Species effects on boreal fire dynamics; Implications for climate feedbacks
North America and Eurasia are covered by vast tracts of boreal forest that experience frequent wildfire. These fires affect climate and ecosystem dynamics through direct emissions and altered vegetation communities for years to decades. Fire dynamics, however, are distinctly different between the two boreal continents and have yet to be comprehensively evaluated. While high-intensity crown fires are known to dominate boreal North America, lower-severity surface fires are typically reported in Eurasia. Here we used over a decade of satellite imagery to characterize variations in circumpolar fire behavior, immediate impacts, longer-term responses, and surface shortwave forcings during the first 11 years after fire. Satellite products were transformed into more relevant metrics, including frequency of crown fires, vegetation combustion, and tree mortality. Results were corroborated by independent datasets on smoke injection height, fire spread rates, and fire sizes. We compared the divergent continental patterns to two widely used global fire models. The origins of contrasting large-scale dynamics were explored using fire weather indices and forest species distribution maps for Alaska, Canada, and Russia.
Satellite-derived metrics of fire intensity (instantaneous fire behavior), fire severity (immediate impacts on the environment), and burn severity (longer-term ecosystem changes) were consistently and significantly lower in boreal Eurasia compared to North America. Eurasian fires were 58 ± 31% less likely to be crown fires, combusted 36 ± 5% less live vegetation, and caused 42 ± 5% less tree mortality. Fires in Eurasia also generated a 69 ± 9% smaller surface shortwave forcing during the initial post-fire decade, suggesting a near-neutral or positive net climate forcing. Current global fire models were unable to capture the continental differences, and fire weather indices were unable to explain the contrasting dynamics. The primary drivers are shown to be species-level adaptations to fire, making this a preeminent example of species effects on continental-scale carbon and energy exchange.