Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Niño year in the Mojave Desert.

Annual and short-lived perennial plant performance during wet years is important for long-term persistence in the Mojave Desert. Additionally, the effects of elevated CO2 on desert plants may be relatively greater during years of high resource availability compared to dry years. Therefore, during an El Niño year at the Nevada Desert FACE Facility (a whole-ecosystem CO2 manipulation), we characterized photosynthetic investment (by assimilation rate-internal CO2 concentration relationships) and evaluated the seasonal pattern of net photosynthesis (A net) and stomatal conductance (g s) for an invasive annual grass, Bromus madritensis ssp. rubens and a native herbaceous perennial, Eriogonum inflatum. Prior to and following flowering, Bromus showed consistent increases in both the maximum rate of carboxylation by Rubisco (V Cmax) and the light-saturated rate of electron flow (J max) at elevated CO2. This resulted in greater A net at elevated CO2 throughout most of the life cycle and a decrease in the seasonal decline of maximum midday A net upon flowering as compared to ambient CO2. Eriogonum showed significant photosynthetic down-regulation to elevated CO2 late in the season, but the overall pattern of maximum midday A net was not altered with respect to phenology. For Eriogonum, this resulted in similar levels of A net on a leaf area basis as the season progressed between CO2 treatments, but greater photosynthetic activity over a typical diurnal period. While g s did not consistently vary with CO2 in Bromus, it did decrease in Eriogonum at elevated CO2 throughout much of the season. Since the biomass of both plants increased significantly at elevated CO2, these patterns of gas exchange highlight the differential mechanisms for increased plant growth. The species-specific interaction between CO2 and phenology in different growth forms suggests that important plant strategies may be altered by elevated CO2 in natural settings. These results indicate the importance of evaluating the effects of elevated CO2 at all life cycle stages to better understand the effects of elevated CO2 on whole-plant performance in natural ecosystems.