Each simulation is run for 500 yr. and includes 75 of the dominant tree species of eastern North America. For the longest gradient the coldest row begins with a mean of 600 GDD while the warmest is at 5600. For a trial with no gradient all cells are given a degree day level of 3100 and WILT of .05. Considering a range of dispersal types is important because it allows us to examine different responses to scale change. To simulate dispersal by gravity we assume that any species producing seed on a site will have potential replacement on that site.
Wind dispersal is modeled as a negative exponential decay away from each source cell.
Zoochory is also modeled using a probability given by a negative exponential decay away from source cells for three types, which we conceptualize as mammal, scrub-bird, and bluejay. For each plot the simulation model calculates whether a seed source is extant based on presence of individuals of reproductive age by species and probability of a seed crop in a given year. The model then calculates the seed rain to surrounding plots. In order to assess the effects of different dispersal distances as well as different amounts of environmental change across the grid, it was necessary to obtain improved estimates of the dispersal functions of the species. Comparisons are made with rates inferred for migrations based on isopols of species range changes in the Holocene.