Comparing the performance of forest gap models in North America

Forest gap models have a long history in the study of forest dynamics, incl uding predicting long-term succession patterns and assessing the potential impacts of climate change and air pollution on forest structure and composi tion. In most applications, existing models are adapted for the specific qu estion at hand and little effort is devoted to evaluating alternative formu lations for key processes, although this has the potential to significantly influence model behavior. In the present study, we explore the implication s of alternative formulations for selected ecological processes via the com parison of several gap models. Baseline predictions of forest biomass, comp osition and size structure generated by several gap models are compared to each other and to measured data at boreal and temperate sites in North Amer ica. The models ForClim and LINKAGES v2.0 were compared based on simulation s of a temperate forest site in Tennessee, whereas FORSKA-2V, BOREALIS and ForClim were compared at four boreal forest sites in central and eastern Ca nada. Results for present-day conditions were evaluated on their success in predicting forest cover, species composition, total biomass and stand dens ity, and allocation of biomass among species. In addition, the sensitivity of each model to climatic changes was investigated using a suite of six cli mate change scenarios involving temperature and precipitation. In the tempe rate forest simulations, both ForClim and LINKAGES v2.0 predicted mixed mes ophytic forests dominated by oak species, which is expected for this region of Tennessee. The models differed in their predictions of species composit ion as well as with respect to the simulated rates of succession. Simulated forest dynamics under the changed climates were qualitatively similar betw een the two models, although aboveground biomass and species composition in ForClim was more sensitive to drought than in LINKAGES v2.0. Under a warme r climate, the modeled effects of temperature on tree growth in LINKAGES v2 .0 led to the unrealistic loss of several key species. In the boreal forest simulations, ForClim predicted significant forest growth at only the most mesic site, and failed to predict a realistic species composition. In contr ast, FORSKA-2V and BOREALIS were successful in simulating forest cover, gen eral species composition, and biomass at most sites. In the climate change scenarios, ForClim was highly sensitive, whereas the other two models exhib ited sensitivity only at the drier central Canadian sites. Although the stu died sites differ strongly with respect to both the climatic regime and the set of dominating species, a unifying feature emerged from these simulatio n exercises. The major differences in model behavior were brought about by differences in the internal representations of the seasonal water balance, and they point to an important limitation in some gap model formulations fo r assessing climate change impacts.