MODEL-BASED ASSESSMENTS OF CLIMATE-CHANGE EFFECTS ON FORESTS - A CRITICAL-REVIEW

While current projections of future climate change associated with inc reases in atmospheric greenhouse gases have a high degree of uncertain ty, the potential effects of climate change on forests are of increasi ng concern. A number of studies based on forest simulation models pred ict substantial alteration of forest composition, forest dieback, or e ven loss of forest cover in response to increased temperatures associa ted with increasing atmospheric carbon dioxide concentrations. However , the structure of these computer models may cause them to overemphasi ze the role of climate in controlling tree growth and mortality. Model functions that represent the influence of climate on tree growth are based on the geographic range limits of a species, predicting maximal growth in the center of the range and zero growth (100% mortality) at the range limits and beyond. This modeling approach ignores the fact t hat the geographic range of a species reflects the influence of both c limate and other environmental factors, including competition with oth er tree species, soil characteristics, barriers to dispersal, and dist ributions of pests and pathogens. These climate-response functions in forest simulation models implicitly assume that tree species occur in all environments where it is possible for them to survive (their funda mental niche or potential habitat) and that these potential habitats a re entirely defined by climate. Hence, any alteration of climate must result in a fairly rapid decline of species near their range limits an d rapid alteration of forest composition and structure. The climate-re sponse functions that lead to these unrealistic conclusions have no ba sis in plant physiology or actual measurements of tree responses to cl imate stressors. Rather, these functions were chosen as a necessary ex pedient for modeling the climatic responses of many tree species for w hich there were limited or no ecophysiological data. There is substant ial evidence, however, that some tree species can survive, and even th rive, in climatic conditions outside their present range limits. This evidence suggests that nonclimatic factors exclude some species from n atural forests beyond their present range limits and that climate may not be the only determinant of these limits. Hence, there is reason to suspect that published projections of forest responses to climate cha nge based on forest simulation models may exaggerate the direct impact of climate on tree growth and mortality. We propose that forest simul ation models be reformulated with more realistic representations of gr owth responses to temperature, moisture, mortality, and dispersal. We believe that only when these models more accurately reflect the physio logical bases of the responses of tree species to climate variables ca n they be used to simulate responses of forests to rapid changes in cl imate. We argue that direct forest responses to climate change project ed by such a reformulated model may be less traumatic and more gradual than those projected by current models. However, the indirect effects of climate change on forests, mediated by alterations of disturbance regimes or the actions of pests and pathogens, may accelerate climate- induced change in forests, and they deserve further study and inclusio n within forest simulation models.