Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/1996
Publication Date: N/A
Citation: Interpretive Summary: Atmospheric carbon dioxide concentrations (C02) have been rising for over a hundred years, and are projected to double over present concentrations of approximately 360 parts per million by the end of the next century. As a result of this, climate is also predicted to change around the globe, with temperatures rising and precipitation patterns being altered. Because plant tgrowth is sensitive to C02, temperature and water, studies are being con- ducted in various ecosystems to better understand how elevated C02 and glo- bal climate change will impact some of the world's major ecosystems. This project investigated how native grasses in the shortgrass prairie of the Central Great Plains respond to elevated C02, variable temperature and dif- ferent water regimes. Large cylinders of native grasses and soils were removed from the prairie in eastern Colorado, transported in a refrigerated truck to Duke University, and placed in large, indoor growth chamber rooms that were controlled at either 350 or 700 ppm C02, and under either normal or elevated temperatures. We found that a doubling of present-day C02 con- centration enhanced productivity of the prairie grasses by 19% over present day conditions, but reduced their tissue N concentrations. The degree of this C02-enhanced growth varied among species depending on the growth tem- perature. This suggests that combined C02 enrichment and global warming warming will likely lead to shifts in species dominance, which could result in major species compositional changes. These results also suggest that while production will likely be enhanced in future C02-enriched atmospheres forage quality may decline due to reduced N in the plant tissues. The end result may be more forage that is more difficult for animals to metabolize.
Technical Abstract: An experiment was carried out to determine the effects of elevated C02, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C4 perennial grass, or Pascopyrum smithii, a C3 perennial grass, were placed in growth chambers with 350 or 700 uL L-1 atmospheric C02, and under either normal or relevatd temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated C02, with no significant difference between the C3 and C4 grass. The effect of elevated C02 on biomass was greatest at the intermediate water level. The positive effect of elevated C02 on shoot bio- mass was greater at normal temperatures in B. gracilis, and greater at ele- -vated temperatures in P. smithii. Neither root-to-shoot ratio nor produc- tion of seed heads was affected by elevated C02. Plant tissue N and soil inorganic N concentrations were lower under elevated C02, but no more so in the C3 than the C4 plant. Elevated C02 appeared to increase plant N limita- tion, but there was no strong evidence for an increase in N limitation or a decrease in the size of the C02 effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated C02, in spite of greater N limitations.