The total biomass response of Stipa occidentalis to elevated CO2 was similar (averaging 18% increase in biomass) to the response found for other grassland species. However, the increase in total biomass, in response to elevated CO2, was dependent on plants not being clipped and receiving urea treatments. Increases in growth as a response to elevated CO2 occurred primarily in crowns, which can serve a storage function in grasses. Plants in Yellowstone National Park evolved in the presence of grazing mammals, but do not exhibit a plastic response to urea treatments. S. occidentalis, which was collected from a high elevation grassland within Yellowstone, could be constrained by the evolution of a slow growth rate and, thus, lack the response to urea. Clipping resulted in increases in aboveground productivity, but also caused a decrease in crown biomass. Since there was no large increases in aboveground biomass, the quantity of forage available to grazers remains unaffected by elevated CO2 levels. Authors suggest that elevated atmospheric CO2 concentrations negatively affect grazing mammals, due to the observed decrease in forage N content, without increase to forage quantity.
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