Drought may initiate western spruce budworm outbreaks, but multi-year periods of increased moisture availability promote widespread defoliation

The western spruce budworm (Choristoneura occidentalis) is one of the most widely distributed native defoliators of coniferous forests in North America, where it plays an important role in shaping ecosystem function. In this study, we use tree-ring and geospatial data to reconstruct periods of past WSB outbreak at 12 sites across central to northern Colorado during the period 1650–2023. We use this record to quantify: (1) temporal synchrony in outbreak history, (2) differences in the dynamics of western spruce budworm outbreaks before and after Euro-American colonization, and (3) the association between climate and outbreak initiation and cessation. We identified eight periods of synchronous outbreak between 1666 and 2023. In the first several decades following Euro-American colonization, outbreaks were shorter, more severe, and less synchronous across the study area, likely due to decreases in host availability that occurred due to widespread burning from 1850 to 1890. In addition, we found outbreaks were often initiated by drought events and sustained by periods of above average moisture availability, consistent with the pulsed stress hypothesis. Collectively, our work highlights the role of broad-scale drivers, including climate and land-use practices, in influencing stand-level disturbance processes.