Effects of dispersal and mortality on diversity in a forest stand model

The diversity of plant communities may be affected by trade-offs among competition, colonization, and longevity. MOSEL (MOdule for Spatially Explicit Landscapes), a version of the JABOWA-FORET model of the dynamic processes of establishment, growth, and death of forest trees that includes dispersal among cells, is used to elucidate the effects of colonization and mortality. Comparisons of diversity are made for simulations of equal or variable dispersal probabilities among species and of the occurrence or absence of density-independent mortality on sites. When diversity is examined between categories of generally high vs. low probabilities of dispersal, it is significantly greater with the former. When even and variable diversity are compared, diversity is greater (less) when the even dispersal probability is greater (less) than the average for variable dispersal probability. The exclusion of density-independent mortality decreases diversity significantly only in the case of generally low dispersal probability. Diversity thus seems to depend on having more movement through space, not necessarily on having different movement, and high dispersal can swamp the effects of low disturbance. In this simulation the relative success of nut-bearing species that are dispersed by both bluejays and mammals may indicate that the trade-off among competition, longevity, and colonization abilities may not be a zero-sum game when plant-animal interactions are considered.