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

Abstract

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.