Large biophysical impacts of forest fire size

<p>Future climate warming is projected to increase the occurrence of large or extreme forest fires, but how fire size impacts forest damage and forest biophysical processes remains unclear. Using multiple datasets including satellite observations, here we reported that the dominant fire size has almost doubled in northern temperate and boreal forest ecosystems over the period of 2003-2016, concomitant with significant surface air warming during the fire season. Increasing forest size is accompanied by enhanced fire behaviour, including faster rate of spread and elongated fire duration. Bigger fires drive enhanced forest mortality and fire intensity. Forest canopy damage measured by decrease in postfire leaf area index, changes in surface albedo and ecosystem evaporation all demonstrate strong linear relationship with the logarithm of fire size (R<sup>2</sup>>0.9), which collectively drive elevated postfire land surface warming with the increase of fire size. Thus, the temporal increase of fire size leads to significantly increasing postfire surface warming, which provides strong positive feedback for further warming. Such a scaling relationship between postfire warming with fire size is, however, strongly regulated by forest type, with surface warming with fire size being most sensitive in evergreen needleleaf forest, followed by deciduous needleleaf forest, and then by forest with deciduous broadleaf trees. Our study thus highlights that important feedbacks exist between fire regime and climate warming and demand the inclusion of explicit representation of fire size in Earth System Models on the one hand, and suggest that increasing broadleaf trees can mitigate such positive feedbacks on the other hand.</p>