Fig. 3. Assimilation of Pascopyrum smithii and Boutleoua gracilis grown in open‐top chambers with ambient (360 µmol mol^–1) and elevated (720 µmol mol^–1) CO₂, and in plots without chambers on the Colorado shortgrass steppe during the growing seasons of 1999 and 2000. Measurements were made at the chamber CO₂ concentrations. Data are means of three replications ± s.e. An asterisk indicates an effect of CO₂ treatment significant at P < 0·05; t, P < 0·10.