How do bark beetle outbreaks drive fire regimes in semi-arid systems?

Abstract:

Bark beetle outbreaks and wildfires are two major reorganizing forces that occur in coniferous forests of North America. In recent decades, they have consumed millions of hectares of forest. Climate change is projected to independently increase the severity and frequency of these two disturbances, however, it is less clear how climate change will drive interactions between them. While in many cases bark beetle outbreaks may increase wildfire activity by increasing surface fuel loads and decreasing fuel moisture, there may also be cases where beetle-caused tree mortality decreases evapotranspiration and thus mitigates climate-driven increases in fuel aridity. The magnitude (and even the direction) of these interactions can vary; some research has found that bark beetle outbreaks increase in fire activity while other studies document no change. This variability has been attributed to factors such as how much time has passed since the beetle outbreak, the percentage of trees that have been killed, and whether the system is fuel- or flammability-limited. It remains unclear however, how specific ecohydrologic and vegetation mechanisms drive fire regime responses to beetle outbreaks and how these mechanisms vary along environmental gradients. To understand how time since beetle attack, outbreak severity, and local fuel conditions interact to drive wildfire activity, we used a coupled eco-hydrological and wildfire model to simulate different outbreak scenarios. We found that fire activity decreased during the red phase (1-5 years after a beetle attack), increased during the grey phase (6-15 years after attack), and showed a non-monotonic behavior during the old phase (16-75 years after attack). Fire responses vary spatially where burn probability increases in fuel-limited area but decreases in flammability-limited area. The variability in responses was driven by competition between increasing fuel loads and decreasing fuel aridity (caused by decreases in leaf area and therefore decreases in PET). When the system is not fuel-limited, fire size and frequency change only with fuel aridity. The mechanistic modeling structure employed here provides a framework for identifying when and where beetle-killed forests are most vulnerable to wildfire.