Fast-spreading conifer fires burn twice the share of their area at high severity

The two area figures in the Eos account of the study do not reconcile as printed. It gives 43 million hectares of burned area across the fires ^[1] and says 11.2 million hectares burned at high severity, calling that more than a third of the area ^[5]. Divided out, 11.2 million of 43 million is about 26 percent ^[1]. For 11.2 million hectares to be a third, the base would need to be about 34 million hectares ^[3]. Either the share was taken over a smaller base than total burned area or one figure is wrong, and the Eos account does not say which.

The speed gradient is cleaner. The fastest fires severely burned about 50 percent of their area and the slowest about 25 percent ^[4], a doubling of the severe share across the range ^[2]. Because fast fires also burned more ground ^[3], the two effects stack in the count of high-severity hectares.

The design, described in Science Advances in August ^[17], is an observational survey at scale: satellite records for each fire, with daily growth rate set against outcome ^[2], across forests in 11 ecological zones ^[8]. That is enough to show speed and severity travel together, though not to separate the effect of speed from the warm, dry conditions that make fires fast. Jonathan Coop, the Western Colorado University ecologist who led the work ^[18], ties bigger, faster fires to those trends ^[19]. "Conditions are becoming more flammable," he said ^[15]. For forecasting damage the causal split matters less, since a day of rapid spread flags severe burning either way ^[3].

Nathan Gill, a landscape ecologist at Texas Tech University who was not involved, said "the evidence connecting fire spread rate, burned area, and fire severity is compelling" ^[9]. He added that "now we can have confidence in saying it's true across much of the forested regions of North America" ^[10]. His endorsement covers spread, area and severity. The prediction about future forests rests on the seed-distance measure ^[6], and a seed distance does not tell you what actually comes up. Eos words the inference carefully: the distance "suggests that forests might not grow back the same after a fast, severe wildfire" ^[7].

Coop draws the line by forest type. "In northern Canada, a big patch of high-severity fire in a jack pine forest is likely to recover exactly towards a jack pine forest again," he said ^[11]. His counter-case is the southwestern United States: "In other settings, like in the southwestern U.S. in a mixed conifer forest that's really well adapted to surface fire, this kind of large patch is unusual, and it's less likely to be able to recover back towards what it was" ^[12]. Adaptations differ by species. Black spruce and lodgepole pine depend on fire to open serotinous cones, and ponderosa pine's thick bark helps it survive surface fires ^[13].

In my view the evidence supports a specific worry: in forests adapted to surface fire, the large severe patches that fast fires produce are the likeliest to regrow as a different forest ^[3][6][12].