Climate stress increases forest fire severity across the western United States

Pervasive warming can lead to chronic stress on forest trees, which may contribute to mortality resultingfrom fire-caused injuries. Longitudinal analyses of forest plots from across the western US show that highpre-fire climatic water deficit was related to increased post-fire tree mortality probabilities. This relationshipbetween climate and fire was present after accounting for fire defences and injuries, and appeared to influencethe effects of crown and stem injuries. Climate and fire interactions did not vary substantially acrossgeographical regions, major genera and tree sizes. Our findings support recent physiological evidence showingthat both drought and heating from fire can impair xylem conductivity. Warming trends have beenlinked to increasing probabilities of severe fire weather and fire spread; our results suggest that warmingmay also increase forest fire severity (the number of trees killed) independent of fire intensity (the amountof heat released during a fire).