Disturbance, Climate, and Management Impacts on US West-Coast Forest Carbon Budgets

Forest net uptake of atmospheric CO2 (net ecosystem production, or NEP) is dependent on climate, disturbance history, management practices, forest age, and forest type. Accurate quantification of NEP and forest carbon budgets is necessary for validation of coupled carbon-climate models and monitoring state carbon budgets. To improve understanding of the influence of disturbance, climate, and management on forest carbon stocks and fluxes in the western U.S., federal inventory data and supplemental field measurements were used to estimate several important components of the carbon balance in forests in Oregon, Washington, and California from 2001-2006. Species- and ecoregion-specific allometric equations and ecoregion-specific lookup tables were used to estimate live and dead biomass stores, net primary productivity (NPP), NEP, and mortality for different age classes. Natural and anthropogenic disturbance impacts on forest carbon accumulation and NPP varied by ecoregion, forest type, and ownership. In the semi-arid East Cascades and mesic Coast Range, mean total biomass was 8 and 24 kg C m-2, and mean NPP was 0.30 and 0.78 kg C m-2 yr-1, respectively. Decrease in NPP with age was not general across ecoregions, with no marked decline in old stands (greater than 200 years) in some ecoregions. Within ecoregions, mean live and dead biomass were usually higher on public lands, primarily because of the younger age class distribution on private lands. In the absence of stand-replacing disturbance, total landscape carbon stocks could theoretically double if forests were managed for maximum carbon storage. Although the theoretical limit is probably unattainable given the timber-based economy and fire regimes in some ecoregions, there is still potential to significantly increase terrestrial carbon storage by decreasing anthropogenic disturbance through increased rotation age and reduction in harvest rates.