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.     

A conceptual framework for comparing species assemblages in native and exotic habitats

Exotic (nonnative) species are known to have a wide variety of impacts on native biota. One potential set of impacts that have been poorly studied are the effects of replacing native habitat-providing species with exotic ones, e.g. when native trees that compose a woodland are replaced by an exotic tree plantation. Here we develop a graphical model that can be used to explore how multiple taxonomic components (such as birds, mammals and plants) respond to such changes. We suggest that four categorical responses are possible, with respect to changes in species richness (or other quantitative measures) of taxonomic groups within species assemblages. First, that each taxonomic group compared between habitats will be relatively unchanged, e.g. have equivalent values of species richness. Second, that a decrease (for example in species richness) of one group will be compensated for by an increase (in species richness) of another group. Third, that one or more groups will decrease without any compensated increases in other groups. Fourth, that one or more groups will increase without any compensated decreases in other groups. We provide empirical support for 3 of these 4 responses, with respect to measures of species richness, with much evidence for equivalency between habitats. These types of comparisons should provide a valuable tool for evaluating 1) the efficacy of environmental mitigation efforts that artificially create or restore habitats and 2) the types of changes that have occurred over time or across space as native habitat-producing species are replaced by exotic ones. Finally, this conceptual framework should help to broaden the range of possible changes considered by ecologists who study the impacts of exotic species.