Pacific Northwest Regional and Ecozone-scale Carbon Cycle Responses to 25 Years of Variation in Climate and Disturbance

Abstract:

Spatial variation in climate, soils, disturbance regime, and forest management − as well as temporal variation in weather − all influence terrestrial carbon sources and sinks. Spatially-distributed, process-based, carbon cycle simulation models provide a means to integrate information from these various influences to estimate carbon pools and flux over large domains. Here we apply the Biome-BGC model over the 4 state (OR, WA, ID, Western MT) Northwest U.S. region for the interval from 1986-2010. Landsat data was used to characterize disturbances and revealed that the overall disturbance rate on forest land across the region was 0.8 % yr^-1, with 49 % as harvests, 28 % as fire, and 23 % as pest/pathogen. A large proportion of the harvested area was on private forestland (62 %) and a large proportion of total burned area was on public forestland (89 %). Net ecosystem production (NEP) for the 2006-2010 interval on forestland was predominantly positive (a carbon sink) throughout the region, with maximum values in the Coast Range, intermediate values in the Cascade Mountains, and relatively low values in the Inland Rocky Mountain ecoregions. Croplands throughout the region had consistently high NEP. Localized negative NEPs were mostly associated with recent disturbances. There was large interannual variation in regional NEP, with notably low values across the region in 2003. In all ecoregions there was a downward trend in NEP over the 25 year study period. The net ecosystem carbon balance was positive in OR, near neutral in ID and WA, and negative (a carbon source) MT. The Northwest region as a whole was a carbon sink in the 2006-2010 period.