Potential response of Pacific Northwestern forests to climatic change, effects of stand age and initial composition

We used an individual-based forest simulator (a gap model) to assessthe potential effects of anthropogenic climatic change on conifer forests of thePacific Northwestern United States. Steady-state simulations suggested that forestzones could be shifted on the order of 500-1000 m in elevation, which could leadto the local extirpation of some high-altitude species. For low-elevation sites,species which currently are more abundant hundreds of kilometers to the southwould be favored under greenhouse scenarios. Simulations of transient responsessuggested that forest stands could show complex responses depending on initialspecies composition, stand age and canopy development, and the magnitude andduration of climatic warming. Assumptions about species response to tempera-ture, which are crucial to the model's behaviors, were evaluated using data onspecies temperature limits inferred from regional distributions. The high level ofwithin-species variability in these data, and other confounding factors influencingspecies distributions, argue against over-interpreting simulations. We suggest howwe might resolve critical uncertainties with further research.