Effects of bark beetle-caused tree mortality on wildfire

Millions of trees killed by bark beetles in western North America have raised concerns about subsequent wildfire, but studies have reported a range of conclusions, often seemingly contradictory, about effects on fuels and wildfire. In this study, we reviewed and synthesized the published literature on modifications to fuels and fire characteristics following beetle-caused tree mortality. We found 39 studies addressing this topic with a variety of methods including fuels measurements, fire behavior simulations, an experiment, and observations of fire occurrence, severity, or frequency. From these publications, we developed a conceptual framework describing expected changes of fuels and fire behavior. Some characteristics of fuels and fire are enhanced following outbreaks and others are unchanged or diminished, with time since outbreak a key factor influencing changes. We also quantified areas of higher and lower confidence in our framework based on the number of studies addressing a particular area as well as agreement among studies. The published literature agrees about responses in many conditions, including fuels measurements and changes in stands with longer times since outbreak, and so we assigned higher confidence to our conceptual framework for these conditions. Disagreement or gaps in knowledge exist in several conditions, particularly in early postoutbreak phases and crown fire behavior responses, leading to low confidence in our framework in these areas and highlighting the need for future research. Our findings resolved some of the controversy about effects of bark beetles on fire through more specificity about time since outbreak and fuels or fire characteristic. Recognition of the type of study question was also important in resolving controversy: some publications assessed whether beetle-caused tree mortality caused differences relative to unattacked locations, whereas other publications assessed differences relative to other drivers of wildfire such as climate. However, some disagreement among studies remained. Given the large areas of recent bark beetle and wildfire disturbances and expected effects of climate change, land and fire managers need more confidence in key areas when making decisions about treatments to reduce future fire hazard and when fighting fires.

► We reviewed studies of bark beetle-caused tree mortality effects on fire.
► Effect can be substantial depending on time since outbreak and fire characteristic.
► Agreement among studies existed for many conditions.
► Disagreement and knowledge gaps occurred in several key areas.