Spatially explicit forecasts of tree insect and disease incidence across North American forests under future climate scenarios.

Shifts in insect and pathogen pressure with climate change present a potential threat to forest health, carbon sequestration and biodiversity. Here we evaluate climatic effects on insect and pathogen incidence across North America and forecast their future trends under climate scenarios. Using Bayesian hierarchical regression fit to records on more than 1 million trees, we show that warming and drought jointly exacerbate insect and disease incidence in historically colder and wetter regions, while reducing incidence in warmer and drier regions. Models allowing contemporary climate effects to depend on historical climate context better predict observed insect and disease incidence than models assuming quadratic functions of contemporary climate alone. Climate effects on incidence further vary among host species, mediated by tree climatic niches and functional traits, and differ across insect and pathogen taxa. Using ensemble climate projections, we forecast that under the Shared Socioeconomic Pathway 2-4.5 scenario, insect and pathogen incidence will increase across, respectively, 58% and 52% of forest land over the coming decades, with the largest impacts at high latitudes. Incorporating the variability of climate impacts across space and taxa, our work will help in predicting climate-driven insect and pathogen risk for North American forests.