Realized versus fundamental niche functions in a model of chaparral response to climatic change

Abstract Simulation models are being developed and used to project the effects of global climatic change, i.e., the greenhouse effect, on the dynamics and pattern of vegetation. Many models use information about species' current ranges relative to climatic variables, i.e., their realized niche, to derive response functions. Using a simulation model for Californian chaparral, we compare the projections of vegetation using response functions representing a calculated realized niche and an assumed fundamental niche. The realized niche response functions are based on the distributions of species along gradients of annual precipitation and mean temperatures of the coldest and warmest months. Then, assuming that moderate increases in precipitation (as projected by some general circulation models for Californian chaparral) will reduce stress, we modified the functions to produce assumed fundamental niche responses. Compared to the projections based on the realized niche, use of the fundamental niche produces more continuous cover, lags in change, and different dominance following climatic change. The effects of increased precipitation per se are lasting using the realized niche responses, but only temporary using the assumed fundamental niche responses. More detailed information on actual responses is needed, but the physiological information now available would be useful only in models that would consume vast computing resources.