Effects of elevated CO(2) on plant-grazer interactions

Atmospheric carbon dioxide levels have been increasing since the beginning of the industrial age, and are expected to reach 700 ppm during the mid- to late 21st century. Most research on plant response to elevated C02 has centered on plant processes in isolation from effects of higher trophic levels, and studies on the effects of elevated CO$\sb2$ on plant-herbivore interaction have concentrated on insects. The objectives of my dissertation were (1) to evaluate the possible effects of elevated CO$\sb2$ on plant-mammal interactions, and (2) to determine whether species from Yellowstone National Park, Flooding Pampa of Argentina, and Serengeti of Tanzania responded similarly to elevated CO$\sb2$ and whether responses were dependent on the mode of photosynthesis (C$\sb3$ or C$\sb4$) of dominant species. To test these objectives, plant species were grown in growth chambers at Syracuse University under ambient (350-370 ppm) and elevated (700 ppm) atmospheric CO$\sb2$, and under clipped and unclipped conditions. Species from the 3 ecosystems responded differently to elevated CO$\sb2$. Significant increases in biomass occurred only in Yellowstone species (all C$\sb3$ plants), and increases occurred mostly in crowns and roots (storage organs). Aboveground biomass, which is the portion that is consumed by grazing mammals, was not affected by elevated CO$\sb2$. There was, in general, no interaction between clipping and CO$\sb2$, and this suggests that defoliation will not affect how plants respond to elevated CO$\sb2$. Reductions in leaf N (an index of forage quality) occurred only in Yellowstone and Flooding Pampas species; no reduction in biomass and nutrient concentration was found in Serengeti species. Since ungulates can not regulate time spent feeding (as insects do), and since passage time is inversely related to food quality, grazing mammals in temperate grasslands may be negatively affected by elevated CO$\sb2$. Data on plant response to elevated CO$\sb2$ was also compiled from the literature, and responses were compared among biomes. Grassland species had smaller responses than forest species. Biome origin accounted for more of the variation in plant response than did mode of photosynthesis or growth form. Thus, biome origin was also a good predictor of plant species response to elevated CO$\sb2$