This doctoral dissertation research project will investigate the effects of warmer temperatures on the susceptibility of spruce trees to infestations by spruce beetles in subalpine environments of western North America. Changes in climate during the late 20th and early 21st centuries are widely viewed as the primary driver of a dramatic increase in bark beetle activity throughout western North America. Although the mountain pine beetle (MPB; Dendroctonus ponderosae) has caused most bark beetle related tree mortality across North America over the past two decades, concern has been growing that outbreaks of the spruce beetle (SB; Dendroctonus rufipennis) could be similarly extensive. The area affected by SB in Colorado and Wyoming nearly doubled from 460 km2 in 2009 to 840 km2 in 2010. The vast extent and substantial mortality of this and other bark beetle outbreaks over the last two decades are consistent with predictions drawn from studies of beetle population dynamics, which predict that a warmer climate stimulates beetle activity through decreased winter mortality and shifts from two-year to one-year life cycles. However, comparatively little is known about the potential constraints on SB activity posed by spatial variation in spruce tree susceptibility, as mediated by physical environmental factors interacting with forest dynamics in response to previous disturbances by fire, wind, and past outbreaks. Early research suggests that variation in tree susceptibility may have limited the extent and severity of previous SB outbreaks, but the feedback of tree susceptibility on outbreak dynamics appears to be greatly reduced under the warmer climate of the current outbreak (1998 - present). The central goal of this project is to determine the susceptibility of Engelmann spruce (Picea engelmannii) trees to SB outbreaks across the complex subalpine environment of northwestern Colorado in the context of climate variability. Using an integration of mapping via GIS as well as tree-ring analysis the research seeks to reconstruct historical and contemporary SB outbreaks. The research seeks answers to the following two research questions: What are the climate conditions related to historic spruce beetle outbreaks in northwestern Colorado? 2) What abiotic and biotic factors determine susceptibility to the current period of SB attack at the tree and stand-level, and how do these relationships compare with historic relationships (pre-1998) between susceptibility to SB and the same variables?
The results of this research will provide insight into how climate variability alters the susceptibility of spruce trees to SB outbreak across the subalpine environment in North America. An increased understanding of the relationships between increased temperatures, beetle infestations and other forest dynamics will help inform management decisions and evaluate hazards to nearby communities related to high tree mortality. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.
Over the past 30 years, severe and extensive bark beetle outbreaks have caused dramatic tree mortality from Alaska to the Southwestern US (Berg et al. 2006, Bentz et al. 2009). Broad-scale tree mortality has been linked to changes in regional carbon dynamics and thus may feedback into future global climate change (Kurz et al. 2008). Future predictions of outbreaks will require a better understanding of how changing climatic processes affect outbreaks through combined effects on both tree defenses and beetle populations. The research project addressed the susceptibility of Engelmann spruce to spruce beetle infestation in the context of climate variability. This research was centered on two questions: Question 1) What are the climate conditions related to historic (1700 CE – present) SB outbreaks in northwestern Colorado? Question 2) What abiotic and biotic factors determine susceptibility to the current period of SB attack at the tree and stand-level, and how do these relationships compare with historic relationships (pre-1998) between susceptibility to SB and the same variables? Preliminary results are presented below, however not all analyses are complete and results should be considered preliminary. To assess the climate conditions related to historic (1700 CE – present) spruce beetle outbreaks in northwestern Colorado we reconstructed periods of broad-scale outbreak using documentary records and tree rings. Periods of outbreak were compared with seasonal temperature, precipitation, vapor pressure deficit (VPD), the Palmer Drought Severity Index (PDSI), and indices of ocean-atmosphere oscillation that include the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). We found outbreaks co-occurred with above average annual AMO values and above average summer VPD values, indicators of drought across Colorado. Notably, we find that spruce beetle outbreaks appear to be predicted best by interannual to multidecadal variability in drought, not by temperature alone. This finding may imply that spruce beetle outbreaks are triggered by decreases in host tree defenses, which are hypothesized to occur with drought stress. To investigate susceptibility of Engelmann spruce to spruce beetle at tree and stand scales, we sampled 32 paired plots. We found spruce beetle infestation in the early 2000s was influenced by tree-level characteristics (e.g. size), but not by topographic variables or stand structure. Our results suggest that tree and stand constraints on spruce beetle outbreak are less restricting in the current period of outbreak as compared to the widespread 1940s outbreak, which was characterized by wetter and cooler conditions. Stand-level hazard ratings systems could thus be improved to consider warm and dry climate conditions, which may remove or relax stand constraints such as in the current outbreak.
