Due to the amplified effects of high latitude climate change, important characteristics of wildfire and epidemic spruce bark beetle (SBB) (Dendroctonus rufipennis) outbreaks in the Alaskan boreal forest are changing. Because natural disturbance is an important determinant of vegetation cover, soil characteristics, and human land use; altering disturbance properties may have large impacts on future ecosystems and human livelihoods. While research has been conducted to project the effects of climate change on future wildfire dynamics in the boreal forest and the implications of those changes for ecosystems, many important factors are not yet adequately incorporated. Recent studies suggest that past SBB outbreaks may either amplify or attenuate wildfire characteristics, a concept known as linked disturbances. The objective of this study was to better understand interactions between SBB outbreaks and subsequent wildfire in the boreal forest. Specifically, we wanted to identify and quantify changes in wildfire ignition and size associated with occurrence and severity of SBB outbreaks on the Kenai Peninsula. Using mapped perimeters of wildfire and SBB outbreaks, we compared the number of fire ignitions and the mean fire size inside and outside SBB outbreaks, as well as between low, medium, and high severity SBB outbreaks.
Results/Conclusions
On the Kenai Peninsula between 2001-2009 there were 1.7 times as many fire ignitions per hectare in non-disturbed areas compared to past SBB outbreaks (1989-2000). However, the mean fire size was three times larger in SBB outbreaks than in non-disturbed areas. When past SBB outbreaks were partitioned by severity class (low, medium, and high severity [data available 1996-2000]), low severity SBB outbreaks were associated with the greatest number of ignitions per hectare. This was followed by high severity, non-disturbed, and medium severity sites. The mean fire size in high and low severity outbreaks was twice as large as fires in non-disturbed areas. We speculate a shift in land cover from spruce to savannah, caused naturally in high severity and facilitated in low severity outbreaks by salvage logging, may largely explain the ignition and fire size patterns. Other mechanisms may include proximity to urban centers and seasonal precipitation. Climatically driven changes in natural disturbance will likely be a critical driver of future land cover and ecosystem function. A better understanding of linked disturbances and their consequences could help to predict and manage future changes.