Thirsty trees die more easily from fire

Photo Credit

USDA Forest Service photo courtesy Sharon Hood

A recent prescribed burn in Montana. Crown scorch is indicated by the orangish-colored needles that were killed during the fire. Some of the smaller trees will likely die in the next 1-2 years from crown scorch. Severe drought may increase the likelihood of tree death after fire even if crown scorch doesn’t increase.

Wildfires leave a mosaic of live and dead trees on the landscape. For trees that initially survived a fire, many die in the following years. Researchers investigated whether trees in burned areas that were stressed by severe drought are more likely to die in the years post fire. Consistent with prior research, they found that fire-caused injuries to the tree crown (such as crown scorch) were the strongest predictor of tree death after fire, with higher levels of scorch being more likely to lead to post-fire tree death. However, they also found that pre-fire drought influenced post-fire death. For trees with the same level of crown scorch, a tree that experienced severe pre-fire drought was 78% more likely to die after fire compared to a tree that burned under less droughty conditions. This impact was most pronounced for the thick-barked tree species Douglas-fir, white fir, and ponderosa pine. For thin-barked species like lodgepole pine, the impacts from crown scorch outweighed those of pre-fire drought.

Tree species with thicker bark are typically the ones managers are trying to maintain on the landscape because they are more resistant to fire. When researchers looked into how the likelihood of post-fire tree death could change under future, hotter climate scenarios in which drought is more frequent, they found that trees died from lower levels of crown scorch compared to non-drought conditions. This was again particularly true for Douglas-fir, white fir, and ponderosa pine, suggesting that these relatively fire-resistant species may become less resilient to fire in the future. This research suggests that higher levels of tree mortality can be expected when fires burn in areas affected by severe drought. In other words, drought is expected to make many western forests less resilient to fire and cause more mortality even if they burn under moderate intensity.

Models that predict which trees will go on to die or survive in the years after a fire are commonly used in fire management decision support systems for developing prescribed burn plans to meet mortality targets, for informing post-fire silvicultural treatments including salvage harvests, and projecting future forest composition and structure. Although current tree mortality models remain fairly accurate, as drought conditions increase in frequency and severity, adding prefire climate conditions to tree mortality models will improve their accuracy. While reducing fire severity from wildfire through prescribed burning and mechanical thinning remains a top priority, this research suggests prescribed burn ignition techniques may need modification to mitigate tree mortality in severe drought years.

This research was the outcome of dozens of smaller studies led by federal and university researchers in close collaboration with Forest Service Forest Health Protection staff. The results will help improve models that predict delayed tree death after fire, which have important applications in both forest and fire management.