The ontogeny of plant defense: general patterns and genetic variation

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Plants change dramatically as they develop from young seedlings to the mature adult stage, a progression called ontogeny. For traits involved in defense against herbivores, these ontogenetic patterns are key aspects of natural community dynamics, driving interactions between plants and herbivores. Plant defense traits include toxic chemicals and physical barriers, such as thorns, spines, and hairs, that deter herbivores, as well as plant traits that reduce the negative impacts of herbivores, such as the ability to regrow following damage. Concentrations of toxins, densities of thorns and spines, and regrowth rate may all change as plants develop, and these changes have consequences for the levels of herbivore damage plants experience. For example, some species of plants produce few defenses against herbivores during the seedling stage, and consequently, damage by herbivores is the highest during this vulnerable period. In contrast, other species of plants are relatively better defended against herbivores as seedlings than they are as adult plants, leading to higher levels of damage in older plants. Ontogenetic patterns in plant defense traits are common and have been documented in over 60 plant species. Yet, a general pattern has failed to emerge. Variation among species is likely to be biologically significant, caused by differences among plant species in their life history strategies (woody vs. herbaceous plants), primary defense traits (toxins, thorns, regrowth, etc.), and plant ages studied. The proposed research addresses two primary goals: 1) To synthesize the current literature using powerful statistical techniques (meta-analysis and graphical vector analysis) in order to reveal general patterns in the ontogeny of plant defense. These statistical analyses will also reveal what factors contribute to the variation observed among species (woody vs. herbaceous, kind of defense traits, plant ages, etc.). 2) To examine genetic variation in the ontogeny of plant defense by using a large-scale field experiment with silver birch in southwestern Finland. The second goal puts this work into a broader evolutionary context by considering whether the ontogeny of defense can respond to natural selection. If ontogenetic patterns in plant defense are to evolve in response to natural selection, there must be genetic variation in the ontogenetic patterns of traits. This key assumption, however, has rarely been tested. This research will be done in collaboration with Julia Koricheva at the School of Biological Sciences, Royal Holloway University of London. Dr. Koricheva is an internationally recognized expert in the use of meta-analysis for synthesis in ecology and evolution. She has used meta-analysis repeatedly to investigate key hypotheses in her primary field of expertise, plant-herbivore interactions. Furthermore, she developed the analytical tool, graphical vector analysis, that will be used to understand how plant defense traits change during ontogeny. Finally, Dr. Koricheva designed and has led the large-scale field experiment in southwestern Finland that will be used to measure genetic variation in silver birch. This collaboration will thus capitalize on my particular expertise in the ontogeny of plant defense, developed during investigations of plant defense for my Ph.D. dissertation at the University of Colorado (USA), and Dr. Koricheva's expertise in meta-analysis and graphical vector analysis. This research will make a significant contribution to our understanding of the evolution of plant defense by elucidating the role of ontogeny in defense and herbivory.