Analysis of silva: A model for forecasting the effects of SO2 pollution and fire on western coniferous forests

Abstract A forest succession simulator, SILVA, has been developed for the mixed-conifer forest type of the Sierra Nevada, California, to simulate the effects of SO 2 and fire on forest dynamics. SILVA was developed by extensively modifying a northeastern U.S. simulator. The state variables of the model are the diameters at breast height (dbh) of each tree on a forest stand. Ponderosa pine is found to be a relatively stable dominant for the site used in the simulations. White fir and sugar pine are relatively stable subordinate species. Incense-cedar shows a slowly fluctuating time-series. Sensitivity analyses suggest that parameters determining growth rates are of major importance and changes in such parameters can often produce a relative effect on basal area larger than their relative change. Factor affecting fire induced mortality are of lesser importance. The effects of competition change the relative magnitude of the calculated sensitivities during the time course of the simulation. Relative rankings of parameters according to their sensitivities also change during the time course of the simulation. Those parameters that exhibit large changes in sensitivity are also important in determining the outcome of competition. We investigated the convergence of the means of the time series of each species. The dominant ponderosa pine converged relatively rapidly with the number of runs. The subordinate species such as white fir converged much more slowly.