In response to atmospheric CO2 concentration changes over seconds to hours, plants either more widely open their stomata (increasing gs) or they reduce their stomatal apertures (lowering gs) in response to decreases or increases in the air's CO2 content, respectively (Farquhar et al., 1978; Katul et al., 2010). Over geologic time intervals on the order of millions of years, plants more slowly evolve to achieve essentially the same end result in terms of altering gs, but they do it by changing stomatal densities and dimensions (Franks and Beerling, 2009). And in between these two time scales, over intervals of decades to centuries, plants may similarly alter their stomatal characteristics via species-specific phenotypic plasticity, as described by de Boer et al. (2011).

In their intermediate time-scale study, Lammertsma et al. "present a high-resolution historical record of nine C3 species that adapted gsmax to the 100 ppm rise in CO2 since approximately AD 1880." These nine species included five woody angiosperms -- Red maple (Acer rubrum), Wax myrtle (Myrica cerifera), Dahoon holly (Ilex cassine), Diamond leaf oak (Quercus laurifolia) and Water oak (Quercus nigra) -- three conifers -- Slash pine (Pinus elliottii), Loblolly pine (Pinus taeda) and Peve Minaret (Taxodium distichum) -- and one fern -- Royal fern (Osmunda regalis). This they did based on analyses of cuticle material obtained from subfossil fragments of leaves found in well-dated young Florida (USA) peat deposits, plus herbarium and modern material collected from various sites throughout the state of Florida.

Based on assessments of stomatal density and/or maximum stomatal dimensions, the six Dutch and U.S. scientists obtained "a highly comparable overall decrease in gsmax to a rise of CO2 from preindustrial to present in angiosperms (slope, -33% per 100 ppm) and conifers (slope, -37% per 100 ppm)." And they say that this expression of their phenotypic plasticity "likely represents the plants' adaptation to increase iWUE by optimizing carbon gain to water loss," citing additionally, in this regard, the work of Katul et al. (2010) and de Boer et al. (2011), while further noting that this phenomenon has already been found to have increased continental run-off (Gedney et al., 2006), thereby making more water available for other important purposes, as had been suggested would be the case -- more than a quarter-century ago -- by Idso and Brazel (1984).

Sherwood, Keith and Craig Idso

References
de Boer, H.J., Lammertsma, E.I., Wagner-Cremerb, F., Dilcherc, D.L., Wassena, M.J. and Dekker, S.C. 2011. Climate forcing due to optimization of maximal leaf conductance in tropical vegetation under rising CO2. Proceedings of the National Academy of Sciences USA: 10.1073/pnas.1100555108.

Farquhar, G.D., Dubbe, D.R. and Raschke, K. 1978. Gain of the feedback loop involving carbon dioxide and stomata: Theory and measurement. Plant Physiology 62: 406-412.

Franks, P.J. and Beerling, D.J. 2009. Maximum leaf conductance driven by CO2 effects on stomatal size and density over geologic time. Proceedings of the National Academy of Sciences USA 106: 10,343-10,347.

Gedney, N., Cox, P.M., Betts, R.A., Boucher, O., Huntingford, C. and Stott, P.A. 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.

Idso, S.B. and Brazel, A.J. 1984. Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature 312: 51-53.

Katul, G., Manzoni, S., Palmroth, S. and Oren, R. 2010. A stomatal optimization theory to describe the effects of atmospheric CO2 on leaf photosynthesis and transpiration. Annals of Botany 105: 431-442.

Lammertsma, E.I., de Boer, H.J., Dekker, S.C., Dilcher, D.L., Lotter, A.F. and Wagner-Cremer, F. 2011. Global CO2 rise leads to reduced maximum stomatal conductance in Florida vegetation. Proceedings of the National Academy of Sciences USA 108: 4035-4040.