STOMATAL DENSITY AND INDEX OF FOSSIL PLANTS TRACK ATMOSPHERIC CARBON-DIOXIDE IN THE PALEOZOIC

It has been demonstrated that the leaves of a range of forest tree spe cies have responded to the rising concentration of atmospheric CO2 ove r the last 200 years by a decrease in both stomatal density and stomat al index. This response has also been demonstrated experimentally by g rowing plants under elevated CO2 concentrations. Investigation of Quat ernary fossil leaves has shown a corresponding stomatal response to ch anging CO2 concentrations through a glacial-interglacial cycle, as rev ealed by ice core data. Tertiary leaves show a similar pattern of stom atal density change, using palynological evidence of palaeo-temperatur e as a proxy measure of CO2 concentration. The present work extends th is approach into the Palaeozoic fossil plant record. The stomatal dens ity and index of Early Devonian, Carboniferous and Early Permian plant s has been investigated, to test for any relationship that they may sh ow with the changes in atmospheric CO2 concentration, derived from phy sical evidence, over that period. Observed changes in the stomatal dat a give support to the suggestion from physical evidence, that atmosphe ric 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 th e end of the Carboniferous. These results suggest that stomatal densit y of fossil leaves has potential value for assessing changes in atmosp heric CO2 concentration through geological time. (C) 1995 Annals of Bo tany Company