Relationships between drought, coniferous tree physiology, and Ips bark beetles under climatic changes

Climate change is associated with increases in temperature and the frequency and intensity of abiotic disturbances such as drought that can affect forest resilience at multiple scales. Higher temperatures leading to extended growing seasons alter the phenology, distribution, and population dynamics of herbivorous insects. Bark beetles (Coleoptera: Curculionidae: Scolytinae) are among the most ecologically and economically important herbivorous insect taxa and can undergo population eruptions and cause widespread coniferous tree mortality. Specifically, Ips bark beetles preferentially attack conifers that are stressed, dying, or dead—such tree conditions are predicted to become more common due to climate change. Our chapter objectives were to synthesize knowledge on the effects of drought on (1) the incidence of Ips beetle outbreaks and (2) changes in conifer host physiology that increase their susceptibility to bark beetles. We documented 132 Ips beetle outbreaks globally between 1919 and 2018 that were reported in association with drought or water stress. However, very few studies quantitatively demonstrated this relationship and/or defined the mechanisms using experiments. Alterations in host physiology that lead to tree stress will increase colonization by Ips beetles, but their responses may be dependent on attributes at the tree, stand to landscape level. Given the shifts in precipitation and temperature regimes worldwide, we argue for more in-depth studies that quantify the damage and drivers of Ips beetle outbreaks, assessing simultaneous changes in host trees and Ips beetles, ways to mitigate damage for beetle outbreaks, and urgently needed proactive adaptive management strategies in drought-prone forests worldwide.