|Milchunas, D - COLORADO STATE UNIVERSITY|
|Parton, W - COLORADO STATE UNIVERSITY|
Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 14, 2003
Publication Date: February 1, 2004
Citation: Morgan, J.A., Mosier, A.R., Milchunas, D.G., Lecain, D.R., Nelson, J.A., Parton, W.J. 2004. CO2 enhances productivity of the shortgrass steppe, alters species composition and reduces forage digestibility. Ecological Applications. 14:208-219. Interpretive Summary: With atmospheric carbon dixoide (CO2) concentrations rising and expected to double over present-day levels by the end of the century, there is a need to understand the effect an increased amount of this atmospheric trace gas will have on the earth's ecosystems. Considerable attention has been paid in the scientific and popular press concerning the potential for such increases in CO2 to cause global warming, but this study investigates the direct effect of CO2 on plants and soils of an important grassland in the western Great Plains, the shortgrass prairie. The study was conducted for five years and utilized large field chambers to double the CO2 concentration over experimental plots on the prairie. We found that while a doubling of CO2 enhanced plant production in the shortgrass prairie when compared with plots maintained at present-day CO2 concentrations, the increase was driven by enhanced growth in just one of the several important species at the site. Further, forage quality declined under elevated CO2. Thus, rising CO2 concentrations may result in a botanically more productive, but less useful prairie.
Technical Abstract: The impact of increasing atmospheric CO2 concentrations has been studied in a number of field experiments, but little information exists on the response of semi-arid rangelands to CO2, and even less on consequences for forage quality. This study was initiated to study the CO2 response of the shortgrass steppe, an important semi-arid grassland on the western edge of the North American Great Plains used extensively for livestock grazing. The experiment was conducted for five years on native vegetation at the USDA-ARS Central Plains Experimental Range in north-eastern Colorado. Three perennial grasses dominate the study site, Bouteloua gracilis (H.B.K.) Lag., a C4 grass and two C3 grasses, Pascopyrum smithii (Rybd.) A. Love and Stipa comata Trin and Rupr. The three species comprise 88% of the aboveground phytomass. To evaluate responses to rising atmospheric CO2, we utilized six open-top chambers, three with ambient air, three with air CO2-enriched to 720 uLL-1, and three unchambered controls. We found that elevated CO2 enhanced production of the shortgrass steppe throughout the study, with 38% greater aboveground phytomass harvested annually in elevated compared to ambient plots. The CO2-induced production response was driven by a single species, S. comata, and was due in part to greater seedling recruitment. The result was species movement towards a composition more typical of the mixed-grass prairie. Growth under elevated CO2 reduced the digestibility of all three dominant grass species, and was lowest in the only species to exhibit a CO2-induced production enhancement, S. comata. The results suggest that rising atmospheric CO2 may enhance production of lower quality, more drought sensitive forage in the shortgrass steppe.