Fecundity and Dispersal in Plant Populations: Implications for Structure and Diversity

Abstract

Demographic models of tree populations assume that seed availability does not depend on the populations themselves. We develop models to assess the consequences of fecun- dity and dispersal for population structure and diversity. Results show that population structure and reproductive success are importantly affected by seed production and dispersal for realistic parameterization of time scales describing thinning, disturbance, maturation, and longevity. Maturation age affects mean and variance in seed rain. Populations with well-dispersed seed have a structure that is most sensitive to maturation age when disturbance is frequent. With restricted dispersal, delayed maturation means increased variability in seed rain, maximized when half of all patches support reproductive individuals. Density-dependent thinning compen- sates for the initial variability conferred by limited dispersal but not enough to permit the neglect of fecundity and dispersal at the disturbance frequencies and thinning rates typical in many forests. Longevity matters most when it is short and disturbance rare. To assess the effects of dispersal on reproductive success, we partition the contributions of seed-rain mean and variance. Fecundity and population structure affect both the mean and the variance in seed rain, albeit in different ways. Dispersal affects only the variance. The partitioned contribution of mean and variance are used to consider two cases: how dispersal consequences for reproductive success depend on life-history schedules and disturbance regime, and boundary growth rates of a globally dispersed population invading a resident population with restricted dispersal. In both cases, restricted dispersal has important consequences on the scales observed in many real forests. Most models of forest tree dynamics assume a globally dispersed seed pool that is disconnected from the populations that should produce that seed. This assumption leads to two opposing (offsetting?) consequences for species diversity: artificially high diversity due to continuous seed supply and artificially low diversity due to lack of sites where good competitors with restricted dispersal should be absent.