Can climate change increase fire severity independent of fire intensity?

Background/Question/Methods

There is a growing realization that regional warming may be linked to increasing fire size and frequency in forests of the western US, a trend occurring in concert with increased fuel loads in forests that historically experienced frequent surface fires. Recent studies have also suggested that warming temperatures are correlated with increased fire severity (post-fire tree mortality). The mechanism whereby fire severity might increase in response to warming is presumed to be increasing probabilities of hazardous fire weather (higher air temperature, lower relative humidity and fuel moisture). While likely true, this view fails to consider the biological context of the fire event. Here we present evidence that trees subject to environmental stress are more sensitive to subsequent fire damage.

Results/Conclusions

Tree growth records, used as an index of health for individuals, show that for two tree species (Abies concolor and Pinus lambertiana) in the Sierra Nevada of California poor growth leads to increased probabilities of mortality following fire. Plot-based fire monitoring databases from over 300 sites across the western US demonstrate that indices of drought stress are strongly predictive of post-fire tree survivorship. In sum, these results suggest that recent climatic trends may lead to a de facto increase in fire severity, even when there is no change in fire intensity.