UREA ADDITIONS AND DEFOLIATION AFFECT PLANT-RESPONSES TO ELEVATED CO2IN 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 d irect effects of elevated CO2 would be affected by defoliation, and ur ea additions simulating the N in a urine hit. Plants were grown in tal l 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 r ates (untreated) or with an additional 40 g N/m(2). Growth increases i n response to elevated CO2 were largest when plants remained unclipped and received urea additions, and were found primarily in crowns and r oots (storage organs). Aboveground biomass, which is the part of the p lant consumed by grazing mammals, was not affected by elevated CO2. Th e elevated CO2 treatment caused a reduction in leaf percent N. However , there was a significant interaction between the CO2 and urea treatme nts, resulting in a larger difference in leaf percent N between urea-t reated 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 affe cted by increases in atmospheric CO2.