Studies have increasingly reported that high-mountain areas are experiencing more rapid increases in temperature than the global average. Changes in the function of forest ecosystems at their altitudinal limit in response to future climate may have important implications for these habitats as well as adjacent lower-elevation ecosystems. Mesocosm experiments, gradient studies and long-term monitoring have been used to explore the impacts of climate change. Application of biogeochemical models to small intensively studied watersheds is also an effective tool to investigate ecosystem in response to the climate change. Process based measurements and time series observations provide for effective model parameterization and testing. Models allow for the investigation of long-term response, and are capable of depicting complex interactions among ecosystem processes including linkages with changes in temperature and soil moisture. Projections of future carbon dynamics of a subalpine forest at Loch Vale, Colorado were made using updated algorithms and parameters in a biogeochemical model, PnET-BGC with downscaled future climate under RCP 4.5 and 8.5 scenarios. We used future meteorological inputs from four General Circulation Models (GCMs) to depict the uncertainty in ecosystem projections to future climate conditions.
Climate projections from GCMs showed less agreement in future precipitation than temperature, especially by season for this site near the continental divide. Our results indicated that future increases in temperature and associated vapor pressure deficit decrease water use efficiency do not necessarily result in soil moisture stress but rather depend on projections of offsetting increases in summer precipitation. Extended growing season and elevated atmospheric CO2 concentrations are projected to increase photosynthesis and accumulation of carbon in wood and internal plant storage if soil moisture stress is absent. Large uncertainty in future conditions of soil moisture during the growing season causes variations in projections of other carbon pool and fluxes. The direction of change in future soil decomposition during the growing season is primarily driven by the quantity of litterfall. Projected decreases in snow depth and cover period are expected to affect subnivean microbial activity and decrease winter soil decomposition. However, this effect may be largely alleviated if increases in litterfall occur. Our findings emphasize the importance of projections of changes in precipitation in assessing the response of Rocky Mountain subalpine ecosystems to future changes in climate.