What was done
The authors grew the C3 perennial forb (Solanum dimidiatum), the C3 annual grass (Bromus japonicus), and the C4 perennial grass (Bothriochloa ischaemum) across an atmospheric CO2 gradient ranging from 200 to 550 ppm to study the effects of atmospheric CO2 enrichment on stomatal functioning in these grassland species.

What was learned
Stomatal conductance declined in all species with increasing atmospheric CO2 concentration.  Indeed, the 350-ppm CO2 increase utilized in this study reduced stomatal conductance by 81, 49 and 46% in Solanum dimidiatum, Bromus japonicus and Bothriochloa ischaemum, respectively.  Simultaneously, stomatal density decreased in Bothriochloa ischaemum, while it increased in Solanum dimidiatum and Bromus japonicus.

What it means
As the air's CO2 content rises, these particular grassland species will likely exhibit significant reductions in stomatal conductance and, consequently, water loss via transpiration.  Moreover, the data suggest that the historic rise in atmospheric CO2 concentration from pre-industrial to current levels may have already caused large changes in grassland evapotranspiration and water-use.  On another note, this study shows that stomatal density may not be an adequate proxy for stomatal conductance, which should be measured directly when determining leaf gas exchange fluxes.