Gregory, K.M., 1996, Are paleoclimate estimates biased by foliar physiognomic
response to elevated atmospheric CO2?: Palaeo-3, v. 124, p. 39-51.
Abstract.-- Physiognomic analysis of fossil angiosperm
leaves has provided an important quantitative database of Tertiary terrestrial
paleoclimate. However, atmospheric CO2 level, a critical control on plant
growth, may have been higher in the Tertiary. It is thus crucial to investigate
whether 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 physiognomic change
noted is an increase in length to width ratio with increasing CO2. In the
literature, leaf size has been observed to increase, decrease or remain
unchanged for woody C3 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 two fossil floras, it is
argued that estimates of mean annual temperature and growing season precipitation
may be biased on the order of 1°C and 20 cm, respectively. These are
relatively small effects, as the values are similar to the standard errors
of the regression models used to estimate paleoclimate. The lack of data,
the variability of response to CO2 associated with microsite and phenotype,
and the question of whether observed short-term trends with elevated CO2
are sustained make it impossible to propose a correction factor. Adequate
sample size and sampling of several sites are the best way to attempt to
compensate for CO2 effects on a given fossil flora until response to CO2
is better understood.