Interactions between bark beetle outbreak and wildland fire in intermountain subalpine forests of the western United States: legacies and future projections under a changing climate

Over the past 30 years, wildland fire and native bark beetle outbreaks have increased in intensity, severity, and extent across the fire-prone forests of the western United States, raising concerns about whether bark beetle outbreaks increase wildfire severity and/or wildfire occurrence. Furthermore, current estimates predict a two-fold increase in area burned by wildland fires over the next 25 years and bark beetles are forecasted to expand in the coming century, shifting toward higher latitudes and elevations. Thus, it is important to better understand how insect-driven tree mortality may affect fire risk and how these disturbance interactions may affect ecosystem structure and dynamics across biophysical settings under current and future climate scenarios. In this dissertation, I investigated the relationships between bark beetle outbreaks, wildfire, and climate across the western United States and within subalpine forests of the Southern Rocky Mountains, CO, USA. The main research questions of this dissertation were: (Chapter II) what is the relative importance of mountain pine beetle (Dendroctonus ponderosae (Hopkins)) outbreaks versus antecedent climatic variability on the occurrence of large wildfires in the western U.S.? (Chapter III) how do pre-outbreak forest conditions mediate the effects of spruce beetle (Dendroctonus rufipennis (Kirby)) outbreaks on fuels complexes in subalpine forests of Colorado? and (Chapter IV) how do changes in fuels following spruce beetle outbreaks affect expected fire potential under current and future climate conditions? Chapter II employed a variety of remotely sensed data and GIS products of fire occurrence, mountain pine beetle outbreaks, physiographic gradients, and climatic condition to test whether prior-disturbance or antecedent climate conditions influenced subsequent wildfire events. Extensive field surveys of stand attributes and fuel arrangements across a chronosequence of spruce beetle outbreaks in the 20th and 21st century were employed to address research questions of Chapter III. Results from Chapter III were used as base inputs for custom fire behavior models in Chapter IV, to test the sensitivity of potential fire behavior across a variety of wind speeds, weather, and climate scenarios. Despite widespread concern that mountain pine beetle outbreaks lead to unprecedented increases in wildfire activity, results from Chapter II demonstrated minimal effects of these pre-fire disturbances on subsequent fire occurrence. Instead, occurrence of large wildfires across the western US has been driven by extreme weather (e.g., hot, dry conditions). Chapter III revealed that the changes to fuels following spruce beetle outbreaks are strongly contingent on pre-outbreak stand structure and disturbance history. For instance, we found that spruce beetle outbreaks reduce canopy fuels in all stands, yet this effect is relatively minor in old spruce-fir stands as compared to young spruce-fir stands. Spruce beetle outbreaks during the 20th and 21st century decreased canopy fuels and increased their heterogeneity, regardless of pre-outbreak conditions. Surface fuel loads were more variable with increased time since spruce beetle outbreak and did not return to pre-outbreak conditions over the 75-year period considered in this study in both young and old stands. Chapter IV concluded that under all weather and climate scenarios, stands affected by spruce beetle had the lowest potential for increased surface fireline intensities, rates of spread, and active crowning among both young and old stands as compared to endemic stands (i.e., non-outbreak). Chapter IV used future climate projections (2016-2100) of RCP 4.5 and RCP 8.5 as proxies for moderate and severe climate change and concluded that moderate climate change will not substantially increase the most important types of fire behavior among young or old stands, nor stands affected by spruce beetle outbreak as compared to current climate (1985-2015). However, under severe climate change projections (RCP 8.5) all characteristics of fire behavior will increase, regardless of stand age, spruce beetle outbreak, and wind and weather scenarios. This research provides much needed insight into the disturbance dynamics in fire-prone forests and informs forest management and policy concerns under a changing climate. Overall, this research highlights the 1) dominant effect of climate, rather than outbreaks, has on fire regimes across the western United States and 2) the importance of accounting for pre-disturbance stand structure and disturbance histories on subsequent disturbance patterns and severities