Are southern pine forests becoming too warm for the southern pine beetle?

Abstract

Climate change is altering the geographic distribution of many species, including numerous forest insect pests, with the potential for severe impacts to forest ecosystems. The southern pine beetle (Dendroctonus frontalis), one of the world's most persistently destructive forest pests, has extended its distribution towards the north, while impacts within its historic range in the southeastern United States have diminished, indicating that a range shift may be occurring. This pattern could be explained by the hypothesis of climatic envelopes if summers in the southern part of the range are warming about as much as winters in the northern part of their range. Here, we tested whether the southeastern United States is becoming too hot for D. frontalis even as the northeastern United States has become climatically permissive. We measured effects of heat on insect survival, and characterized the thermal environment of D. frontalis, including estimating the frequency of potentially lethal heat waves and testing whether the intensity or duration of heat waves has increased over the past 80 years. We found that temperatures warm enough to kill D. frontalis larvae, pupae, or callow adults have been rare or absent and that there has been no change in the duration or severity of heat waves in the southern pine forest region over the last 80 years. Therefore, alternative explanations for the reduced activity of D. frontalis within its historic range must be considered. The broad conclusions are not restricted to D. frontalis because they flow from an asymmetry in climate change: at least for now in eastern North America, the coldest winter temperatures have been becoming less extreme without summer heat becoming more extreme. This creates broad potential for northward range expansions without expectation of corresponding contractions in southern distribution limits.