The occurrence, severity, and interaction of mountain pine beetle and wildfire in the Colorado Front Range
In the Colorado Front Range, recent mountain pine beetle (MPB; Dendroctonus ponderosae) and wildfire activity have garnered much attention, causing concern about the safety of humans and property, aesthetics, and real estate values (Flint 2006, Price et al. 2010, Fettig et al. 2014). Since climate warming is the underlying driver of increases in wildfire and bark beetle activity (Raffa et al. 2008, Dennison et al. 2014), there is an urgent need to address biophysical drivers of disturbance occurrence and severity as well as the interactions between disturbances. We investigate the influences, including previous fire activity, on the susceptibility of ponderosa pine to MPB during the recent outbreak and examine the temporal stability of these relationships. Additionally, we examine a recent fire that burned in the Wildland Urban Interface (WUI) to assess if the effects of that event are within the historical range of variability (HRV).
To examine the influences of previous fire activity on ponderosa pine susceptibility to MPB, we collected tree- and stand-level characteristics within previously sampled fire history sites (Chapter 2). We found recent (~5 years) fires injury increased tree-level susceptibility and stands that experienced high-severity fires at least 100 years ago had greater severity of MPB infestation. In order to develop a method to detect MPB-killed trees in the montane zone with complex vegetation patterns, we compared remote sensing image classification methods in a representative study area and specifically examined the classification accuracies across high-, moderate-, and low-forest densities (Chapter 3). We developed a classification method that resulted in high and consistent accuracies across forest densities. In order to assess how the outbreak progressed across the landscape from early (2009) to more advanced (2011) outbreak stages, we used classifications of MPB infestation derived from high-resolution aerial imagery covering Boulder County (Chapter 4). We found MPB activity during the early outbreak to occur more often than expected on xeric slopes and transition to more mesic slopes later in the outbreak suggesting changes to the constraints on beetle activity as MPB populations increase. In 2010, the most costly fire in Colorado’s history at the time provided an opportunity to assess if the fire was within the Historical Range of Variability (HRV) for the area (Chapter 5). We used remotely-sensed burn severity data to examine biophysical (i.e. terrain and vegetation) and human (i.e. fuel treatments) influences on burn severity. We found burn severity was primarily driven by terrain, and was within the HRV for the forest type. Specifically, the research in this dissertation addresses the susceptibility of ponderosa pine to MPB and wildfire in order to improve our understanding of disturbance occurrence, severity, and interactions in the WUI of the Colorado Front Range.