Urea additions and defoliation affect plant responses to elevated CO2 in a C-3 grass from Yellowstone National Park

A common grass from Yellowstone National Park, Stipa occidentalis, was grown in a factorial experiment to determine if its response to the direct effects of elevated CO2 would be affected by defoliation, and urea additions simulating the N in a urine hit. Plants were grown in tall pots (to mimic rooting depth in the field) in growth chambers under elevated (700 ppm) and ambient (370 ppm) CO2, were defoliated or left undefoliated, and given N-supply rates based on field mineralization rates (untreated) or with an additional 40 g N/m(2). Growth increases in response to elevated CO2 were largest when plants remained unclipped and received urea additions, and were found primarily in crowns and roots (storage organs). Aboveground biomass, which is the part of the plant consumed by grazing mammals, was not affected by elevated CO2. The elevated CO2 treatment caused a reduction in leaf percent N. However, there was a significant interaction between the CO2 and urea treatments, resulting in a larger difference in leaf percent N between urea-treated and control plants under elevated than under ambient CO2. Hence, elevations in atmospheric CO2 may cause an increase in the amount of urine-hit-induced spatial variability in temperate grasslands. Since food quantity remained largely unchanged in response to elevated CO2, and forage N content went down, grazing mammals may be negatively affected by increases in atmospheric CO2.