DISSERTATION RESEARCH: The Ontogeny of Defense in Tropical Rainforest Trees: Leaf Secondary Metabolites across Canopy-Understory Gradients of Resources and Stressors

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This goal of this study is to examine how air pollution in a tropical forest affects the way in which plants produce a special class of chemicals in their leaves. Because plants cannot run from the insects seeking to eat them ("herbivores"), they produce these chemicals as a means of defense. The spice rack at a supermarket offers but a taste of the staggering diversity of plant defense compounds; tens of thousands have been cataloged, with many more still undescribed. The variety of chemical defenses arises from the fact that herbivores and pathogens have evolved resistance to particular defenses. Thus in what has been called an evolutionary arms-race, plants are constantly under pressure to evolve new defenses, while consumers are constantly under pressure to get around them. Tropical forests contain the highest diversity of plants and herbivores on the planet, so it is no surprise that these forests also contain the highest diversity of plant chemical defenses. Not just an artifact of the number of species in tropical forests, the diversity of plant chemical defenses is thought to be a central factor in both producing and maintaining tropical forest biodiversity. Thus, any large-scale alteration of the chemical landscape of a tropical forest may have wide-ranging effects.Air pollution, particularly ozone pollution, may constitute just such a landscape alteration. The focus of this study is ozone, which is produced as a result of fossil fuel combustion or biomass burning. In areas of the tropics where these activities are concentrated, ozone levels have been recorded above thresholds for impacts on leaf defense chemistry observed in other ecosystems. In temperate ecosystems, ozone pollution has been found to affect both the concentrations of defenses within leaves and the ability of herbivorous insects to locate appropriate host plants. Ozone affects the production and stability of certain chemical compounds more than others, so ozone pollution in a chemically diverse tropical forest is likely to affect some plant and herbivore species more than others. Based in a forest adjacent to the heavy shipping traffic of the Panama Canal, this study will investigate how the defense chemistry of an array of tropical tree species is affected by ambient ozone levels. Specifically, it will test how oxidative stress, foliar secondary metabolites, and herbivory interact. It will do so by mapping ozone levels in a forest and conducting a 2x2 factorial ozone and light manipulation experiment to assess how these variables affect production of foliar secondary metabolites.