A Negative Fire–Vegetation Feedback Substantially Limits Reburn Extent Across the North American Boreal Biome

The North American boreal biome (NAB) is warming at 2–4 times the mean global rate, contributing to increasing wildfire activity. The degree to which this trend alters biome-level feedbacks to global climate depends on how strongly bottom-up feedbacks between fire and vegetation dampen the effects of climate drivers. As young vegetation recovering from fire covers a growing portion of the landscape, it could resist reburning, buffering against further increases in fire. Resistance to reburning could be particularly strong in the NAB, where slow post-fire fuel accumulation is sometimes combined with a fire-driven shift from conifers toward less-flammable, deciduous trees. However, continued warming could eventually override the feedback. To quantify the strength of the feedback throughout the biome, we divided the NAB into 27 Fire Regime Units (FRUs) and used fire data from 1986 to 2018 to determine the area expected to have burned more than once (that is, reburned) within each FRU under the null assumption that recent fire does not influence burn probability. Then, we ran a spatial simulation to quantify the strength of departure from the null value while accounting for variation driven by stochastic fire patterns. Reburn extent was 5 Mha less than expected without the feedback. Departure from the null model was strongest in the most fire-prone FRUs, suggesting that the feedback will continue to dampen climate-driven increases in wildfire activity. These results provide a sound baseline from which to identify potential weakening of the feedback under continued warming, and our approach could be expanded to other biomes.