A Proxy for Air

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Science  20 Jun 2003:
Vol. 300, Issue 5627, pp. 1849
DOI: 10.1126/science.300.5627.1849b

The frequency and density of stomata (the pores in the surfaces of leaves, through which gases and water vapor transit) are known to vary with the atmospheric CO2 concentration present when the leaves developed. Also, the carbon isotope ratios in leaf cuticles vary according to the microclimatic conditions (sun, shade, and precipitation) experienced by leaves. On the basis of these relationships, stomata have been used as proxies to quantitate past CO2 concentrations by measuring their variation in museum specimens and fossils and comparing these values with those of extant plants of the same species.

“Living fossils” such as Ginkgo (and Metasequoia, see Editors' Choice, 30 May, p. 1343) are especially prized for such studies because their fossil record extends back through the Mesozoic and therefore provides a good record of atmospheric conditions over the past 200 million years. However, it is important to establish that the variation observed in fossils can be interpreted in the same way as that observed in present-day plants. In a study of Ginkgo leaves from a variety of positions (sun and shade) and climates in China and the United Kingdom, Sun et al. show that stomatal index (the percentage of leaf epidermal cells that are stomata) is almost constant under different climates and that the variation in carbon isotopes matches that of fossil Ginkgo cuticles. These results suggest that photosynthetic physiology and leaf morphology have been conserved over the evolutionary history of these trees, strengthening their reliability as proxies for atmospheric change.—AMS

Proc. Natl. Acad. Sci. U.S.A. 100, 7141 (2003).

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