



Atmospheric concentrations of carbon dioxide have been steadily rising, from approximately 315 ppm (parts per million) in 1959 to a current atmospheric average of approximately 385 ppm (Keeling et al.,2009). Current projections are for concentrations to continue to rise to as much as 500–1000 ppm by the year 2100 (IPCC 2007).



While a great deal of media and public attention has focused on the effects that such higher concentrations of CO 2 are likely to have on global climate, rising CO 2 concentrations are also likely to have profound direct effects on the growth, physiology, and chemistry of plants, independent of any effects on climate (Ziska 2008). These effects result from the central importance of CO 2 to plant metabolism. As photosynthetic organisms, plants take up atmospheric CO 2 , chemically reducing the carbon. This represents not only an acquisition of stored chemical energy for the plant, but also provides the carbon skeletons for the organic molecules that make up a plants’ structure. Overall, the carbon, hydrogen and oxygen assimilated into organic molecules by photosynthesis make up ~96% of the total dry mass of a typical plant (Marschner 1995). Photosynthesis is therefore at the heart of the nutritional metabolism of plants, and increasing the availability of CO 2 for photosynthesis can have profound effects on plant growth and many aspects of plant physiology.



Our knowledge of plant responses to future CO 2 concentrations rests on the results of experiments that have experimentally increased CO 2 and then compared the performance of the experimental plants with those grown under current ambient CO 2 conditions. Such experiments have been performed in a wide variety of settings, including greenhouses and chambers of a variety of sizes and designs. However plants grown in chambers may not experience the effects of increasing CO 2 the same way as plants growing in more natural settings. For this reason, techniques of Free-Air Carbon dioxide Enrichment (FACE) have been developed that allow natural or agricultural ecosystems to be fumigated with elevated concentrations of CO 2 in the field without use of chambers (Figure 1). As these experiments are the most naturalistic, they should provide the best indication of the responses of plants to increased CO 2 under the real-world conditions of the future. This article therefore focuses on data from FACE experiments wherever these are available. Whenever possible, to ensure the generality of conclusions, reference is made to analyses that have incorporated data from multiple experiments independently conducted at various research facilities.



Figure 1: Free-air carbon dioxide enrichment allows experiments with controlled atmospheric concentrations of carbon dioxide to be conducted in the field and avoids potential experimental artifacts from growing plants in enclosed chambers. Courtesy of David F. Karnosky.