Climate change alters host tree physiology and drives plant-insect interactions in forests of the southwestern United States of America

Abstract The climate of the southwestern United States of America (USA) is projected to become warmer and drier as atmospheric CO2 concentrations continue to increase, and these environmental shifts are likely to impact the ecology of plant-insect interactions in regional forest ecosystems. Increasing atmospheric CO2 concentrations can enhance tree growth rates but may also reduce nutritional quality of tree tissues and alter insect chemical signaling and tree defense processes. Elevated temperatures are likely to alter tree and insect phenology, with consequences for duration of tree exposure to herbivory and number of generations-per-year that some insect populations achieve in the southwestern USA. Increased rates of herbivory early in the growing season resulting from expedited insect emergence may be especially damaging. Conversely, some forest insect species with obligate cold diapause requirements may experience population declines as a result of warming, especially in high-elevation areas. Increasingly, xeric conditions and more frequent droughts are likely to disproportionately impact interactions, as forest trees under drought stress may lose the ability to mount appropriate induced defenses in response to herbivory. In general, cambium feeders such as bark beetles are expected to positively respond to warmer and drier conditions as water limitations mediate the defenses of their host trees. Other feeding guilds such as defoliators, phloem-feeders, and galling insects are more variable in their responses to environmentally mediated changes in host tree physiology and may either increase or decrease in population size. Collectively, very few studies have simultaneously examined the effects of multiple climate drivers on plant-insect interactions in southwestern forest tree species, and there is a need to incorporate population variation of both host trees and insect herbivores into the research framework to determine which populations may be vulnerable to rapidly changing conditions.