Responses in stomatal conductance (g
st
) and leaf xylem pressure potential (
leaf
) to elevated CO
2 (2x ambient) were compared among 12 tallgrass prairie species that differed in growth form and growth rate. Open-top chambers (OTCs, 4.5 m diameter, 4.0 m in height) were used to expose plants to ambient and elevated CO
2 concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August,
leaf
was usually higher in all species at elevated CO
2 and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO
2 resulted in decreased g
st
in 10 of the 12 species measured. Greatest decreases in g
st
(ca. 50%) occurred in growth forms with the highest potential growth rates (C
3 and C
4 grasses, and C
3 ruderals). In contrast, no significant decrease in g
st
was measured in the two C
3 shrubs. During a dry period in September, reductions in g
st
at elevated CO
2 were measured in only two species (a C
3 ruderal and a C
4 grass) whereas increased g
st
at elevated CO
2 was measured in the shrubs and a C
3 forb. These increases in g
st
were attributed to enhanced
leaf
in the elevated CO
2 plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in g
st
were measured in response to elevated CO
2. Thus, there was significant interspecific variability in stomatal responses to CO
2 that may be related to growth form or growth rate and plant water relations. The effect of growth in the OTCs, relative to field plants, was usually positive for g
st
and was greatest (>30%) when water availability was low, but only 6–12% when
leaf
was high.The results of this study confirm the importance of considering interactions between indirect effects of high CO
2 of plant water relations and direct effects of elevated CO
2 on g
st
, particularly in ecosystems such as grasslands where water availability often limits productivity. A product of this interaction is that the potential exists for either positive or negative responses in g
st
to be measured at elevated levels of CO
2.
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