Plant responses to the projected future levels of CO₂ were first characterized in short term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO₂ were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO₂ Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO₂ on plants over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO₂ under completely open-air conditions for nearly two decades. This presentation describes some of the lessons learned from the long-term investment in these experiments.

Results/Conclusions

First, elevated CO₂ stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO₂ improves water use efficiency at both the leaf and canopy scale. Third, elevated CO₂ stimulates dark respiration via a transcriptional reprogramming of metabolism. Fourth, elevated CO₂ does not directly stimulate C₄ photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, genotypic variation in response to elevated CO₂ is the rule, not the exception. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.