Climatic stress increases forest fire severity across the western United States

Pervasive warming can lead to chronic stresson forest trees, which may contribute to mortality resulting from fire-caused injuries. Longitudinal analyses of forest plotsfrom across the western US show that high pre- fireclimatic water deficit was related to increased post- firetree mortality probabilities. This relationship between climate and firewas present after accounting for firedefences and injuries, and appeared to influence the effects of crown and stem injuries. Climate and fireinteractions did not vary substantially across geographical regions, major genera and tree sizes. Our findings support recent physiological evidence showing that both drought and heating from firecan impair xylem conductivity. Warming trends have been linked to increasing probabilities of severe fireweather and firespread; our results suggest that warming may also increase forest fireseverity (the number of trees killed) independent of fireintensity (the amount of heat released during a fire).