Harvey, Brian J. 2015. Causes and consequences of spatial patterns of fire severity in Northern Rocky Mountain forests: the role of disturbance interactions and changing climate. PhD dissertation, University of Wisconsin-Madison.

Bark beetle outbreaks have affected fire-prone forests across North America in recent decades, raising concern about whether outbreaks increase fire severity and/or reduce postfire tree regeneration. Wildfire activity has also increased over this period, but little is known about whether spatial patterns of fire severity are changing, and/or how spatial patterns of fire and warming climate affect postfire tree establishment. In this dissertation, I investigated relationships between beetle outbreaks, wildfire, climate, and postfire tree regeneration in the US Northern Rocky Mountains. I tested whether prefire beetle outbreaks affected fire severity and/or postfire tree regeneration, using field plots distributed region-wide. I field-validated satellite maps of fire severity, and tested whether spatial patterns of fire severity changed over the period 1984-2010. I also used extensive field surveys to test the effect of postfire drought severity and distance to seed source on tree establishment following stand-replacing fires.

Effects of beetle outbreak severity on fire severity were minimal, regardless of the interval between outbreaks and fires. Fire severity was instead largely driven by topography and extreme weather conditions. Effects of beetle outbreaks on postfire tree regeneration depended on regeneration mechanisms of dominant host trees. For tree species that retain a persistent seedbank in serotinous cones on live and dead trees, postfire tree regeneration was not affected by prefire beetle outbreak severity. However, for tree species that lack a persistent seedbank, postfire tree regeneration decreased with greater prefire beetle outbreak severity. Most metrics of spatial heterogeneity of fire severity were affected by fire size and the proportion of fires burning as stand replacing, but few metrics changed over time from 1984 to 2010. The proportion of fires burning as stand replacing increased from 0.22 to 0.27 over this time period, and further increases may cause rapid shifts in the configuration of burn severity. Postfire tree regeneration for tree species that currently dominate subalpine forests was substantially reduced toward the interior of stand-replacing patches (i.e., far from seed sources) and in fires that were followed by severe drought. Overall, this research provides insight into disturbance dynamics in fire-prone forests and informs forest management and policy concerns.