Genotype, temperature, and defoliation interact to influence tree growth and defense traits

In recent decades, tree mortality has increased at regional and global scales, due to multiple environmental stressors, including climate warming and insect pests. While the links between these stressors and tree mortality is certain, our understanding of how future climate change may interact with insect pests to affect trees and forest health is limited. In particular, little is known about the influence of temperature on plant defense, with respect to genotype by environment effects. In this study, we examined growth and phytochemical responses of four aspen (Populus tremuloides) genotypes to elevated temperature and gypsy moth (Lymantria dispar) defoliation. Trees were grown in replicate, climate-controlled environments under simulated ambient or elevated (+ 5°C) temperatures. After two months, we implemented a defoliation treatment (two weeks of gypsy moth herbivory + clipping to reduce foliage by ~75%). Then, we evaluated growth and phytochemical composition after defoliation. Results indicated tree growth was influenced by genotype and negatively affected by defoliation, but temperature had no effect. Elevated temperature, however, did alter biomass allocation, resulting in shorter trees with higher root:shoot ratios. Defoliation led to a ~20% reduction in condensed tannins. Elevated temperature had an even stronger effect, yielding a ~55-80% decrease, depending on genotype. Similarly, temperature affected phenolic glycosides more strongly than did defoliation, and tree response to warming depended on genotype and defoliation treatment. Our study indicates climate warming may interact with herbivory to differentially influence tree growth and defense, with variable responses among genotypes.