, Volume 19, Issue 3, pp 251-265
Date: 22 Dec 2004

Reconstructing atmospheric carbon dioxide with stomata: possibilities and limitations of a botanical pCO2-sensor

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Stomatal frequency is often observed to vary inversely with atmospheric CO2 concentration (pCO2). The response is due to (1) individual phenotypic plasticity and (2) evolutionary change, depending on the time scale. Evolutionary responses occur more frequently than individual responses and individual responses are more pronounced under subambient pCO2 levels than under elevated pCO2 (“CO2 ceiling”). The evolutionary response appears therefore to be a valuable device for determining past pCO2. Since tree leaves often represent a conspicuous and rich resource of fossil material, they are increasingly important in this respect. Additionally, certain tree species are considered to represent “living fossils” and therefore valuable sources of ancient stomatal data. There are, however, numerous difficulties which have to be considered such as: (1) high variance of the data, especially for fossil material, (2) interspecific differences of the response, (3) the CO2 ceiling and (4) differences between short-term and long-term responses. Whereas the qualitative pCO2 signal of stomatal frequency appears to be reliable, quantitative pCO2 reconstruction has to be performed with caution. The results of a number of studies which used stomatal frequency as a pCO2 sensor demonstrate good agreement with the results obtained with other proxy data. Current techniques are based on “transfer functions” which calibrate the fossil data with extant material. It is suggested that a mechanistic approach including physical as well as physiological processes could improve pCO2 reconstruction. Furthermore, the topic of the influence of pCO2 on stomatal frequency is significant not only for reconstructing past pCO2 but also with respect to the climate-biosphere interrelationship.