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 resulting
from fire-caused injuries. Longitudinal analyses of forest plots from across the western US show that high
pre-fire climatic water deficit was related to increased post-fire tree mortality probabilities. This relationship
between climate and fire was present after accounting for fire defences and injuries, and appeared to influence
the effects of crown and stem injuries. Climate and fire interactions 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 fire can impair xylem conductivity. Warming trends have been
linked to increasing probabilities of severe fire weather and fire spread; our results suggest that warming
may also increase forest fire severity (the number of trees killed) independent of fire intensity (the amount
of heat released during a fire).