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Stomatal Density (Proxy for Past CO2 Levels) -- Summary
Stomatal density (SD; the number of stomata per unit leaf area) and stomatal index (SI; the number of stomata divided by the sum of the numbers of stomatal and epidermal cells) are often used as proxies of past atmospheric CO2 concentrations.  Some of the evidence for the validity of this technique may be found in our Subject Index under the headings Stomatal Density (Response to CO2 - Herbaceous Plants and Woody Plants).  Because the relationships between atmospheric CO2 concentration and SD and SI terminate at close to the air's current CO2 concentration and are often not evident in short-term CO2 enrichment studies (Reid et al., 2003), however, the experimental basis for the technique rests primarily upon paleontological data, most of which have been derived from research conducted on woody angiosperm taxa (Woodward, 1987; Pe˝uelas and Matamala, 1990; Paoletti and Gellini, 1993; Kurschner et al., 1996; Wagner et al., 1996; Wagner, 1998; Retallack, 2001); and these data indicate that the maximum effect of the historical increase in atmospheric CO2 concentration on SD and SI has already been reached.

In an attempt to expand the data base for this new means of reconstructing past levels of atmospheric CO2 concentration and see if other woody plants might enable the technique to be useful at higher CO2 concentrations, Kouwenberg et al. (2003) constructed plots of needle SD as a function of atmospheric CO2 concentration for four native North American conifer species - Tsuga heterophylla, Picea mariana, Picea glauca and Larix laricina - based on measurements made on needles collected from living trees, herbarium samples and well-dated peat cores that could be assigned atmospheric CO2 concentrations corresponding to the times of the needles' creation on the basis of historical CO2 measurements and CO2 measurements of air bubbles trapped in shallow Antarctic ice cores.  This work revealed that the four conifer species exhibited "a significant reduction in stomatal frequency as a response to a CO2 rise of 80 ppm over the last century."  Specifically, they found a 4.49% decrease in stomatal density per unit needle length for Tsuga heterophylla and decreases of 7.34% and 8.09% for Picea mariana/glauco and Larix laricina, respectively, which responses are of the same order as those observed for angiosperm trees.  In addition, they note that "Tsuga heterophylla and Picea glauca/mariana have not reached their response limit yet at the current CO2 level of 370 ppm," and they conclude that "because of their sensitive response to CO2, combined with a high preservation capacity, fossil needles of Tsuga heterophylla, Picea glauca, P. mariana, and Larix laricina have great potential for detecting and quantifying past atmospheric CO2 fluctuations," even those "well above present levels (Royer et al., 2001)."

In an actual application of the new technique, McElwain et al. (2002) derived high-resolution (20- to 37-year accuracy) atmospheric CO2 histories from stomatal frequency measurements of subfossil leaves of Dryas integrifolia, Picea mariana, P. glauca and Larix laricina obtained from sediment cores extracted from two different sites in New Brunswick, Canada - Pine Ridge Pond (45░34'N, 67░06'W) and Splan Pond (45░15'N, 67░20'W) - that contained material spanning the period of time from approximately 13,000 to 10,500 years ago.  The data revealed an abrupt drop in atmospheric CO2 concentration of approximately 75 ppm at the onset of the Younger Dryas cold event.  This drop in CO2 lagged the event-defining temperature drop by approximately 130 years.  Then, at the end of the Younger Dryas, there was a rapid rise in atmospheric CO2 concentration that was (within the time-resolution error bounds of the data) essentially coeval with the increase in temperature that brought an end to the Younger Dryas and initiated the Holocene.  In absolute terms, the pre-Younger Dryas CO2 concentration was something on the order of 300 to 320 ppm, the concentration during the Younger Dryas interval approximately 235 ppm, and the concentration immediately afterwards somewhere in the range of 285 to 300 ppm.

In comparing their results with atmospheric CO2 concentrations derived from polar ice core data, McElwain et al. noted that the best such data available had time resolutions on the order of 200 to 550 years.  Degrading (averaging) their data accordingly, they were able to closely match the ice core results (a steady increase in atmospheric CO2 from the beginning to the end of the Younger Dryas interval); but there was no way the ice core data could mimic the much-finer-scale story told by their data, including the dramatic decline in atmospheric CO2 concentration that commenced about 130 years after the inception of the Younger Dryas and the dramatic increase in the air's CO2 content at the conclusion of the cold event.

