Henk Visscher

STOMATAL FREQUENCY RESPONSES IN HARDWOOD- SWAMP VEGETATION FROM FLORIDA DURING A 60-YEAR CONTINUOUS CO2 INCREASE'

In a stomatal frequency analysis of leaf remains of Quercus nigra, Acer rubrum, Myrica cerifera, Ilex cassine, and Osmunda regalis that were preserved in precisely dated peat deposits of north-central Florida, the stomatal index decreased as a response to an atmospheric CO(2) increase from 310 ppmv to 370 ppmv over the past 60 years. The observations indicate that CO(2) responsiveness may occur in different canopy levels of hardwood-swamp vegetation. Apart from common woody plants, long-lived ferns of the undergrowth appear to be affected by CO(2) changes. Response rates are most pronounced in M. cerifera, I. cassine, and O. regalis. The potential of these species for quantifying past atmospheric CO(2) levels is assessed by a combined analysis of the well-dated buried leaf record and herbarium material collected during the past century. Leaf remains of the widely occurring species M. cerifera and I. cassine are concluded to be highly suitable for CO(2) reconstructions, by which the application range of the stomatal frequency proxy is extended into the warm-temperate to subtropical realm of North America.