RESPONSES IN STOMATAL CONDUCTANCE TO ELEVATED CO2 IN 12 GRASSLAND SPECIES THAT DIFFER IN GROWTH FORM

Responses in stomatal conductance (g(st)) and leaf xylem pressure pote ntial (psi(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, psi(leaf) was usually higher in all species at el evated CO2 and was lowest in adjacent field plots (without OTCs). Duri ng June, when water availability was high, elevated CO2 resulted in de creased g(st) in 10 of the 12 species measured. Greatest decreases in g(st) (ca. 50%) occurred in growth forms with the highest potential gr owth rates (C-3 and C-4 grasses, and C-3 ruderals). In contrast, no si gnificant decrease in g(st) was measured in the two C-3 shrubs. During a dry period in September, reductions in g(st) at elevated CO2 were m easured in only two species (a C-3 ruderal and a C-4 grass) whereas in creased g(st) at elevated CO2 was measured in the shrubs and a C-3 for b. These increases in g(st) were attributed to enhanced psi(leaf) in t he elevated CO2 plants resulting from increased soil water availabilit y and/or greater root biomass. During a wet period in September, only reductions in g(st) were measured in response to elevated CO2. Thus, t here was significant interspecific variability in stomatal responses t o CO2 that may be related to growth form or growth rate and plant wate r relations. The effect of growth in the OTCs, relative to field plant s, was usually positive for g(st) and was greatest (> 30%) when water availability was low, but only 6-12% when psi(leaf) was high. The resu lts of this study confirm the importance of considering interactions b etween indirect effects of high CO2 of plant water relations and direc t effects of elevated CO2 on g(st), particularly in ecosystems such as grasslands where water availability often limits productivity. A prod uct of this interaction is that the potential exists for either positi ve or negative responses in g(st) to be measured at elevated levels of CO2.