Assessing the potential for the stomatal characters of extant and fossil Ginkgo leaves to signal atmospheric CO2 change

The stomatal density and index of fossil Ginkgo leaves (Early Jurassic to Early Cretaceous) have been investigated to test whether these plant fossils provide evidence for CO2-rich atmosphere in the Mesozoic. We first assessed five sources of natural variation in the stomatal density and index of extant Gingko biloba leaves: (1) timing of leaf maturation, (2) young vs. fully developed leaves, (3) short shoots vs. long shoots, (4) position in the canopy, and (5) male vs. female trees. Our analysis indicated that some significant differences in leaf stomatal density and index were evident arising from these considerations. However, this variability was considerably less than the difference in leaf stomatal density and index between modern and fossil samples, with the stomatal index of four species of Mesozoic Ginkgo (G, coriacea, G. huttoni, G. yimaensis, and G. obrutschewii) 60-40% lower than the modern values recorded in this study for extant G. biloba. Calculated as stomatal ratios (the stomatal index of the fossil leaves relative to the modern value), the values generally tracked the CO2 variations predicted by a long-term carbon cycle model confirming the utility of this plant group to provide a reasonable measure of ancient atmospheric CO2 change.