Background/Question/Methods We tested the hypothesis that native plants acclimate to elevated CO₂ resulting in no additional growth and no additional carbon assimilation by the ecosystem. Open top chambers (OTC, 0.5m by 1.0m) were established in stands dominated by C3, C4  and mixed stands of these in a tidal marsh at the Smithsonian Environmental Research Center, Edgewater, MD  in 1987. Large OTCs (3.0m by 2.8m) were established in scrub oak forest on the Merritt Island Wildlife Refuge, Kennedy Space Center, FL(1996-2007). There were 5 plots of each CO₂ treatment (ambient , ambient + 340ppm, and unchambered) in Maryland and 8 plots for each treatment (ambient and ambient + 350 ppm) in Florida. Non-destructive estimates of standing shoot biomass were made in the marsh at peak growing season (August) and in January of in the forest using allometric relationships. Gas exchange studies were made periodically in each study. Results/Conclusions In the tidal wetland, shoot number, above and belowground biomass increased at elevated compared with plants at ambient CO₂ each year throughout the study. However, large inter-annual variation was observed and was mostly attributable to variation in precipitation. Nitrogen concentration of shoots and roots declined throughout the study with a large inverse relationship to inter-annual variation in precipitation. In the scrub oak ecosystem, we saw a 70% stimulation of shoot growth and a 60% stimulation of coarse root biomass in 10 years. Overall, scrub oak biomass increased (18%) throughout the study.  In both studies,Ecosystem gas exchange showed a consistent increase in both studies (ca 35% increase in the tidal marsh and more than 50% increase in the scrub oak forest), with a strong interaction between carbon assimilation and precipitation. Evapotranspiration was reduced by the CO₂ treatment in the Florida study resulting in a near doubling of ecosystem water use efficiency (carbon gain/water loss). We will review evidence for a decline in measured plant (growth and  photosynthesis) or  ecosystem responses (NEE, NEP, and ET) to elevated CO₂. We noted a strong inter-annual variation in all responses to elevated CO₂, mainly attributable to variation in available precipitation but there was also some evidence for N limitation.