RESPONSES OF A C-3 AND A C-4 PERENNIAL GRASS TO ELEVATED CO2 AND TEMPERATURE UNDER DIFFERENT WATER REGIMES

An experiment was carried out to determine the effects of elevated CO2 , elevated temperatures, and altered water regimes in native shortgras s steppe. Intact soil cores dominated by Bouteloua gracilis, a C-4 per ennial grass, or Pascopyrum smithii, a C-3 perennial grass, were place d in growth chambers with 350 or 700 mu L L(-1) atmospheric CO2, and u nder either normal or elevated temperatures. The normal regime mimicke d field patterns of diurnal and seasonal temperatures, and the high-te mperature regime was 4 degrees C warmer. Water was supplied at three d ifferent levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under e levated CO2, with no significant difference between the C-3 and C-4 gr ass. The effect of elevated CO2 on biomass was greatest at the interme diate water level. The positive effect of elevated CO2 on shoot biomas s was greater at normal temperatures in B. gracilis, and greater at el evated temperatures in P. smithii. Neither root-to-shoot ratio nor pro duction of seed heads was affected by elevated CO2. Plant tissue N and soil inorganic N concentrations were lower under elevated CO2, but no more so in the C-3 than the C-4 plant. Elevated CO2 appeared to incre ase plant N limitation, but there was no strong evidence for an increa se in N limitation or a decrease in the size of the CO2 effect from th e first to the second growing season. Autumn samples of large roots pl us crowns, the perennial organs, had 11% greater total N under elevate d CO2, in spite of greater N limitation.