Background/Question/Methods
Outbreaks of bark beetles are key drought-sensitive disturbances affecting North American forests, where synchronous outbreaks have caused extensive tree mortality across 6.6 Mha over the 1997-2012 period. Given that future changes in climate are expected to alter patterns of drought across much of the western United States, it is critical to understand how drought affects the initiation and spread of bark beetle outbreaks. In the current study, we use Landsat data to map spruce beetle outbreak over the 1999-2013 period in the Southern Rocky Mountains and quantify the relative contributions of exogenous (e.g. drought, temperature, forest habitat) and endogenous factors (spruce beetle dispersal) in driving spatiotemporal patterns. Specifically, we seek to answer the following three questions: (1) Did the current spruce beetle outbreak originate at multiple locations or diffuse from a single location? (2) At what spatial scale is the temporal synchrony of spruce beetle infestation spatially dependent and how does it compare with the spatial dependence of regional drought and temperature conditions favorable for outbreak? (3) How is outbreak initiation and spread associated with timing and severity of drought, temperature conditions favorable for outbreak, forest habitat, and beetle dispersal?
Results/Conclusions
Cluster analysis on time series of spruce beetle activity from 1999-2013 revealed the outbreak did not simply diffuse from a single origin point, but instead from multiple disjunct areas of infestation, indicative of a regional driver. We further found that time series of spruce beetle activity were spatially dependent at spatial scales greater than annual spruce beetle dispersal distances. Multi-year drought, summer drought and peak snow water equivalent variables showed high regional correlation, suggesting that drought may play an important role in synchronizing infestation. Spatial overlay analysis and Random Forest modeling show infestation is strongly linked to summer, winter, and multi-year drought. Notably, we find that spruce beetle infestation is associated with low peak snow water equivalent, not just summer drought. Given that recent warming has already affected snowpack regimes across the West and as shown here these changes have promoted broad-scale tree mortality, future alterations to precipitation regimes are likely to drive important changes in subalpine forests.