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Elevated CO2 Doubly Stimulates Growth in Grasslands at Low Soil Moisture
Volk, M., Niklaus, P.A. and Korner, C.  2000.  Soil moisture effects determine CO2 responses of grassland species.  Oecologia 125: 380-388.

What was done
The authors grew plant assemblages similar to those of calcareous grasslands of northwest Switzerland in controlled environment chambers for three months at atmospheric CO2 concentrations of 360 and 600 ppm.  In addition, plant assemblages were subjected to four different irrigation regimes in order to study the interactive effects of elevated CO2 and soil moisture on plant growth.  By these means the authors hoped to separate direct plant responses to elevated CO2 from indirect responses resulting from CO2-enhanced soil moisture contents.

What was learned
Elevated CO2 consistently reduced stomatal conductance, regardless of irrigation regime.  Consequently, plant assemblages grown at 600 ppm CO2 always exhibited higher amounts of soil moisture than control assemblages grown at ambient CO2 concentrations.  In fact, this effect was so pronounced that the authors inadvertently generated eight, instead of four, irrigation regimes.

Atmospheric CO2 enrichment increased whole-community biomass by 18 and 40% at the wettest and driest irrigation regimes, respectively, while the maximum CO2-induced biomass enhancement of 51% occurred in the second driest irrigation regime.  Furthermore, when the biomass data were analyzed with respect to actual soil moisture content, it was found that the direct CO2-induced increases in biomass production were augmented by the extra growth that resulted from the CO2-induced enhancements of soil moisture.

What it means
As the CO2 content of the air rises, it is likely that calcareous grasslands will exhibit increasing rates of photosynthesis and biomass production, even under conditions of low soil moisture.  In fact, this paper demonstrates that elevated CO2 often has its greatest effect on biomass production when soil moisture levels are less than optimal, as its transpiration-reducing properties tend to conserve soil moisture and make it last longer, thereby enabling plants to grow productively for longer periods of time.