Latitudinal and Elevational Gradients in Plant Defences and Herbivory in Temperate Trees: Recent Findings, Underlying Drivers, and the Use of Genomic Tools for Uncovering Clinal Evolution

Abstract

Environmental gradients serve as powerful settings to elucidate the ecological and evolutionary processes driving changes in species diversity, trait evolution, and ecosystem function. Classic theory holds that stronger plant-herbivore interactions under more stable and warmer climates towards the equator and sea level have resulted in stronger selection on plant defences. We hereby address latitudinal and elevational gradients in plant defences and herbivory follow these predictions for a number of dominant taxa of temperate trees. Many of these taxa include species that span broad latitudinal and elevational ranges and thus represent useful models for testing clinal variation in plant defences and herbivory. First, we review recent studies testing for latitudinal and elevational gradients in temperate tree defences and herbivory. Second, we analyse these results in the light of classical theory and discuss potential deviations from expected patterns and candidate mechanisms. Third, we analysed the use of genomic tools for assessing the genetic basis of clinal evolution in plant defences, a promising alternative toward reducing inconsistencies and identifying commonalities in ecological and evolutionary processes. Our review indicates considerable variation in the strength and direction of elevational and latitudinal gradients in temperate tree defences and herbivory. Strikingly, patterns that are opposite to classic predictions are equally common and, in some cases, even more common than expected patterns. In light of these findings, we argue for a need to apply consistent methods across studies, conduct more comprehensive assessments of plant defensive phenotypes, and explicitly consider the role of abiotic factors. Furthermore, as future research closes these gaps, the adoption of genomic tools will open an unprecedented opportunity to launch a new generation of studies. To achieve this, there is a need to merge research on landscape genetics and ecological studies of plant-intraspecific clines in plant-herbivore interactions to unveil the genetic basis of clinal evolution in plant defences. Likewise, analyses of the molecular level evolution of target genes associated with plant defence also hold a large potential for assessing plant defence macro-evolutionary patterns along environmental clines. Applying these tools will help elucidate the mechanisms of adaptive evolution in plant defence along environmental clines and contribute to develop new theory by uncovering patterns not apparent previously from studies based solely on measurements of plant phenotypes and species interactions.