Forest fire impacts on carbon uptake, storage, and emission; The role of burn severity

This study quantifies the short-term effects of low-, moderate-, and high-severity fire on carbon pools and fluxes in the Eastern Cascades of Oregon. We surveyed 64 forest stands across four fires that burned 41,000 ha (35%) of the Metolius Watershed in 2002 and 2003, stratifying the landscape by burn severity (overstory tree mortality), forest type (ponderosa pine [PP] and mixed-conifer [MC]), and prefire biomass. Stand-scale C combustion ranged from 13 to 35 % of prefire aboveground C pools (area- weighted mean = 22%). Across the sam-pled landscape, total estimated pyrogenic C emis-sions were equivalent to 2.5 % of statewide anthropogenic CO2 emissions from fossil fuel com-bustion and industrial processes for the same 2-year period. From low- to moderate- to high-severity ponderosa pine stands, average tree basal area mortality was 14, 49, and 100%, with parallel pat-terns in mixed-conifer stands (29, 58, 96%). Despite this decline in live aboveground C, total net primary productivity (NPP) was only 40 % lower in high-versus low-severity stands, suggesting strong com-pensatory effects of non-tree vegetation on C up-take. Dead wood respiratory losses were small relative to total NPP (range: 10–35%), reflecting decomposition lags in this seasonally arid system. Although soil C, soil respiration, and fine root NPP were conserved across severity classes, net ecosys-tem production (NEP) declined with increasing severity, driven by trends in aboveground NPP. The high variability of C responses across this study underscores the need to account for landscape pat-terns of burn severity, particularly in regions such as the Pacific Northwest, where non-stand-replace-ment fire represents a large proportion of annual burned area. Key words: carbon balance; Cascade Range; dis-turbance; fire emissions; heterotrophic respiration; mixed-severity fire regime; net ecosystem produc-tion; net primary productivity; Pinus ponderosa; wildfire