A shrinking envelope? Climate warming across the Pacific coastal temperate rainforest and its projected impact on a native defoliator

Temperature regulates the location, frequency, and extent of irruptive forest insect herbivore outbreak cycles. Across the Pacific coastal temperate rainforest, recent outbreaks by a native defoliator, western blackheaded budworm, have impacted the greatest land area recorded since the advent of aerial detection programs and led to widespread losses of canopy leaf area and forest growth. Evidence suggests that the geographic distribution of budworm outbreaks has tracked a poleward shift in suitable temperature across the ecoregion. In this manuscript, we compile aerial observer estimates of insect defoliation, forest inventory data, and historical and projected climate data under three emissions scenarios to hind- and forecast the distribution of budworm outbreaks from 1901 to 2100. Climate data indicate that seasonal temperatures have warmed and are projected to warm further across the ecoregion, while seasonal precipitation has and will remain relatively constant. Models indicate that a range of spring and summer temperatures primarily constrain the biogeography of budworm outbreaks, while minimum host availability, autumn and winter temperatures, and seasonal precipitation further contribute. Projected warming will shift a substantial portion of regional forestland beyond the upper temperature threshold of historic outbreaks. Thus, our forecasts suggest that budworm outbreak distribution will narrow under all three future climatic scenarios tested. Across much of the ecoregion, the distribution of forestlands suitable for budworm outbreaks is projected to shift poleward and upslope, eventually eclipsing its host’s elevational distribution. The possible disruption of periodic defoliator outbreak disturbances in this system may have important ramifications for primary productivity, forest dynamics, and forest structure.