Geographic variation in the effect of drought on southern pine beetle outbreaks

Background/Question/Methods

Climatic variation can trigger strong nonlinear responses in species, such as population crashes or outbreaks. For example, drought conditions have been hypothesized to facilitate outbreaks of the southern pine beetle (Dendroctonus frontalis Zimm.) (SPB) in the southeastern U.S. SPB outbreaks are defined here as rapid increases in population density that result in high mortality of host trees over a contiguous area. While physiological evidence suggests that severe drought conditions lower the resistance of pine trees (Pinus spp.) to southern pine beetle attack, evidence for a drought-outbreak relationship is difficult to quantify.  This study evaluated drought effects on SPB outbreaks over 23 USDA climate divisions across five states in the southeastern U.S. from 1991-2004. Monthly drought intensity was based on Palmer’s Drought Index (PDI). The relationship between drought intensity and two measures of outbreak intensity (area of outbreak in a given year [I] & change in area of outbreak from the previous year [ΔI]) were quantified in each climate division. Discrete outbreak regions were grouped using two methods: principal components analysis and correlation analysis. Geographic differences in outbreak response to drought were tested through comparison of regression among the five outbreak regions.   

Results/Conclusions

 Drought intensity (PDI) had a greater effect on the change in outbreak intensity (ΔI) than on outbreak intensity (I) in a given year. The June PDI had the strongest effect on ΔI, so this relationship was used in all further analyses. Across all 23 climate divisions, there was a weak positive effect of drought on SPB outbreak. The PCA identified three loosely defined groups of SPB outbreak patterns, but no geographic structure was evident, suggesting that this is not a useful explanatory measure. In contrast, when climate divisions were grouped by correlation coefficients with a cutoff of ρ >0.8, five distinct geographic regions of SPB outbreak were identified :(A) Alabama, (B) Central & Southern Georgia, (C) Northern Georgia/Western South Carolina, (D) Eastern Tennessee/Western North Carolina, and (E) Central North Carolina. The effect of drought on SPB outbreak intensity was strong in region D, moderate in region A, and weak to nil in the other regions. This is the first study to demonstrate geographic variation in SPB response to drought. These results are a first step in building a predictive model of a SPB outbreak intensity that includes effects of both climate and geographic region.