Abstract
Insects impact local- to landscape-scale dynamics across many forests in western North America. The western spruce budworm (Choristoneura freemani; WSB) is the most damaging defoliating insect in the western United States, affecting multiple species of native conifers. Outbreaks of WSB are influenced by multiple factors such as stand composition and climate. WSB activity can be reconstructed over centuries with dendrochronological techniques. However, few studies have used long-term reconstructions to evaluate how WSB outbreak dynamics are affected by topographically created variability in local climate conditions. Here, we reconstruct WSB activity in Douglas-fir (Pseudotsuga menziesii) at six sites in the mountains of central Idaho, USA, at low and high elevations using dendrochronological techniques. We first tested the sensitivity of metrics describing WSB activity from established dendroecological methods by varying input parameters such as duration and magnitude of growth suppression of individual trees. We found that the timing tree-scale defoliation was less sensitive to parameter choice than metrics used to categorize stand-scale outbreak periods, such as number and duration of outbreaks. To evaluate the accuracy of our reconstructions, we compared our results to multiple sources of WSB activity, including historical reports of WSB activity, previously published dendroecological reconstructions, and aerial survey data. We found agreement between several of our reconstructions and aerial survey records to be between 46% and 66% depending on site. To assess topo-climatic influences on WSB activity, we compared WSB tree-scale defoliation and outbreak metrics at local low- and high-elevation edges of Douglas-fir distributions. Over three centuries, years of WSB tree defoliation were more common at high-elevation sites, which had cooler and wetter average conditions. This difference in low- and high-elevation defoliation may be due to reduced host availability and warm spring and fall temperatures at low-elevation sites. We examined the influence of interannual variability in climate on WSB outbreak initiation and found neutral climate conditions prior to outbreak initiation at low and high-elevation sites. Our results suggest reduced WSB populations at the low-elevation edge of host distribution, indicating climate change may limit WSB in warmer and drier forests.