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Effects of Elevated CO2 on Photosynthesis in a C4 Tallgrass Prairie Plant
Adams, N.R., Owensby, C.E. and Ham, J.M.  2000.  The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie.  Photosynthesis Research 65: 121-129.

What was done
Open-top chambers were established on a tallgrass prairie located in Kansas, USA.  Chambers containing mixtures of C3 and C4 grasses were fumigated with ambient air and air enriched to twice the ambient concentration of atmospheric CO2.  During the relatively wet and dry years of 1993 and 1994, respectively, gas exchange measurements were made on the dominant C4 grass, Andropogon gerardii Vitman, to determine the effects of elevated CO2 and soil moisture on this perennial prairie species.

What was learned
In the wet year, there were minimal differences in the photosynthetic rates of plants grown at the two atmospheric CO2 concentrations.  However, in the dry year, plants grown at elevated CO2 exhibited significantly greater rates of photosynthesis than plants grown at ambient CO2.  Regardless of soil moisture conditions, plants grown at elevated CO2 consistently displayed reduced rates of stomatal conductance and transpirational water loss, which consequently enhanced their daily average
water-use efficiency throughout the entire two-year study.

What it means
As the CO2 content of the air increases, it is likely that tallgrass prairie ecosystems - especially those dominated by the C4 grass Andropogon gerardii - will experience increases in both photosynthetic productivity and overall water-use efficiency.  These phenomena will likely lead to increased availability of soil moisture, which can positively influence soil microbial activity and eventually improve soil fertility to further increase plant productivity.  The essentially inescapable consequence is that tallgrass prairies will become increasingly more productive as the air's CO2 content continues to rise.