EFFECTS OF ELEVATED CO2 AND DEFOLIATION ON GRASSES - A COMPARATIVE ECOSYSTEM APPROACH

Three plant species from each of three grassland ecosystems were grown under elevated (700 mL/m(3)) and ambient (350 mL/m(3)) CO2 and were d efoliated or left undefoliated to test whether species response to ele vated CO2 and grazing is related to evolutionary grazing history or to mode of photosynthesis. The three ecosystems represented a tropical g rassland dominated by C-4 species (the Serengeti of Africa), a tempera te grassland dominated by a mixture of C-3 and C-4 species (Flooding P ampa of South America), and a northern temperate grassland dominated b y C-3 species (Yellowstone National Park of North America). Plants wer e grown in growth chambers under common conditions to compare relative responses to grazing and elevated CO2. Elevated CO2 caused an increas e in total biomass and total productivity (biomass + clippings) only i n Yellowstone species, and increases in growth occurred primarily in c rowns and roots (storage organs). There were no significant CO2 effect s on biomass or productivity in Serengeti or Flooding Pampa species, a nd no CO2 effects on aboveground biomass or productivity (aboveground biomass + clippings) in species from any of the three ecosystems. Sinc e aboveground plant parts are the portions that are available to grazi ng mammals, this suggests that increased atmospheric CO2 may not affec t food quantity in these three grasslands. There was no interaction be tween CO2 and defoliation for any species; thus, it appears that herbi vores will not affect how grasses respond to elevated CO2 (at least un der average nutrient conditions). Elevated CO2 caused a reduction in l eaf percentage of N in species from Yellowstone and Flooding Pampa (es pecially the C-3 species, Briza subaristata), but not in Serengeti spe cies. Because the quantity of food was unaffected by the CO2 treatment s and forage N was reduced, grazing mammals in Yellowstone (elk, Cervu s elaphus, and bison, Bison bison) and the Flooding Pampa (cattle) may be negatively affected. Responses to defoliation were fairly consiste nt among ecosystems in aboveground productivity, which did not differ between defoliated and undefoliated plants, and in leaf water potentia ls and percentage of N, both of which increased in response to defolia tion. However, differences among ecosystems were found for crown and r oot biomass in response to defoliation: Serengeti species, on average, had higher crown and similar root biomasses after defoliation, wherea s defoliated species from the other two ecosystems had reduced crown a nd root biomass. We suggest that the lower intensity and increased tem poral variance in grazing pressure in Yellowstone vs. the Serengeti, s elected for plants that shift allocation away from roots and crowns in order to compensate for aboveground herbivory.