Estimating post-fire organic soil depth in the Alaskan boreal forest using the Normalized Burn Ratio

As part of a long-term moose browse/fire severity study, we used the Normalized Burn Ratio (NBR) with historic Landsat Thematic Mapper (TM) imagery to estimate fire severity from a 1983 wildfire in interior Alaska. Fire severity was estimated in the field by measuring the depth of the organic soil at 57 sites during the summer of 2006. Sites were selected for field sampling from five fire severity classes based on threshold NBR values. The linear relationship between post-fire NBR and organic soil depth among sites within the burn was weak (r(2)=0.26), and improved substantially (r(2)=0.66) when restricted to non-wetland black spruce sites. The relationship between NBR and aspen/willow counts was non-linear. Sites with high densities of aspen stems consistently occurred in the high fire severity classes, and sites with high willow stem densities consistently occurred in the moderate fire severity class. However, NBR varied substantially from sites with low aspen or willow reproduction and therefore predicting aspen or willow regeneration based on post-fire NBR values would be difficult.