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.
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.