Fire weather drives daily area burned and observations of fire behavior in mountain pine beetle affected landscapes

Abstract

In the western United States, mountain pine beetles (MPBs) have caused tree mortality across 7% of the forested area over the past three decades, leading to concerns of increased fire activity in MPB-affected landscapes. While fire behavior modeling suggests MPB-associated changes in fuels may influence fire behavior, retrospective studies have generally found negligible or weak effects of pre-fire MPB outbreak on fire activity. This apparent disagreement may arise from differences in fire weather, fuels, or scale and highlights the need for empirical studies that examine the influence of MPB outbreak on fire activity at finer spatiotemporal scales. Here we use a novel combination of geospatial data and firefighter observations to test the relative influences of red and gray stage MPB outbreak on two measures of daily wildfire activity—daily area burned (DAB) and observed fire behavior. We analyzed 2766 large wildfires that burned in the West over the 2003–2012 period. We found 329 fires that intersected prior MPB outbreak, however most burned in areas affected by MPB for only a few days (median = 4 d). We modeled DAB and the occurrence of observation of high-extreme fire behavior in 57 large (>1140 ha) wildfire events that burned for long time periods (>10 d) in landscapes affected by MPB. Under these conditions, we found no effect of red or gray stage MPB outbreak on either DAB or observed fire behavior. Instead, greater DAB and observations of high-extreme fire behavior occurred during warmer, drier, and windier weather conditions and where pre-outbreak fuels were characterized by lower canopy base heights and greater canopy bulk densities. The overriding influence of weather and pre-outbreak fuel conditions on daily fire activity observed here suggest that efforts to reduce the risk of extreme fire activity should focus on societal adaption to future warming and extreme weather.