INTRA- AND INTERSPECIFIC VARIATION IN STOMATAL PROXIES FOR QUERCUS AND NYSSA IN THE SUBTROPICAL SOUTHEASTERN USA

Since the industrial revolution, Earth has experienced a warming climate linked to rising concentrations of atmospheric gases. Observations from the Mauna Loa Observatory over the past 55 years show that pCO₂ has risen ~60 ppm. It is thought that a 1% pCO₂ increase will result in a warming between 1-4.5C, with serious global implications. It is uncertain whether the observed pCO₂ rise is a direct result of anthropogenic activity or natural variability. Evaluating recent and deep past pCO₂ records will help to resolve these uncertainties. Stomatal density (SD) and stomatal index (SI) are used as proxies for past atmospheric concentrations of CO₂ based on empirical and laboratory data. Both have been used to reconstruct Holocene pCO₂ trends and shifts in the deep past. Previous studies evaluated the response of one taxon over time, while others used the nearest living relative (NLR) or nearest living equivalent (NLE) taxon to calibrate stomatal features. The present study identifies SD and SI variations between two genera and species of commonly fossilized taxa, the leaves of which were collected under a known pCO₂.

Sun leaves of 12 species of Quercus and 3 species of Nyssa were collected in the Auburn University Arboretum in the 1991 growing season (pCO₂ = 357 ppm). SD and SI were calculated for each taxon based on counts of a 0.8 mm2 area, extrapolated to the 1 mm2 standard. Confidence intervals (95%) were calculated for sample means and results compared with published values of Quercus evaluated in the same year. SD and SI varied significantly between taxa at both the specific and generic levels, with greater variation found in SD than in SI. Results for Nyssa were lower than published and empirical values for Quercus, while data for Quercus species ranged from below to above minima and maxima reported in the literature. Several taxa plotted outside the range of sun-and-shade leaf limits for Q. petraea. The present study confirms the conclusion that SI displays a more consistent relationships within and between genera than SD, but variation amongst species of the same genus grown in the same locality under the same climatic conditions is greater than expected. This observation casts serious doubt on the validity of using stomatal characteristics of NLRs or NLEs in the reconstruction of past atmospheric CO₂ levels.