Urea additions and defoliation affect plant responses to elevated CO₂ in a C₃ 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/m². 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.