In recent decades, tree mortality has increased at regional and global scales in response to multiple environmental stressors, including climate warming and insect pests. Yet, little is known about how climate warming will affect tree resistance and tolerance to insect herbivores, in particular, with respect to genotype by environment effects. For this study, we conducted a greenhouse experiment to examine the independent and interactive effects of elevated temperature and defoliation on physiological, growth, and phytochemical responses of aspen (Populus tremuloides) and birch (Betula papyrifera). For aspen, we evaluated intraspecific variation in responses among three genotypes. Two-year old aspen (N=192) and birch (N=64) trees were grown under one of four temperature (ambient, elevated) by defoliation (no defoliation, 75% defoliation) treatment combinations. Treatments were arranged in a split-plot design with four replicate ambient and four replicate elevated temperature greenhouse rooms. Defoliation treatments were randomly assigned to trees within greenhouse rooms. Thus, greenhouse rooms comprised the whole-plot replicates and individual trees were considered subplots. Our primary research goals were to assess how future climate warming might influence tree tolerance and resistance to herbivory and to determine if tree species and genotypes demonstrate similar responses to warming and defoliation.
Prior to defoliation treatments, warming enhanced photosynthesis and decreased stomatal conductance for aspen and birch. Several weeks after defoliation, the effect of warming on photosynthesis and conductance was not statistically significant. Defoliation, however, significantly enhanced photosynthesis for both tree species. The influence of warming and defoliation on new leaf growth after defoliation depended on species and genotype. Overall, elevated temperatures had a positive effect on tree growth rates and final biomass. Warming also altered biomass allocation, resulting in shorter trees with higher root:shoot ratios. While trees grown under elevated temperatures were generally less tolerant of defoliation, we did not detect a significant effect of temperature on tolerance metrics. Defoliation led to a ~20% reduction in condensed tannins and warming had an even stronger effect, yielding a ~55-80% decrease, depending on tree species or genotype. Similarly, elevated temperature reduced phenolic glycosides in aspen more strongly than did defoliation, and tree response to warming depended on genotype and defoliation treatment. Our study indicates that climate warming may interact with herbivory to differentially influence tree growth and defense, with variable responses among tree species and genotypes. Further, results demonstrate that warming may reduce tree chemical defenses with little to no added tolerance trade-off.