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CO2 Effects on Shortgrass Steppe Grassland Productivity
Ferretti, D.F., Pendall, E., Morgan, J.A., Nelson, J.A., LeCain, D., and Mosier, A.R.  2003.  Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: Seasonal variations and ecosystem implications of elevated atmospheric CO2Plant and Soil 254: 291-303.

What was done
From May 1999 to October 2001, within open-top chambers maintained at ambient and twice-ambient atmospheric CO2 concentrations, the authors investigated "the dynamics of soil water isotopes and water cycling in a mixed C3/C4 grassland in the western Great Plains region of the USA," which region is predominantly carpeted by the C4 grass Bouteloua gracilis (H.B.K.) Lag. (blue grama), and the C3 grasses Stipa comata Trin and Rupr. (needle-and-thread grass) and Pascopyrum smithii (Rydb.) A. Love (western wheatgrass).

What was learned
In addition to documenting a mean plant biomass increase of 50% in the elevated CO2 treatment over the two years of their study, the authors observed significantly wetter soils in the elevated CO2 treatment, which they say were "most likely a result of improved soil-water conservation as a result of reduced stomatal conductance under elevated CO2."

What it means
Noting that "elevated CO2 had the effect of increasing soil-water conservation as has been previously found (e.g., Morgan et al., 2001; Volk et al., 2000)," and that "reduced evaporation was mainly responsible for greater soil water content under elevated CO2," Ferretti et al. remark that "the most significant effect of elevated CO2 on the hydrologic budget in water limited ecosystems is likely to be an increase in soil water storage (Jackson et al., 1998)," such as that which they found in their study.  It is also therefore likely that this phenomenon contributed significantly to the large growth enhancement observed in the CO2-enriched treatment, which was more than what might have been expected from the aerial fertilization effect of elevated CO2 operating alone.

Jackson, R.B., Sala, O.E., Paruelo, J.M. and Mooney, H.A.  1998.  Ecosystem water fluxes for two grasslands in elevated CO2: A modeling analysis.  Oecologia 113: 537-546.

Morgan, J., LeCain, D., Mosier, A. and Milchunas, D.  2001.  Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.  Global Change Biology 7: 451-466.

Volk, M., Niklaus, P.A. and Korner, C.  2000.  Soil moisture effects determine CO2 responses of grassland species.  Oecologia 125: 380-388.

Reviewed 31 December 2003