The effects of seed dispersal on the simulation of long-term forest landscape change

The study of forest landscape change requires an understanding of the compl ex interactions of both spatial and temporal factors. Traditionally, forest gap models have been used to simulate change on small and independent plot s. While gap models are useful in examining forest ecological dynamics acro ss temporal scales, large, spatial processes, such as seed dispersal, canno t be realistically simulated across large landscapes. To simulate seed disp ersal, spatially explicit landscape models that track individual species di stribution are needed. We used such a model, LANDIS, to illustrate the impl ications of seed dispersal for simulating forest landscape change. On an ar tificial open landscape with a uniform environment, circular-shaped tree sp ecies establishment patterns resulted from the simulations, with areas near seed sources more densely covered than areas further from seed sources. Be cause LANDIS simulates at 10-y time steps, this pattern reflects an integra tion of various possible dispersal shapes and establishment that are caused by the annual variations in climate and other environmental variables. On real landscapes, these patterns driven only by species dispersal radii are obscured by other factors, such as species competition, disturbance, and la ndscape structure. To further demonstrate the effects of seed dispersal, we chose a fairly disturbed and fragmented forest landscape (approximately 50 0,000 ha) in northern Wisconsin, We compared the simulation results of a ma p with tree species (seed source locations) realistically parameterized (th e real scenario) against a randomly parameterized species map (the random s cenario). Differences in the initial seed source distribution lead to diffe rent simulation results of species abundance with species abundance startin g at identical levels under the two scenarios. This is particularly true fo r the first half of the model run (0-250 y). Under the random scenario, inf requently occurring and shade tolerant species tend to be overestimated, wh ile midabundant and midshade tolerant species tend to be underestimated. Th e over- and underestimation of species abundance diminish when examining lo ngterm (500 y) landscape dynamics, because stochastic factors, such as fire , tend to make the landscapes under both scenarios converge. However, diffe rences in spatial patterns, and especially species age-cohort distributions , can persist under the two scenarios for several hundred years.