Regional variation in interior Alaskan boreal forests is driven by fire disturbance, topography, and climate

Abstract High latitude regions are warming rapidly with important ecological and societal consequences. Utilizing two landscape‐scale data sets from interior Alaska, we compared patterns in forest structure in two regions with differing fire disturbance, topography, and summer climate norms. Our goal was to evaluate a set of hypotheses concerning possible warming‐driven changes in forest structure suggested by recent literature. We found essentially consistent habitat associations for the tree flora across two disparate study areas concomitant with considerable differences in observed patterns of forest structure and composition. Our results confirmed expected increases in broadleaved species occupancy and abundance in the warmer, more fire‐affected study region along with considerably higher tree occupancy and abundance in high elevation areas there. However, contrary to our predictions, we found no evidence of expected reductions in conifer occupancy or increases in non‐fire related tree mortality. Instead, both individual and combined tree species occupancy, density, abundance, and richness were considerably higher in the warmer, more fire‐influenced region, except in the warmest, driest areas (steep and south‐facing slopes at low elevation). Our comparison of two landscape‐scale data sets suggests that changes in tree distribution and forest structure in interior Alaska will proceed unevenly, governed by a mosaic of site‐dependent influences wherein forest community composition and species dominance will shift along different trajectories and at different rates according to variation in underlying landscape attributes. Although there were clear differences in forest structure between the two areas that were likely attributable to differences in growing season warmth and fire disturbance, we found scant support for the concept of an incipient, ongoing biome shift in interior Alaska resulting from impending diminution of boreal forest cover over the short to medium term. Indeed, we suggest that (depending on severity of disturbance dynamics and the rapidity of future warming) cooler areas of interior Alaska's forest may reasonably be expected to sustain marginal increases in forest cover with additional warming, at least in certain topographic positions (such as poorly drained basins and cool treeline sites) and/or geographic regions, prior to any landscape‐scale diminution of forest cover due to warming.