A Density-Dependent Growth Model of a Perennial Herb, Viola fimbriatula

The growth of a population of the perennial herb Viola fimbriatula was simulated using a modified stage-matrix model. The model takes into account the following characteristics of the species: there exists a seed pool with seeds capable of surviving up to 100 yr; seedling survival is a function of adult density, but adult survival is independent of the density of other violet plants; survival and fecundity are functions of size rather than age; transition values (i.e., the probability of going from size i to size j in one time interval) vary normally around a mean. All survival and fecundity values and their variances used in these calculations were empirically obtained from a 5-yr study of naturally growing populations of the species. The principal finding is that the population modeled exhibits a stable oscillation resulting from the combined effect of density dependence of seedling survival and the one oscillation-cycle lag in incorporating surviving seedlings into the adult population. Increasing survival and reducing fecundity reduces the amplitude of the oscillations but does not eliminate them, although it makes them less predictable. Increased survival and reduced fecundity also result in increased adult population size and a decreased seed-pool size. These conclusions are valid for the equation used to describe density dependence (probability of seedling survival = 500 - density/500) and may not be valid for other density-dependence equations.