Meteorological versus spatial drivers of the spatial synchrony of forest insect pest outbreaks in North America

Spatial synchrony of population fluctuations has major consequences for the impacts of forest insect pest outbreaks at regional scales. We tested the predictions that the strength and drivers of this synchrony would differ among species according to their dispersal abilities and feeding guilds. Using a matrix regression approach, we statistically partitioned the importance of spatial and environmental drivers of synchrony in six species of phloem‐feeding bark beetles and six species of defoliating Lepidoptera in North America. Potential drivers included in the regressions were synchrony of weather conditions and spatial proximity. Overall, model selection operations on the matrix regressions indicated that synchrony in tree damage caused by forest insect pests arises from a combination of drivers including synchrony in weather and spatial drivers such as dispersal. Weather appeared to be more important in driving the synchrony of bark beetles than defoliators, possibly because weather (e.g. drought) has stronger impacts on bark beetle population dynamics. Spatial correlation functions showed that defoliators exhibited stronger synchrony over short distances than bark beetles, possibly because the cyclical nature of defoliator populations allows them to be more easily synchronized. The greater influence of weather on the synchrony of bark beetles compared to defoliators, coupled with climate‐change driven increases in synchrony of weather, may lead to more widespread events of high tree mortality due to bark beetle epidemics.