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 E arly Cretaceous) have been investigated to test whether these plant fossils provide evidence for CO2-rich atmosphere in the Mesozoic. We first assesse d five sources of natural variation in the stomatal density and index of ex tant Gingko biloba leaves: (1) timing of leaf maturation, (2) young vs. ful ly developed leaves, (3) short shoots vs. long shoots, (4) position in the canopy, and (5) male vs. female trees. Our analysis indicated that some sig nificant differences in leaf stomatal density and index were evident arisin g from these considerations. However, this variability was considerably les s 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. Calcula ted as stomatal ratios (the stomatal index of the fossil leaves relative to the modern value), the values generally tracked the CO2 variations predict ed by a long-term carbon cycle model confirming the utility of this plant g roup to provide a reasonable measure of ancient atmospheric CO2 change.