niches on a landscape and the thinning rates of cohorts on patches created by adult mortality or larger disturbances. The theory is then extended to different types of disturbances that operate at different scales and are interdependent, such as treefalls that only become important as the early—successional trees that initially colonize an area affected by a larger scale disturbance become mature. In general, landscapes that provide frequent regeneration niches support high—density young stands. Density distributions are negatively skewed. Decreasing frequency of regeneration niches results in lower mean density, higher variance, and increased (less negative) skewness. When regeneration niches are rare, density is low, variance is low, skewness is positive, and the age classes are highly variable. In more complex cases, regeneration niches may depend on the time since the last large disturbance; for example, canopy gaps can become more frequent as postfire cohorts become senescent. Then age class distributions on a given disturbed area become increasingly platykurtic with time; skewness is negative for early—successional species and positive for late—successional species. The exponential distribution of age classes commonly observed in late—successional species is the asymptotic result of a more general distribution that depends on time since the last disturbance. Across a landscape that supports these interdependent disturbance processes, infrequent disturbances result in density distributions for late—successional species with large mean, negative skewness, and low variance. An early—successional species is present at low density with positive skewness and low variance. Density distributions for both species types are more platykurtic and have higher variance when disturbance frequency ° thinning rate. Frequent disturbances produce higher densities of early—successional species with positive skewness and lower densities of late—successional species. The landscape distributions of age classes are J—shaped for both species. With frequent disturbance, age distributions are leptokurtic for both species, but more so for late—successional species. Age class distributions are increasingly platykurtic with less frequent disturbance. The "intermediate" disturbance frequency that maximizes the probability of being reproductively mature at the time of the next disturbance event (Clark 1991a) is also that which maximizes the density of reproductive individuals on this shifting mosaic landscape.