Herbivory and photosynthesis of oaks under elevated atmospheric CO2

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Trees and other plants are under constant assault by insects. It is estimated that on average 18 percent of forest productivity is lost to insect herbivores each year. The investigators hypothesize that this value underestimates the true impact of herbivores, and furthermore, that it will change substantially as the composition of the atmosphere changes. Widespread combustion of fossil fuels is causing a dramatic increase in the concentration of carbon dioxide in the atmosphere. Plants respond to this increase with higher rates of photosynthesis. However, other changes caused by rising carbon dioxide, including the formation of thicker leaves with higher concentrations of sugars and lower concentrations of nitrogen, are very likely to change the nutritional quality of leaves with respect to insect herbivores. In North Carolina the investigators are exposing plots in an intact pine forest to elevated carbon dioxide to simulate predicted atmospheric composition in the year 2050. Preliminary data show that among different kinds of oaks growing in the shade of the pines the amount of leaf tissue removed by insects was different in the high- and low-carbon dioxide plots. However, not all oak species were affected the same way. The investigators hypothesize that herbivory will be greatest for those species with greatest leaf nutritional quality under future atmospheric conditions. To test this hypothesis measures of leaf structure and chemistry for oaks in elevated and ambient carbon dioxide plots will be made. These measurements will be paired with measurements of herbivory and of the numbers and types of insects. Estimates of the amount of productivity lost to herbivores typically are based on measurements of the amount of leaf tissue removed. An assumption inherent in these estimates is that the remaining leaf tissue operates normally. When a caterpillar devours a quarter or half of a leaf, does the remaining tissue have normal rates of photosynthesis? To examine this question, the investigators have designed a unique camera system that measures light fluorescing from leaves. Fluorescence provides a direct measure of the rate of photosynthesis, but unlike other methods the measurement of fluorescence allows determination of how the rate of photosynthesis varies across the surface of the leaf. This instrument can be used to test the hypotheses that insects with different feeding behaviors will have different effects on photosynthetic rates of the remaining leaf tissue. These new measurements of the effect of herbivory on photosynthesis together with the experiments on the effects of elevated carbon dioxide on herbivory will provide new insights into how plant-herbivore interactions will alter ecosystem productivity in a future high-carbon dioxide world. In addition to training doctoral research scientists, this project also will provide the opportunity for undergraduate students, including those of underrepresented groups, to participate in scientific research through a NSF-sponsored Undergraduate Mentoring in Environmental Biology grant administered by the University of Illinois.