Abstract
Summary Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO 2 . Three chambers were maintained at current CO 2 concentration (ambient treatment), three at twice ambient CO 2 , or approximately 720 μmol mol −1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C 3 and C 4 grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO 2 , with no differences in the relative responses of C 3 /C 4 grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO 2 on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C 3 ) and Bouteloua gracilis (C 4 ) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO 2 ‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO 2 , but only Stipa comata (C 3 ) plants exhibited significant reductions in N under elevated compared to ambient CO 2 chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO 2 enriched environments.