Knapp, A.K., Hamerlynck, E.P., Ham, J.M. et al. Vegetatio (1996) 125: 31. doi:10.1007/BF00045202
Responses in stomatal conductance (gst) and leaf xylem pressure potential (ψleaf) 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, ψleafwas 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 gstin 10 of the 12 species measured. Greatest decreases in gst(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 gstwas measured in the two C3 shrubs. During a dry period in September, reductions in gstat elevated CO2 were measured in only two species (a C3 ruderal and a C4 grass) whereas increased gstat elevated CO2 was measured in the shrubs and a C3 forb. These increases in gstwere attributed to enhanced ψleafin the elevated CO2 plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in gstwere 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 gstand was greatest (>30%) when water availability was low, but only 6–12% when ψleafwas 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 gst, 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 gstto be measured at elevated levels of CO2.