02902nas a2200217   4500008004100000245010500041210006900146300001100215490000800226520215900234653001702393653001602410653002502426653002202451653002002473100002002493700002102513700001402534700001802548856011802566       1996                        eng d00aResponses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form0 aResponses in stomatal conductance to elevated CO2 in 12 grasslan  a31 -410 v1253 aResponses in stomatal conductance (g st ) and leaf xylem pressure potential (ψ leaf ) to elevated CO2 (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 CO2 concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August, ψ leaf was usually higher in all species at elevated CO2 and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO2 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 (C3 and C4 grasses, and C3 ruderals). In contrast, no significant decrease in g st was measured in the two C3 shrubs. During a dry period in September, reductions in g st at elevated CO2 were measured in only two species (a C3 ruderal and a C4 grass) whereas increased g st at elevated CO2 was measured in the shrubs and a C3 forb. These increases in g st were attributed to enhanced ψ leaf in the elevated CO2 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 CO2. Thus, there was significant interspecific variability in stomatal responses to CO2 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 ψ leaf was high.
The results of this study confirm the importance of considering interactions between indirect effects of high CO2 of plant water relations and direct effects of elevated CO2 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 CO2.10aelevated CO210aGrowth form10aStomatal Conductance10atallgrass prairie10aWater relations1 aKnapp, Alan, K.1 aHamerlynck, E.P.1 aHam, J.M.1 aOwensby, C.E.  uhttp://129.130.186.12/content/responses-stomatal-conductance-elevated-co2-12-grassland-species-differ-growth-form