Responses of tree-killing bark beetles to a changing climate

Bark beetles cause widespread tree mortality, so understanding how climate change will influence the distribution and magnitude of outbreaks by this group of herbivores is important. We first develop a framework of outbreak dynamics that emphasizes transitions from states dominated by negative feedback to those dominated by positive, density-dependent feedback. We then consider mechanisms by which temperature and precipitation changes can allow populations to breach critical thresholds, and the empirical data relating outbreaks to weather. Finally, we consider how anticipated climatic change, and relationships with new hosts and natural enemy guilds, may influence dynamics in new habitats. There is strong evidence that elevated temperature can increase overwintering survival and decrease generation times of bark beetles, although evolved traits can also constrain response to warming in some habitats. Moreover, combinations of phenotypic plasticity, genotypic variation and physiological thresholds yield a broad range of conditions under which adults emerge synchronously, and thus can mass attack trees. There is likewise strong evidence that severe drought reduces tree defences against attack. Drought stress can occur through lower precipitation and/ or higher temperatures that reduce soil moisture and/or raise vapour pressure defi- cit. We also delineate three categories of range modifications: increased and more persistent establishment in areas where trees experienced only intermittent exposure historically; establishment in areas dominated by host species but where local populations experienced little or no pressure historically; and movement into new areas containing susceptible species that have not been exposed previously. Each of these has been documented for bark beetles. Trees in areas that experienced minor or no exposure generally had lower defences than their more historically exposed counterparts. However, there can be lags in beetle behavioural responses. In some cases, more heterogeneous forest structure, more abundant local predators and competitors and low host nutritional quality could potentially lessen risk in new habitats. Direct comparative studies are needed to evaluate outbreak potentials following range expansions driven by climate warming.