ESTIMATING VEGETATION CARBON CHANGES IN THE WESTERN UNITED STATES DUE TO LAND USE CHANGE, CLIMATE CHANGE, AND NATURAL DISTURBANCE: 1951-2006

During the twentieth century, continuing human-induced land cover changes and increasing size and frequency of wildland fires have resulted in increasing losses of carbon from terrestrial ecosystems in the western USA. However, vegetation growth may have been enhanced by CO2 fertilization and increasing growing season length, contributing to faster vegetation recovery and offsetting carbon losses. Estimation of the net carbon change of ecosystems at landscape to continental scales is determined by the quality of land cover disturbance data and the ability of process-based ecosystem models to correctly capture growth enhancement and disturbance effects. We used the Integrated Biosphere Simulator (IBIS) with a set of 30- to 60-meter resolution wildland fire and land cover change data to assess vegetation carbon changes in response to recent trends in climate, CO2 concentration, land use change and disturbances for 1951–2006. Our analyses suggest that the effects of vegetation growth enhancement and land disturbance were heterogeneous over time and space. Land cover change and disturbances caused significant fluctuations in ecosystem carbon density at county and ecoregion levels whereas climate variability played an important role at the state and national level. Further analysis indicates that growth enhancement effects were greater in high elevation forests than in low elevation forests.