As the air's CO2 content rises, many plants reduce their stomatal apertures, because with more CO2 in the air, they don't need to open their stomates as wide as they do at lower atmospheric CO2 concentrations to allow for sufficient inward diffusion of CO2 for use in photosynthesis. As a result, plants growing in CO2-enriched air typically exhibit reduced rates of transpirational water loss, smaller productivity losses attributable to the indiscriminate uptake of aerial pollutants, and increased water-use efficiency. For much the same reason (and producing similar effects), many plants also reduce the density of stomates on their leaf surfaces at higher atmospheric CO2 concentrations. In this brief summary, we review the findings of studies that have addressed this subject in woody plants such as trees and shrubs.
In a study that may appear to challenge the norm, Apple et al. (2000) grew two-year-old Douglas fir seedlings for three years in controlled-environment chambers maintained at atmospheric CO2 concentrations of either 350 or 550 ppm and ambient or elevated (ambient plus 4°C) air temperatures, finding that neither elevated CO2 nor elevated air temperature, acting alone or together, significantly affected needle stomatal density. On the other hand, in a study of the long-term effects of elevated CO2 on various leaf properties of mature white oak trees growing at different distances from CO2-emitting springs in central Italy, Paoletti et al. (1998) found that elevated CO2 significantly decreased leaf stomatal density by a factor of nearly 1.5 as the air's CO2 concentration rose from 350 to 750 ppm. From that point on, however, there were no further reductions in stomatal density, even for CO2 concentrations as great as 2600 ppm.
In another study that evaluated the impact of an increase in the air's CO2 concentration from 350 to 750 ppm, Lin et al. (2001) grew seven-year-old Scots pine seedlings in the field in open-top chambers for a period of four years, applying no additional nutrients or irrigation waters to the soils in which the young trees were rooted. After the fourth year of the experiment, a detailed analysis revealed that the extra CO2 had reduced needle stomatal density by an average of 7.4%, which finding indicates that Scots pines may be better able to conserve water and cope with periods of drought and water stress in a high-CO2 world of the future.
Finally, in a study related to both the distant past and the years and decades just ahead, Beerling et al. (1998) grew one-year old Ginkgo biloba saplings in greenhouses maintained at atmospheric CO2 concentrations of 350 and 560 ppm for a period of three years, finding that the leaves of plants grown at 560 ppm CO2 exhibited significant reductions in both stomatal density (number of stomates per leaf area) and stomatal index (ratio of stomata to epidermal cells). Interestingly, the stomatal density of the CO2-enriched leaves was similar to that measured on fossilized Ginkgo leaves dating back to the Triassic and Jurassic time periods; and because the CO2-induced reductions in stomatal density and index did not impact rates of photosynthesis, it can be inferred that the water-use efficiencies of ancient Ginkgo species were much higher than those of their modern counterparts. Consequently, as the CO2 content of the air continues to rise, it may, as Beerling et al. remark, "contribute to restoring the function of this 'living fossil' species back to that more representative of its long geological history."
In summary, much like what has been learned about herbaceous plants, studies of the effects of atmospheric CO2 enrichment on the leaf stomatal density of woody species reveal a wide range of responses - from no change at all to a single-digit percentage decrease to a double-digit percentage decrease that ceases after a few hundred ppm increase in CO2. All of these changes are beneficial to the species that exhibit them, including the cessation of the response at a species-specific critical atmospheric CO2 concentration, which we described in our editorial of 27 Dec 2000.
Apple, M.E., Olszyk, D.M., Ormrod, D.P., Lewis, J., Southworth, D. and Tingey, D.T. 2000. Morphology and stomatal function of Douglas fir needles exposed to climate change: elevated CO2 and temperature. International Journal of Plant Science 161: 127-132.
Beerling, D.J., McElwain, J.C. and Osborne, C.P. 1998. Stomatal responses of the 'living fossil' Ginkgo biloba L. to changes in atmospheric CO2 concentrations. Journal of Experimental Botany 49: 1603-1607.
Lin, J., Jach, M.E. and Ceulemans, R. 2001. Stomatal density and needle anatomy of Scots pine (Pinus sylvestris) are affected by elevated CO2. New Phytologist 150: 665-674.
Paoletti, E., Nourrisson, G., Garrec, J.P. and Raschi, A. 1998. Modifications of the leaf surface structures of Quercus ilex L. in open, naturally CO2-enriched environments. Plant, Cell and Environment 21: 1071-1075.Last updated 20 April 2005