Summary Arid and semiarid climates comprise roughly 40% of the earth’s terrestrial surface. Deserts are predicted to be extremely responsive to global change because they are stressful environments where small absolute changes in water availability or use represent large proportional changes. Water and carbon dioxide fluxes are inherently coupled in plant growth.  No documented global change has been more substantial or more rapid than the increase in atmospheric CO2. Free Air CO2 Enrichment (FACE) technology permits manipulation of CO2 in intact communities without altering factors such as light intensity or quality, humidity or wind. The Nevada Desert FACE Facility (NDFF) consists of three 491 m2 plots in the Mojave Desert receiving 550 μL L–1 CO2, and six ambient plots to assess both CO2 and fan effects. The shrub community was characterized as a Larrea–Ambrosia–Lycium species complex. Data are reported through 12 months of operation.

Seeds were collected and compared from parent plants of Bromusrubens L. (Poaceae), an exotic Mojave Desert annual grass, grown in ambient (360 μmol mol) and elevated (700 μmol mol) CO to determine if parental CO growth conditions affected seed quality. Performance of seeds developed on the above plants was evaluated to determine the influence of parental CO growth conditions on germination, growth rate, and leaf production. Seeds of B. rubens developed on parents grown in elevated CO had a larger pericarp surface area, higher C:N ratio, and less total mass than ambient-developed seeds. Parental CO environment did not have an effect on germination percentage or mean germination time, as determined by radicle emergence. Seedlings from elevated-CO-developed seeds had a reduced relative growth rate and achieved smaller final mass over the same growth period. Elevated-CO-developed seeds had smaller seed reserves than ambient seeds, as determined by growing seedlings in sterile media and monitoring senescence. It appears that increased seed C:N ratios associated with plants grown under elevated CO may have a major effect on seed quality (morphology, nutrition) and seedling performance (e.g., growth rate and leaf production). Since the invasive success of B. rubens is primarily due to its ability to rapidly germinate, increase leaf area and maintain a relatively high growth rate compared to native annuals and perennial grasses, reductions in seed quality and seedling performance in elevated CO may have significant impacts on future community composition in the Mojave Desert.

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The photosynthetic response of Larrea tridentata Cav., an evergreen Mojave Desert shrub, to elevated atmospheric CO 2 and drought was examined to assist in the understanding of how plants from water-limited ecosystems will respond to rising CO 2 . We hypothesized that photosynthetic down-regulation would disappear during periods of water limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased water availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 µmol mol -1 CO 2 in a glasshouse; and (2) from intact adults exposed to 360 and 550 µmol mol -1 CO 2 at the Nevada Desert FACE (Free Air CO 2 Enrichment) Facility. FACE and glasshouse well-watered Larrea significantly down-regulated photosynthesis at elevated CO 2 , reducing maximum photosynthetic rate (A max ), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. A max and CE were lower in droughted Larrea compared with well-watered plants, and CO 2 had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO 2 . Operating C i increased at elevated CO 2 in droughted plants, resulting in greater photosynthetic rates at elevated CO 2 as compared with ambient CO 2 . In well-watered plants, the changes in operating C i , CE and A max resulted in similar photosynthetic rates across CO 2 treatments. Our results suggest that drought can diminish photosynthetic down-regulation to elevated CO 2 in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO 2 .