Background/Question/Methods As increasing atmospheric CO₂ and warming alter plant communities, one particular concern is that they may lead to plant invasion.  Experimental CO₂ enrichment has facilitated plant invasions in a number of ecosystems.  It is unclear, however, whether this represents a general pattern, either in the responses of invasive species, or of fast-growing weedy species.  Little is known about the influence of warming on invasion.  Increases in temperature are expected to shift  invasive species distributions, increasing invasion in some areas while decreasing it in others.  Warming may also lead to consistent trends in invasion by particular species types (e.g., C₄ species), or within particular ecosystems.  Because both CO₂ and warming influence water availability, CO₂ by decreasing transpiration and warming by increasing desiccation, we hypothesize that they will have particularly strong, and potentially opposing effects on invasion in semi-arid ecosystems.  To test this possibility, we seeded four native forbs and four introduced forbs with similar life histories into the Prairie Heating and CO₂ Enrichment (PHACE) Experiment, which exposes northern mixed-grass prairie to a factorial combination of free air CO₂ enrichment (to 600 ppm) and warming with ceramic heaters (+ 1.5C in the day, + 3C at night).
Results/Conclusions Planted forbs responded strongly to both CO₂ and warming over the first two growing seasons.  The nature of those responses depended on life history, but within life history types, responses of native and introduced species were quite similar.  For native and introduced perennial species, total height (number of individuals * average height) increased with elevated CO₂, but decreased with warming, leading to little net effect of CO₂ and warming.  Biennials were similarly favored by elevated CO₂, but only responded negatively to warming in a dry year.  In contrast summer annuals, including the C4 species Salsola tragus, were favored by warming but not influenced by CO₂.  Winter annuals were inhibited by warming and not influenced by CO₂.  These results suggest that future climates may favor summer annuals, and possibly biennials in this semi-arid grassland.  Despite qualitatively similar responses between native and exotic species, future climates might also favor introduced over native species, due to strong responses among the more abundant introduced species to CO₂ (Centaurea diffusa) or warming (Salsola tragus).