Potential impact of future climate changes on net carbon exchange in an old-growth temperate rainforest

Net Ecosystem Exchange of carbon (NEE) under an array of future climate change scenarios is estimated using a sophisticated land-surface process model for a late-stage temperate rainforest in southern Washington, using the UC Davis Advanced Canopy-Atmosphere Soil Algorithm (ACASA). Model simulations included alterations to the canopy and the forcing meteorology, at 30-minute intervals, to mimic future climate and land-use change scenarios. Meteorological forcing data from a number of climate model runs were used to drive the model.

Results from these tests suggest that old growth ecosystems such as the one at WRCCRF are sensitive to anticipated redistributions in precipitation, changes in temperature, and alterations to mean carbon dioxide concentrations that may occur on the coming decades. Model results of net primary production (NPP) of carbon also suggest that regeneration of young trees may not occur under current and anticipated future climate conditions. The drier, shallower root zone that would result increases the likelihood of moisture stress in dry season. The implications of this are potentially worrisome, as ongoing and future climate changes may increase forest susceptibility to disease and mortality through reductions in available photosynthate. Forests that are normally understood to operate mostly as sinks of CO2 may become sources as the system shifts into higher respiration.