Climate variability and historical spruce beetle outbreaks across northwestern Colorado

Background/Question/Methods

Large infrequent disturbances are essential in dictating ecosystem structure and composition, which in turn may affect a forest’s susceptibility to subsequent disturbances. The major coarse-scale disturbances affecting spruce-fir forests in Colorado and Wyoming are wildfire and spruce beetle (SB; Dendroctonus rufipennis) outbreak, both of which are conditioned by weather and climate. Here we make use of existing and new site-specific tree-ring reconstructions of historical (1700 CE – present) SB outbreak to better describe the synchrony of outbreak across NW Colorado and examine the influence of climate teleconnections on SB outbreak. We examined the synchrony between two dendroecological sources of SB outbreak, death dates and periods of increased radial growth, using a modified Ripley’s K function for a single dimension, time. Records of SB outbreak derived from growth releases were then used to examine the synchrony of SB outbreak across NW Colorado. We tested relationships between large-scale climate drivers and historical SB outbreak using both bivariate event analysis and superposed epoch analysis (SEA). 

Results/Conclusions

We found that growth releases lagged death dates associated with SB outbreaks by 7 years (95% significance). Since the frequency of death dates is heavily skewed towards more recent time periods, the confirmation that SB mortality and release periods occur synchronously statistically confirms that release periods can be used reconstruct SB outbreak in NW Colorado. We found that SB outbreaks were temporally aggregated within a 5-21 year period. This suggests that SB outbreaks in NW Colorado are related to large-scale driver, such as climate. Indeed, results from bivariate event analysis and SEA showed that SB events were related to both large-scale climate drivers and phase combinations of large-scale climate drivers.