Gas exchange analysis in fossil leaves and climate change

Next month, experts in climate reconstruction will meet to create a consensus record of atmospheric CO2 over the past 66 million years.  Stomata will play an important role in the coming meeting when paleoclimatologists reconcile their calculation using leaf gas exchange in fossil leaves with other sources1.

It was Woodward in 1987 who first observed that a significant inverse relationship existed between plant stomatal density (number of stomata per mm2) and atmospheric CO2 concentration. Using herbarium specimens, he demonstrated that the stomatal densities and ratio in 8 temperate woody specimens collected 200 years ago were significantly higher than those of the same plant species today2.

In 2014, Franks et al.3 applied the same idea of counting stomata to fossil leaves and integrated their counts with the available models for leaf gas exchange. Another important parameter used in common leaf gas exchange analysis is the internal CO2 content. In fossil leaves, this concentration can be obtained using the isotopic signal of δ13C to discriminate between leaf and atmospheric carbon. A worrisome finding from this method is that the Earth’s climate could be more sensitive to CO2 concentrations than previously thought.



Conceptual idea for the analysis of leaf gas exchange in fossil leaves (taken from Franks et al. 2014)


Understanding the sensitivity of climate to carbon dioxide concentrations is a key point in the discussions of climate change. Scientists are trying to come up with better methods, with less uncertainty, which can help arrive at more accurate conclusions about our past climate. Hopefully, the stomata in fossil leaves can help in this effort. Another promised result from the meeting is an open source paleo-pCO2 database.

Thanks to the people in the plant bio journal club for pointing out this news.

  1. Hand, E. Fossil leaves bear witness to ancient carbon dioxide levels. Science 355, 14–15 (2017).
  2. Woodward, F. I. Stomatal numbers are sensitive to increases in CO2 from pre-industrial levels. Nature 327, 617–618 (1987).
  3. Franks, P. J. et al. New constraints on atmospheric CO2 concentration for the Phanerozoic. Geophys. Res. Lett. 41, 4685–4694 (2014).

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