Liverworts, along with their close relatives the mosses and hornworts, are one of the oldest groups of land plants, first appearing in the early Paleozoic. They are short, have no tissue to conduct fluids (i.e. no xylem or phloem), and their photosynthetic tissue typically either lacks stomata altogether or has pores that remain fixed in size and shape. This differs from most other land plants, whose leaves have stomatal pores that can be opened and closed (see Stomatal frequencies).

Because liverworts lack functional stomata, they are largely passive with regards to leaf conductance. That is, unlike most other plants, they cannot change their leaf conductance (see Leaf gas-exchange proxies) by changing the sizes of their stomatal pores. CO₂ diffuses into the plant either across cell membranes or through fixed pores. Similar to marine phytoplankton, the carbon isotopic fractionation during photosynthetic carbon assimilation is partly controlled by the amount of surrounding CO₂. In this regard, the liverwort δ¹³C proxy is a terrestrial “cousin” to the marine phytoplankton δ¹³C proxy (Marine phytoplankton δ¹³C). The higher the atmospheric CO₂ concentration, the stronger the air-to-leaf carbon isotopic fractionation (i.e. δ¹³C of their tissues decreases).

Similar to the Marine phytoplankton δ¹³C proxy, the largest challenge with the liverwort proxy is modeling the growth rate, which also affects the plant’s carbon isotopic fractionation. This is done with a photosynthesis model (for details, see Fletcher et al., 2006) which requires inputs such as irradiance. Liverwort fossils are relatively rare, which limits the general applicability of this proxy.

The figure above shows a simple flow chart of the different parameters required to calculate CO₂ from this proxy. A template for entering new paleo-CO₂ estimates from liverworts can be found here. For adding new data, please fill out the template and submit it to the paleo-CO₂ database at the NOAA National Centers for Environmental Information (formerly known as the National Climatic Data Center, NCDC), using email address paleo@noaa.gov.

References cited

Fletcher, B.J., Brentnall, S.J., Quick, W.P. and Beerling, D.J. (2006) BRYOCARB: A process-based model of thallose liverwort carbon isotope fractionation in response to CO₂, O₂, light and temperature. Geochimica et Cosmochimica Acta 70, 5676-5691.