The importance of these observations resides in the fact that the drop in the air's CO2 content could not have caused the drop in temperature that initiated the Younger Dryas event, because the temperature decline preceded the CO2 decline.  Much more likely, the temperature drop - or whatever caused it - was responsible for the drop in the air's CO2 concentration; and there is no reason not to believe that the same sequence of events occurred at the end of the Younger Dryas, i.e., an increase in temperature followed by an increase in atmospheric CO2, although even the good time resolution of McElwain et al.'s data was not sufficient to demonstrate that fact.  Nevertheless, this order of events has been demonstrated for a number of glacial-to-interglacial transitions (see many of the Journal Reviews and Editorials filed under CO2-Temperature Correlations in our Subject Index).  Hence, it would appear that the scientific community is gingerly edging its way toward the inevitable conclusion we have long espoused on this subject, i.e., that temperature changes drive changes in the air's CO2 content and not vice versa.

An even earlier use of stomatal density data to reconstruct past changes in atmospheric CO2 concentration was conducted by Wagner et al. (1999), who derived a record of early Holocene atmospheric CO2 concentration based upon an analysis of the stomatal frequency of birch tree leaves buried in peat deposits near Denekamp, The Netherlands.  They discovered that atmospheric CO2 concentrations 10,000 years ago hovered between 260 and 265 ppm, after which they rose to a value near 330 ppm over the course of a century.  Thereafter, concentrations remained in the 330 ppm range over the next 300 years, whereupon they declined to about 300 ppm.  A second sharp increase in atmospheric CO2 concentration to a maximum value of 348 ppm then followed, with concentrations hovering between 333 and 347 ppm for the duration of the record.

The results of this study challenge the notion that atmospheric CO2 concentrations were relatively stable throughout the Holocene, as Wagner et al.'s data clearly indicate there were large shifts on a century timescale.  Their results also suggest that the atmospheric CO2 concentrations of today might not be unprecedented in the current interglacial, as some of the values they report approached 350 ppm in the mean, and possibly 365 ppm when potential errors are considered.  Thus, since atmospheric CO2 concentrations may have risen by as much as 90 ppm over the course of a few centuries 10,000 years ago in the absence of human influence, it is possible that the rise in atmospheric CO2 since 1800 may contain a significant non-anthropogenic-induced component.

In summary, there is considerable evidence to suggest that leaf stomatal density may be a reasonable proxy for past concentrations of atmospheric CO2; and certain applications of this approach produce results that suggest that anthropogenic CO2 emissions are not the strong drivers of climate change that climate alarmists make them out to be.

Kouwenberg, L.L.R., McElwain, J.C., Kurschner, W.M., Wagner, F., Beerling, D.J., Mayle, F.E. and Visscher, H.  2003.  Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO2American Journal of Botany 90: 610-619.

Kurschner, W.M., van der Burgh, J., Visscher, H. and Dilcher, D.L.  1996.  Oak leaves as biosensors of late Neogene and early Pleistocene paleoatmospheric CO2 concentrations.  Marine Micropaleontology 27: 299-312.

McElwain, J.C., Mayle, F.E. and Beerling, D.J.  2002.  Stomatal evidence for a decline in atmospheric CO2 concentration during the Younger Dryas stadial: a comparison with Antarctic ice core records.  Journal of Quaternary Science 17: 21-29.

Paoletti, E. and Gellini, R.  1993.  Stomatal density in beech and holm oak leaves collected over the last 200 years.  Acta Ecologica 14: 173-178.

Pe˝uelas, J. and Matamala, R.  1990.  Changes in N and S leaf content, stomatal density and specific leaf area of 14 plant species during the last three centuries of CO2 increase.  Journal of Experimental Botany 41: 1119-1124.

Reid, C.D., Maherali, H., Johnson, H.B., Smith, S.D., Wullschleger, S.D. and Jackson R.B.  2003.  On the relationship between stomatal characters and atmospheric CO2Geophysical Research Letters 30: 10.1029/2003GL017775.

Retallack, G.J.  2001.  A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles.  Nature 411: 287-290.

Royer, D.L.  2001.  Stomatal density and stomatal index as indicators of paleoatmospheric CO2 concentration.  Review of Palaeobotamy and Palynology 114: 1-28.

Wagner, F.  1998.  The Influence of Environment on the Stomatal Frequency in Betula.  Ph.D. Thesis.  Laboratory of Palaeobotany and Palynology, Utrecht University, Utrecht, The Netherlands.

Wagner, F., Below, R., de Klerk, P., Dilcher, D.L., Joosten, H., Kurschner, W.M. and Visscher, H.  1996.  A natural experiment on plant acclimation: lifetime stomatal frequency response of an individual tree to annual atmospheric CO2 increase.  Proceedings of the National Academy of Science USA 93: 11,705-11,708.

Wagner, F., Bohncke, S.J.P., Dilcher, D.L., Kurschner, W.M., van Geel, B. and Visscher, H.  1999.  Century-scale shifts in early Holocene atmospheric CO2 concentration.  Science 284: 1971-1973.

Woodward, F.I.  1987.  Stomatal numbers are sensitive to increases in CO2 from preindustrial levels.  Nature 327: 617-618.

Last updated 27 April 2005