The Varying Strength and Impact of Negative Fire-Vegetation Feedbacks Across the North American Boreal Forest Biome

Annual area burned in North American boreal forests is projected to increase 50-300% by 2100 in response to increasing aridity. These projections, however, generally do not account for post-fire vegetation changes, which alter landscape flammability and can change the probability of future burning, setting up fire-vegetation feedbacks. Here we use fire-history records from 1950-2016 and spanning the North American boreal forest biome, MODIS data characterizing tree cover (MOD44B), and annual summaries of the Fire Weather Index (FWI) from the Canadian Forest Fire Danger Rating System, to characterize the strength of and impact of fire-vegetation feedbacks in boreal forest ecosystems. Our study domain was the North American boreal forest biome (4.24108 ha). In this study, we focused on exploring how the strength of negative fire-vegetation feedbacks varies among ecoregions, and feedback strength was estimated as the relative difference between observed fire rotation periods and expected fire rotation periods under a null hypothesis that there is no feedback. We found that historical fire activity led to tree-cover loss of up to 40% in some areas, with reductions from pre-fire levels lasting decades (35-55 yr). In concert with these vegetation changes, we found evidence for negative fire-vegetation feedbacks when considering our entire study domain. By breaking our study area into individual 100 km2 hexagons (i.e., hexels), we found the distributions of area burned between single- and re-burned hexels differed significantly (p < 0.05), and the magnitude of the feedback strength was 4-5%. We found significant variability in feedback strength among ecoregions. For example, the feedback strength was 2.5% and 6% in Alaska and Central Canada, respectively. Variability among ecoregions was better explained by regional differences in FWI (p < 0.05, r2 = 0.64) rather than vegetation (p > 0.05, r2 = 0.04), with warmer and drier conditions leading to stronger feedback effects. In summary, while climate change may lead to broadscale burning, increased fire activity may also predicate a stronger negative fire-vegetation feedback. Exploring how feedback strength may mediate fire regimes under a range of future climate change scenarios is a key next step in projecting the future state of boreal forest ecosystems.