HISTORICAL RECORDS OF STOMATAL INDICES FOR QUERCUS AND NYSSA FROM THE SOUTHEASTERN U.S

Modeling atmospheric carbon dioxide concentration (pCO₂) is critical to understanding the Carbon Cycle over geologic time. Recently, biological proxies have become commonly used including stomatal frequencies on leaves. An inverse relationship exists between epidermal stomata and the pCO₂ under which the leaf grew and expanded; this provides a record of atmospheric gases as accurate as ice core data, another proxy, but at a higher temporal resolution. The two most commonly used leaf parameters are Stomatal Density (SD - # of stomata/unit area) and Stomatal Index (SI - the ratio between # stomata and total epidermal cells/unit area). However, stomatal frequencies have some limitations. A CO₂ “ceiling” exists where plants stop responding to gas concentration in a linear fashion above a certain threshold. Additionally, transfer functions calibrated from extant plants do not always correspond to fossil equivalents.

Another recent consideration is that taxonomically related plants may not exhibit similar growth responses under the same pCO₂ conditions. To test this, 12 species of Oak (Quercus) and 3 of Tupelo Gum (Nyssa) were evaluated with respect to SD and SI over an 83 year interval based on trees grown under a humid subtropical climate in Lee County, AL. A previous data set was based on collections made in 1991 from the Auburn University Arboretum, Auburn, AL. Herbarium specimens dating from 1924 to the 1980s were used to supplement the historical record; a collection made during June 2007 extended the record to the present. Herbarium samples were taken from collections made in the same year, if available. Materials were cleared in chromic acid, mounted on slides, where after stomata and epidermal cells were counted using a Zeiss Axioskop and AxioVision software.

Initial results appear to demonstrate that both Oak and Tupelo Gum intragenerically vary as pCO₂ has increased during the past century. While some species, such as Q. nigra and Q. laurifolia, exhibit very similar responses which are inversely proportional to pCO₂, others, such as Q. velutina, show a mixed response over time. The present study suggests that approaches correlating data from extant taxa with data from extinct taxa are debatable and should be considered for their qualitative observations rather than quantitative measures.