EFFECT OF ELEVATED ATMOSPHERIC CARBON-DIOXIDE AND OPEN-TOP CHAMBERS ON TRANSPIRATION IN A TALLGRASS PRAIRIE

Increasing concentrations of atmospheric carbon dioxide (CO2) may infl uence plant-water relations in natural and agricultural ecosystems. A tallgrass prairie near Manhattan, KS, was exposed to elevated atmosphe ric CO2 using open-top chambers (OTCs). Heat balance sap Bow gauges we re used to measure transpiration in ironweed [Vernonia baldwini var. i nterior (Small) Schub.], a C-3 forb, and on individual grass culms of big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash], both C-4 grasses, in each of three treatments: (i) CE (chamber enriched, 2x ambient CO2); (ii) CA (chamber ambient, no C O2 enrichment); and (iii) NC (no chamber, no CO2 enrichment). Sap Bow data were coupled with measurements of stomatal conductance, plant/can opy resistance, and whole-chamber evapotranspiration (ET) to determine the effect of elevated CO2 on water use at different scales. Because of frequent rainfall during the study, all data were collected under w ell-watered conditions, Comparisons of CE and CA showed that sap Bow w as reduced by 33% in ironweed, 18% in big bluestem, and 22% in indiang rass under CO2 enrichment. Whole-chamber ET was reduced by 23 to 27% u nder CO2 enrichment. Comparisons of CA and NC showed that the environm ental effect of the OTCs caused a 21 to 24% reduction in transpiration , Stomatal conductance decreased from 7.9 to 3.6 mm s(-1) in big blues tem and from 5.3 to 3.2 mm s(-1) in indiangrass under CO2 enrichment. Soil water was consistently highest under elevated CO2, reflecting the large reductions in transpiration, During sap flow measurements, whol e-plant stomatal resistance to water vapor Bur in big bluestem increas ed from 103 to 194 s m(-1) under elevated CO2.