The influence of burn severity on postfire vegetation recovery and albedo change during early succession in North American boreal forests
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Severity of burning can influence multiple aspects of forest composition, carbon cycling, and climate forcing. We quantified how burn severity affected vegetation recovery and albedo change during early succession in Canadian boreal regions by combining satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Canadian Large Fire Database. We used the MODIS-derived difference Normalized Burn Ratio (dNBR) and initial changes in spring albedo as measures of burn severity. We found that the most severe burns had the greatest reduction in summer MODIS Enhanced Vegetation Index (EVI) in the first year after fire, indicating greater loss of vegetation cover. By 5-8 years after fire, summer EVI for all severity classes had recovered to within 90%-108% of prefire levels. Spring and summer albedo progressively increased during the first 7 years after fire, with more severely burned areas showing considerably larger postfire albedo increases during spring and more rapid increases during summer as compared with moderate-and low-severity burns. After 5-7 years, increases in spring albedo above prefire levels were considerably larger in high-severity burns (0.20 +/- 0.06; defined by dNBR percentiles greater than 75%) as compared to changes observed in moderate- (0.16 +/- 0.06; for dNBR percentiles between 45% and 75%) or low-severity burns (0.13 +/- 0.06; for dNBR percentiles between 20% and 45%). The sensitivity of spring albedo to dNBR was similar in all ecozones and for all vegetation types along gradients of burn severity. These results suggest carbon losses associated with increases in burn severity observed in some areas of boreal forests may be at least partly offset, in terms of climate impacts, by increases in negative forcing associated with changes in surface albedo.