Stomatal Index

The density of stomata (gas exchange openings) on leaves can vary with the CO2 level in the atmosphere (Woodward, 1987). The density also varies with changes in water availability, called "water stress." A simple way to eliminate the water stress effect is to normalize the density of stomata to the number of epidermal cells, which corrects for changes in cell size (Salisbury, 1927). The result is called stomatal index. Besides water stress, stomatal index is insensitive, also to illumination and temperature (Beerling, 1999). The selective sensitivity to CO2 makes stomatal index a potentially powerful tool for deducing ancient CO2

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Figure 5.17. Plots of RCO2 versus time for the paleosol carbon isotope method (vertical bars) and the stomatal index method (square boxes) compared to the standard GEOCARB III curve. Data for the paleosol carbon isotope method from Yapp and Poths (1992, 1996), Mora et al. (1996), Mora and Driese (1999), and Cox et al. (2001). Stomatal index data from McElwain and Chaloner (1995, 1996). All methods indicate that a large drop in CO2 occurred at the same time as the rise of large vascular land plants.

Stomatal Index Gingko

Figure 5.17. Plots of RCO2 versus time for the paleosol carbon isotope method (vertical bars) and the stomatal index method (square boxes) compared to the standard GEOCARB III curve. Data for the paleosol carbon isotope method from Yapp and Poths (1992, 1996), Mora et al. (1996), Mora and Driese (1999), and Cox et al. (2001). Stomatal index data from McElwain and Chaloner (1995, 1996). All methods indicate that a large drop in CO2 occurred at the same time as the rise of large vascular land plants.

levels from the study of fossil leaves, but it is necessary to choose leaves for study that are either conspecific with modern representatives or that have nearest living ecological and morphological equivalents. In this way the CO2 response of the stomata can be calibrated. Also, fossil leaves need to be collected that were derived from trees that were tall or in sparse stands so that they did not sample excess CO2 that builds up at shallow levels in forests, and the trees must not have been at high elevations because stomata respond to the partial pressure of CO2 and not to concentration (Royer et al., 2001). At sea level concentration and partial pressure are identical.

McElwain and Chaloner (1995, 1996) and McElwain (1998) have used the nearest-living-equivalent approach for the study of the stomatal index of fossil leaves ranging from the Devonian period to the present. They calculate paleo-CO2 levels by the "stomatal ratio" method, which assumes that the ratio of stomatal index for two different times is directly proportional to the inverse of their CO2 levels. They calibrate their method by choosing a paleo-CO2 level that shows rough agreement between different methods (RCO2 = 1 at 300 Ma; see figure 5.17). This method is useful in showing semiquantitative effects, but it is not calibrated against a known level of CO2. A plot of results for RCO2 derived via the stomatal ratio method is compared to the standard curve for GEOCARB III in figure 5.18, and the overall trend with time is similar to that based on modeling. Also, the large drop in CO2 based on modeling and paleosol results (figure 5.17) is corroborated by the stomatal ratio method.

Stomatal Index
Figure 5.18. Plots of RCO2 versus time derived by the stomatal index method compared to the standard GEOCARB III curve. Stomatal results from McElwain (1998) and Retallack (2001) (5-point running average).

Quantitative calibration of stomatal index (SI) against CO2 change has been done via both laboratory experiments and the history of CO2 over the past 150 years as recorded in ice cores and actual atmospheric measurements. Both approaches have been used by Royer et al. (2001b) to calibrate Ginkgo biloba and Metasequoia glyptostroboides leaves. Values of SI were measured on samples from herbarium collections that had been collected at different times over the past 150 years at atmospheric CO2 levels ranging from 270 ppm to 360 ppm. These results were combined with Ginkgo and Metasequoia experiments conducted at elevated CO2 levels. An excellent linear negative correlation between SI and CO2 concentration was found for the ginkgo and Metasequoia leaves between 270 and 360 ppm, but at higher CO2 levels, based on laboratory experiments, the response diminished and approached constant SI values. This shows that the SI method may have an upper CO2 limit, at least on the time scale (few years) of laboratory experiments.

Retallack (2001) extrapolated from the SI versus CO2 experimental calibrations of Beerling et al. (1998) for a single ginkgo species to obtain values of RCO2 over the past 300 million years based on the study of other species. Although the extrapolation is not theoretically justified, and CO2 response can vary greatly between species (Royer et al., 2001), Retallack's results, in terms of 5-point moving averages, show overall semiquantitative agreement with those of GEOCARB modeling and the stomatal ratio results of McElwain (1998; figure 5.18).

The results of Royer et al. (2001) and other well-calibrated SI studies based on single species (van der Burgh et al., 1993; Kurschner, 1997) are summarized for the Tertiary below in figure 5.20. Results for the Eocene disagree with those based on the plankton fractionation and boron isotope methods (described below), but there is general agreement for the Miocene.

Organic Gardening

Organic Gardening

Gardening is also a great way to provide healthy food for you and your loved ones. When you buy produce from the store, it just isnt the same as presenting a salad to your family that came exclusively from your garden worked by your own two hands.

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Responses

  • isabel
    How to calculate stomatal index?
    8 years ago
  • lemlem
    How to calibrate stomata?
    8 years ago
  • Marcus
    Why are CO2 levels and stomatal index are inversely related?
    8 years ago
  • piper
    How to compute the stomatal index?
    8 years ago
  • Frank
    How to measure stomatal indices?
    7 years ago
  • Largo Boffin
    How do stomata index change?
    2 years ago
  • Zane
    How does stomatal index change in different climates?
    2 years ago

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