Spatial and temporal patterns of bark beetle and defoliator outbreaks, and their interactions, in the Pacific Northwest.

The dynamics of many forest insects are changing in response to climate warming; however, patterns are not always consistent among or within taxa. Changes in outbreak dynamics for individual species have received much recent attention, but the potential for interactions among species has received less. With climate-mediated shifts in the outbreaks of individual species, it is probable that changes in interactions between and among species will follow. We used historical aerial survey data (1960-2019) from conifer-dominated forests of Oregon, Washington, and British Columbia to quantify spatial dynamics of bark beetles, defoliators, and their interactions across local and regional scales and to measure how these dynamics have changed through time. We identified common patterns of increasing spatial autocorrelation through time for insect species that inhabit higher latitudes and elevations (mountain pine beetle, spruce beetle, western balsam bark beetle, and 2-year cycle budworm), suggesting that regional warming is likely a main driver of these landscape scale changes. We also observed greater spatial overlap of bark beetle and defoliator outbreaks in more recent decades, particularly in historically colder ecosystems at higher elevations and latitudes. However, these patterns were also observed in more temperate forests, most notably in spatial overlap among Douglas-fir beetle, western spruce budworm, and Douglas-fir tussock moth. Increases in overlapping disturbances can compromise forest resilience in vulnerable or sensitive ecosystems with important implications for forest management. Understanding how the spatial and temporal patterns of individual and interacting forest insect outbreaks have changed is vital to identifying threatened ecosystems, developing effective management plans for these areas, and understanding how patterns may continue to change under a changing climate.