What was done
Large 23-m diameter circular plots were established in an intact Mojave Desert ecosystem located in Nevada, USA, as part of a FACE study.  Plots were fumigated with air containing atmospheric CO2 concentrations of 350 or 550 ppm to study the effects of elevated CO2 on plant productivity and growth in this native desert community.  In this particular paper, the authors report the effects of elevated CO2 on the functional ecology of the drought deciduous shrub Lycium andersonii.

What was learned
Plants grown in air of elevated CO2 exhibited stomatal conductances that were consistently about 27% lower than those observed in ambiently-grown plants.  However, elevated CO2 had little impact on midday leaf water potential values until the last month of the spring growing season (March-June), during which time plants in elevated CO2 plots displayed values that were more positive than those exhibited by control plants.  In contrast, elevated CO2 did not significantly impact rates of photosynthesis, regardless of time within the growing season.  Nonetheless, plants grown in elevated CO2 displayed photosynthetic acclimation, as indicated by maximum rates of rubisco activity that were 19% lower than those exhibited by plants growing in ambient CO2 concentrations.

What it means
As the CO2 content of the air increases, the desert shrub Lycium andersonii will likely respond by increasing its ability to more effectively deal with the highly variable precipitation and temperature regimes of the Mojave Desert.  Indeed, the present data indicate that these shrubs will likely enhance their water-use efficiencies as atmospheric CO2 concentrations rise, due to persistent CO2-induced reductions in stomatal conductance.  Moreover, acclimation within the shrubs' photosynthetic apparatus may allow them to reallocate more resources to producing and sustaining greater amounts of biomass, even under conditions of limited soil moisture availability.  Thus, it is highly likely that future increases in the air's CO2 content will favor a greening of the American Mojave Desert.