What was done
The authors studied how the annual net photosynthesis integrals, above ground productivities and water use efficiencies of three woody perennials of the Mojave Desert - creosote bush (Larrea tridentata), white burrobush (Ambrosia dumosa) and littleleaf ratany (Krameria erecta) - respond to elevated CO2 "as a function of infrequent and unpredictable precipitation inputs that characterize this ecosystem." The study was conducted at the Nevada Desert Free-Air CO2 Enrichment (FACE) facility in southern Nevada, USA, where over four growing seasons - 1998, which the researchers characterize as "an anomalously wet year," and 1999, 2000 and 2001, which they describe as "below-average rainfall years" - the CO2 concentration of the air surrounding the shrubs in their natural habitat was increased by approximately 170 ppm, or about 45% above then-current ambient concentrations.

What was learned
In the wet year (1998), annual integrated CO2 assimilation was 26, 103 and 42% higher in the CO2-enriched treatment than in the ambient-air treatment for L. tridentata, A. dumosa and K. erecta, respectively, while in the last of the three dry years (2001) it was 37, 32 and 43% greater in the CO2-enriched treatment. In the case of aboveground biomass production, the corresponding CO2-induced increases were 131, 122 and 59% (1998) and 31, 64 and 57% (2001), while in the case of the shrubs' mean instantaneous water use efficiencies the CO2-induced increases were 38, 23 and 40% (1998) and 80, 76 and 33% (2001). Adjusting these figures to what they would likely have been for a 300-ppm increase in the air's CO2 concentration, the CO2-induced increases in the important aboveground productivity parameter rise to 230, 215 and 105% in the wet year and to 55, 43 and 100% in the dry year.

What it means
Housman et al. conclude that their findings suggest that "elevated CO2 can enhance the productivity of Mojave Desert shrubs, but this effect is most pronounced during years with abundant rainfall when soil resources are most available." Interestingly, this latter observation is just the opposite of what many studies have found for the plants of more humid environments, i.e., that the percentage growth enhancement due to atmospheric CO2 enrichment is greater when water resources are less available. Nevertheless, we have no reason to doubt their findings on this important matter, which portend good things indeed for the future of earth's desert shrublands in an era of rising atmospheric CO2 concentrations.