Stomatal density and index of fossil plants track atmospheric carbon dioxide in the palaeozoic

Abstract

It has been demonstrated that the leaves of a range of forest tree species have responded to the rising concentration of atmospheric CO2 over the last 200 years by a decrease in both stomatal density and stomatal index. This response has also been demonstrated experimentally by growing plants under elevated CO2 concentrations. Investigation of Quaternary fossil leaves has shown a corresponding stomatal response to changing CO2 concentrations through a glacial-interglacial cycle, as revealed by ice core data. Tertiary leaves show a similar pattern of stomatal density change, using palynological evidence of palaeo-temperature as a proxy measure of CO2 concentration. The present work extends this approach into the Palaeozoic fossil plant record. The stomatal density and index of Early Devonian, Carboniferous and Early Permian plants has been investigated, to test for any relationship that they may show with the changes in atmospheric CO2 concentration, derived from physical evidence, over that period. Observed changes in the stomatal data give support to the suggestion from physical evidence, that atmospheric CO2 concentrations fell from an Early Devonian high of 10–12 times its present value, to one comparable to that of the present day by the end of the Carboniferous. These results suggest that stomatal density of fossil leaves has potential value for assessing changes in atmospheric CO2 concentration through geological time. © 1995 Annals of Botany Company.