Leaf stomatal frequency in the Australian tropical rainforest tree Neolitsea dealbata (Lauraceae) as a proxy measure of atmospheric pCO2

A putative relationship has been demonstrated for European and North American woody dicots and gymnosperms between leaf stomatal frequency and historical levels of atmospheric CO2. However, hitherto no such study has been presented for Australian tropical broadleaved evergreen trees. In this study, variation of stomatal index (SI) along environmental gradients is examined for the broadleaved evergreen tropical rainforest tree Neolitsea dealbata (Lauraceae). Historical herbarium samples from natural populations in northeastern and southeastern Queensland were analysed. Leaf SI for Neolitsea dealbata is shown to be insensitive to mean annual rainfall or seasonal totals, or to temperature variables, indicating the climatic factors that influence the water budget of the plants were not a factor controlling SI. Contrary to the pattern most commonly observed in temperate areas, SI decreased with elevation for collections within a single year for 3 out of 4 years surveyed, a pattern consistent with environmental factors other than pCO2 acting as the control over SI. However, an overall decrease in SI was recorded for samples collected in Queensland (30°–17°S) over the period 1899–1988, corresponding to a Southern Hemisphere increase in pCO2 from 295 to 350 ppm. Restricting the analysis to sites within an altitudinal band of 640–1120 m demonstrated a significant relationship between SI and pCO2 (r2=0.8942, F11=84.5, P<0.001). This data set was used to estimate atmospheric pCO2 from fossil leaf cuticle for the closely related genus Litsea, giving pCO2 values comparable to those estimated using Ginkgo from Northern Hemisphere fossil sites.