Changes in prairie vegetation under elevated carbon dioxide levels and two soil moisture regimes

Abstract

Abstract. It is important to know how increasing levels of atmospheric CO2 will affect native vegetation. The objective of this study was to determine the effect of elevated CO2 concentrations on species composition in a tallgrass prairie kept at a high water level (730 mm of water in a 2000 mm soil profile) and a low water level (660 mm of water in 2000 mm). 16 cylindrical plastic chambers were placed on the prairie to maintain two levels of CO2 (ambient or twice ambient) during two growing seasons in 1989 and 1990. Frequency of species was determined on 25 July 1989 and on 5 and 10 October 1990.At the beginning of the study, Poa pratensis (Kentucky bluegrass), the dominant C3 species, had the highest frequency of 43.3%, but decreased with time. However, at the end of the experiment and under the high soil‐water level, there were more P. pratensis plants in the elevated CO2 treatment (frequency: 13.5%) than in the ambient CO2 treatment (1.0%). Under the low soil water regime, the reverse occurred (frequencies: 3.6% and 11.0% for high and low CO2, respectively).The frequency of major C4 plants, Andropogon gerardii (big bluestem), A. scoparius (little bluestem) and Sorghastrum nutans (Indian grass) was not affected by CO2. However, water did affect their frequency. Under low water, the frequency of A. gerardii decreased between 1989 and 1990. Under both soil moisture levels, the frequencies of S. nutans and A. scoparius increased. At the end of the study, Indian grass grown with high water had the highest frequency of all species on the prairie (frequency at the end of the study in October, 1990, of 44.4% and 47.4% for the high and low CO2 levels, respectively). Unlike Indian grass, little bluestem grew better under low water conditions than under high water conditions.These results suggest that, if the climate becomes drier, A. scoparius will flourish more than S. nutans or A. gerardii, and P. pratensis may die out. Elevated CO2 might not increase survival of C3 plants under dry conditions, if temperatures are too high for them.