Tree mortality from drought, insects, and their interactions in a changing climate

William R L Anderegg, Jeffrey A Hicke, Rosie A Fisher, Craig D Allen, Juliann Aukema, Barbara Bentz, Sharon Hood, Jeremy W Lichstein, Alison K Macalady, Nate McDowell, Yude Pan, Kenneth Raffa, Anna Sala, John D Shaw, Nathan L Stephenson, Christina Tague, Melanie Zeppel

Climate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate-induced tree mortality and biotic disturbance agents are largely absent from process-based ecosystem models. Using data sets from the western USA and associated studies, we present a framework for determining the relative contribution of drought stress, insect attack, and their interactions, which is critical for modeling mortality in future climates. We outline a simple approach that identifies the mechanisms associated with two guilds of insects - bark beetles and defoliators - which are responsible for substantial tree mortality. We then discuss cross-biome patterns of insect-driven tree mortality and draw upon available evidence contrasting the prevalence of insect outbreaks in temperate and tropical regions. We conclude with an overview of tools and promising avenues to address major challenges. Ultimately, a multitrophic approach that captures tree physiology, insect populations, and tree-insect interactions will better inform projections of forest ecosystem responses to climate change.