Evaluating Effects of Harvest, Fire, and Insect Disturbances on Ecoregion Scale Net Ecosystem Exchange of CO2

Forest disturbance regimes are generally a function of both human interventions ─ such as wood harvesting ─ and background events such as wildfire. In this study, we employed 25 years of Landsat data (1986-2011) to characterize the disturbance regime over the predominantly forested West Cascades ecoregion in Oregon, U.S.A. Disturbances were labeled by year and classified by cause, duration, and intensity. That data-set was input to the Biome-BGC carbon cycle model (applied in a spatially-distributed mode) to evaluate the spatial and temporal patterns in ecoregion net ecosystem production (NEP), net ecosystem exchange (NEE), and net ecosystem carbon balance (NECB). The model simulated disturbance events in terms of tree mortality, carbon removals, fire emissions, and transfers from live to dead carbon pools. The disturbance regime in this ecoregion was dominated by harvesting, with lower levels of fire and insect mortality. Two apparent trends over the 25 year study period were a decrease in the harvest level on public lands, and an increase in the area burned on public lands. Ecoregion total net ecosystem production (the balance of net primary production and heterotrophic respiration) was positive (a carbon sink) in all years, with greater carbon uptake in relatively cool years. Localized carbon source areas were associated with recent harvests and fire. Net ecosystem carbon exchange, which includes the effect of wildfire emissions, showed greater interannual variation than NEP, and approached zero in high fire years. The consistently positive NECB for the ecoregion over the last 25 years has provided a significant offset to the concurrent state-level fossil fuel emissions.