The Response of Vegetation Zonation in Rocky Mountain Ecotones to Climate Change

Abstract:

Mean annual temperatures in the western United States have increased in the last few decades, and during the 21^st century, it is predicted that this warming trend will continue. This change in climate may create shifts in the optimal ranges of vegetation within the Rocky Mountains, requiring species migration. For a species at the top of a mountain there may be little room for upward migration. These forests are a crucial part of the US’s carbon budget, thus it is important to analyze how climate change will affect the zonation and species composition of vegetation in Rocky Mountain landscapes. UVAFME is an individual-based gap model that simulates biomass and species composition of a forest. Originally developed for northeast China and applied across all of Russia, this model has accurately simulated diverse forests in a range of climates, as well as the response of these forests to climate change. UVAFME is first calibrated to several sites along the Colorado and Wyoming Rocky Mountains using species, soil, and climate data from the US Forest Service. The initial model output of biomass and species composition is tested against forest inventory data and expected forest type ecotone along an elevational gradient. The model is then run with a linear increase in temperature of 3°C over 200 years, corresponding to the A1B IPPC climate scenario. These results are compared to current forest inventory data and to model runs without climate change. We project that with climate change species ranges will shift up the mountain, leading to an increase in the deciduous species Populus tremuloides, and a decrease in coniferous species at high elevations. These results are an important step in evaluating the response of Rocky Mountain vegetation to climate change and will help predict the future of these crucial ecosystems.