Dead forests burning: the influence of beetle outbreaks on fire severity and legacy structure in sub‐boreal forests

Recent regional mountain pine beetle (MPB) outbreaks have generated unprecedented tree mortality across the fire‐prone landscapes of western North American forests and could potentially modify fire severity and postfire ecological effects. In 2012, 2013, and 2014, three fires burned through high mortality, gray‐phase lodgepole pine‐dominated forests in the plateau regions of central interior British Columbia, Canada, providing an opportunity to test for interactions between MPB outbreaks and wildfires. We inventoried 63 plots that spanned gradients of outbreak severity, fire severity, and burning conditions in a wilderness setting. Our objective was to evaluate the influence of outbreak severity on fire severity by assessing typical first‐order fire effects as well as legacy structure related to the consumption of woody biomass on snags/trees. We found no evidence of a relationship between outbreak severity and fire severity for six of seven first‐order fire effects, with the exception of deep charring. We found evidence that legacy structure in the form of consumed branch structure and deep char development had greater odds of occurrence on MPB‐killed snags compared to trees killed during wildfire. Our results indicate two key findings. First, fire severity as it relates to most first‐order fire effects measures is not influenced by outbreak severity, instead it is more strongly influenced by the interaction of fuels, weather, and topography during fire events. Second, our results highlight how the interaction between outbreak severity and fire severity alters postfire structural legacies and their functional attributes, which could have important ecosystem implications. Forest ecosystems across western North America are increasingly experiencing ecological disturbances from wildfires burning through landscapes with abundant tree mortality from insect outbreaks. The recent mountain pine beetle (Dendroctonous ponderosae, hereafter MPB) outbreaks circa the 1990s and 2000s are responsible for tree mortality in forests that span over 25 million hectares across the western United States and Canada (Raffa et al. 2008, Bentz et al. 2010, Meddens et al. 2012), and British Columbia (BC) houses nearly 20 million of those hectares (Axelson et al. 2009, Perrakis et al. 2014). The spatial extent and high mortality rates associated with recent outbreaks alter standing woody fuels in affected forests from mostly alive to mostly dead, which changes the composition of the fuel profile and raises concerns for increased fire severity (Hicke et al. 2012, Jenkins et al.