What was done
The authors constructed model grasslands representative of the Negev of Israel and placed them in growth chambers with atmospheric CO2 concentrations of 280, 440 and 600 ppm for five months to determine the effects of elevated CO2 on these semi-arid plant communities.

What was learned
Elevated CO2 reduced rates of evapotranspiration and increased soil moisture contents in model grassland communities exposed to atmospheric CO2 concentrations of 440 and 600 ppm.  Between two periods of imposed drought, for example, soil moisture was 22 and 27% higher in communities exposed to 440 and 600 ppm CO2, respectively, than it was in control communities exposed to pre-industrial levels of atmospheric CO2.  Such increases in soil moisture content possibly contributed to peak ecosystem CO2 uptake rates that were 21 and 31% greater at 400 and 600 ppm CO2 than they were at 280 ppm CO2.  In addition, atmospheric CO2 enrichment had no effect on nighttime respiratory carbon losses from the ecosystems.  Thus, these semi-arid grasslands were acting as carbon sinks under CO2-enriched conditions.  In fact, elevated CO2 (440 and 600 ppm) increased total community biomass by 14% over that produced by communities exposed to the subambient CO2 concentration.  Also, when the total biomass produced was related to the total amount of water loss via evapotranspiration, communities grown at atmospheric CO2 concentrations of 440 and 600 ppm exhibited CO2-induced increases in water-use efficiency that were 17 and 28% higher than those displayed by control communities exposed to air of 280 ppm CO2.

What it means
As the atmospheric CO2 concentration increases, semi-arid grasslands common to the Negev of Israel will likely exhibit increases in photosynthesis and biomass production.  Moreover, such increases in biomass will likely occur while using less water.  Indeed, model ecosystems exposed to elevated atmospheric CO2 concentrations lost less water through evapotranspiration and consequently had greater soil moisture contents than ecosystems that were not CO2-enriched.  Hence, one would anticipate that the water-use efficiencies of these grasslands will likely increase in future years with further increases in the air's CO2 content.  Most importantly, the gas exchange measurements -- including the null effect of elevated CO2 on dark respiration -- and the biomass data obtained for the ecosystems demonstrate that these grasslands will likely become ever stronger carbon sinks that remove ever greater amounts of carbon from the air as time progresses.