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Stomatal density (SD) has been shown to change with CO2 concentration (at the same atmospheric pressure; Lake et al., 2001), and SD of fossilized leaf material or leaf imprints can be used to indicate past CO2 concentrations (Beerling et al., 1998). However, the results of the analytical approach described by McElwain (2004) do not include an important parameter influencing the effect of altitude on CO2 diffusion and uptake by a plant. Although the partial pressure of carbon dioxide does decrease in a predictable manner with decreasing atmospheric pressure, the diffusion of all gas-phase molecules increases at lower atmospheric pressure (Fig. 1). As a result, photosynthetic CO2 uptake does not appear to become more rate-limited at greater altitudes, due primarily to the compensatory effects of increased diffusion rates (e.g., Smith and Donahue, 1991; Terashima et al., 1995). Thus, there would be no selective force for increasing SD in plant species evolving at higher altitudes, and the relationship suggested by McElwain (2004) is not expected to be generally applicable.

Analytically, diffusion is often expressed according to Fick's Law as  
where Dj is the diffusion coefficient for the molecule in question, ΔCj is the gradient in the concentration or partial pressure of molecule j, and Δx is the diffusing distance (Nobel, 1999). As seen in Figure 1, both Dj and ΔCj influence flux density proportionally. Moreover, both terms change in similar proportions with increasing altitude, but are inversely related. For example, an increase in altitude from sea level to 4 km results in a decrease in the partial pressure of CO2 of –38% (0.37–0.23 kPa), and an increase in DC of 53% (1.65–2.53 m−2 s−1 x 10−5) based on a relatively dry adiabatic lapse rate of 8 °C per 1000 m (Fig. 1).

A host of abiotic factors may be associated with changes in altitude, most of which can strongly influence stomatal density (e.g., Tichá, 1982; Lockheart et al., 1998; Qiang et al., 2003). Numerous studies have found increases in SD with elevation (e.g., Hovenden and Brodribb, 2000), decreases (e.g., Hultine and Marshall, 2000), or have been inconclusive (e.g., Qiang et al., 2003), suggesting that CO2 concentration is not a primary altitudinal factor driving SD.

Although SD has been used to estimate historical CO2 concentrations on a geological time scale, the similar proportional changes in DCO2 and CO2 partial pressure would indicate a compensating influence of the two acting in concert. As mentioned above, this compensating relationship has already been addressed in other publications dealing with the impact of altitude on the photosynthetic process. Similarly, there is no convincing evidence that greater altitude results in greater SD as a general rule. In this regard, it is actually total pore area per unit leaf area (which incorporates both stomatal size and frequency) that is the most pertinent parameter, and one which is rarely measured (e.g., Smith and Knapp, 1990). However, others have also concluded that the greatest impact of altitude is probably on plant transpiration, primarily because the increasing dryness of the air with altitude is often exacerbated by the concomitant increase in the diffusion coefficient of water vapor in air (Dwv) (Gale, 1972; Smith and Geller, 1979; Smith and Knapp, 1990; Terashima et al., 1995).