ARE PALEOCLIMATE ESTIMATES BIASED BY FOLIAR PHYSIOGNOMIC RESPONSES TOINCREASED ATMOSPHERIC CO2

Physiognomic analysis of fossil angiosperm leaves has provided an impo rtant quantitative database of Tertiary terrestrial paleoclimate. Howe ver, atmospheric CO2 level, a critical control on plant growth, may ha ve been higher in the Tertiary. It is thus crucial to investigate whet her elevated CO2 affects leaf physiognomy. In this study, leaves were collected from white oak (Quercus alba L.) seedlings grown in open-top growth chambers at Oak Ridge National Laboratory. The only physiognom ic change noted is an increase in length to width ratio with increasin g CO2. In the literature, leaf size has been observed to increase, dec rease or remain unchanged for woody C-3 species grown in elevated CO2. Typically, one sees more variation due to microsite or phenotype than due to CO2 level. By applying these observed physiognomic trends to t wo fossil floras, it is argued that estimates of mean annual temperatu re and growing season precipitation may be biased on the order of 1 de grees C and 20 cm, respectively. These are relatively small effects, a s the values are similar to the standard errors of the regression mode ls used to estimate paleoclimate. The lack of data, the variability of response to CO2 associated with microsite and phenotype, and the ques tion of whether observed short-term trends with elevated CO2 are susta ined make it impossible to propose a correction factor. Adequate sampl e size and sampling of several sites are the best way to attempt to co mpensate for CO2 effects on a given fossil flora until response to CO2 is better understood.