What was done
The authors measured seasonal gas exchange in an annual grass (Bromus madritensis ssp. rubens) and a perennial forb (Eriogonum inflatum) growing naturally within FACE plots established in the Mojave Desert of Nevada, USA, which were maintained at atmospheric CO2 concentrations of 350 and 550 ppm.  Gas exchange measurements were made during an unusually wet year characterized by abundant moisture delivered via rain showers during an El Niņo regime.

What was learned
Atmospheric CO2 enrichment consistently increased net photosynthetic rates in the annual grass without inducing any signs of photosynthetic acclimation.  Indeed, even as seasonal photosynthetic rates declined post-flowering, the decline was much less in CO2-enriched plants than in control plants.  However, elevated CO2 had no consistent effect on stomatal conductance in this species.

In contrast, Eriogonum plants growing at 550 ppm CO2 displayed significant signs of photosynthetic acclimation, especially late in the season, which lead to similar rates of net photosynthesis in these plants in both CO2 treatments.  Also, atmospheric CO2 enrichment did reduce stomatal conductance in this perennial species throughout most of the growing season.

Even though both of these desert plants exhibited quite different photosynthetic and stomatal conductance responses to elevated CO2, they both experienced significant CO2-induced increases in water use efficiency and biomass production, thus highlighting the existence of different species-specific mechanisms for responding positively to atmospheric CO2 enrichment.

What it means
As the air's CO2 content rises, it is likely that most of earth's plants will utilize combinations of various physiological strategies to take advantage of the additional CO2 and increase their biomass.  The current paper highlights two such different sets of responses to atmospheric CO2 by plants utilizing different life-cycle strategies, which both achieve the same final outcome -- enhanced biomass production.  Moreover, it also documents the positive growth responses of two additional desert plants to elevated concentrations of atmospheric CO2.