Photosynthetic rate and thallus size are not related in alpine yellow-green Rhizocarpon crustose lichens: Implications for lichenometry and growth

Lichenometry, first proposed at the beginning of the XXth century, is a technique that uses growth rates of saxicolous crustose lichens to date exposed surfaces over an age range of 500 years from present. Despite of the wide use of the methodology, it has been strongly criticized by several authors who consider that biological aspects involved in growth rates of lichens used are not sufficiently considered and that this will contribute to uncertainty in the final surface dating. The assumption, important for direct lichenometric measurements, that crustose lichens have constant growth rates through all their life span, is controversial, with some works pointing to a change in growth rate with thallus size in yellow-green Rhizocarpon sp. samples, the most widely lichens used for surface dating. This change in growth rate with size would contribute to inaccuracy in dating. In this work, we contribute to the discussion with a novel approach in which we measure several physiological parameters linked to carbon gain in 42 Rhizocarpon sp. samples of different sizes collected in the same locality. We found no significant relationship between thallus photosynthetic rate and thallus size indicating that possible growth rate variations over Rhizocarpon life span are not linked to carbon gain. The experiment performed provides a new data set to include in lichenometric modelling with the aim to obtain a better understanding of crustose lichens biology before attempting more feasible and accurate surface dating strategies. Measurements of length of activity periods and carbon allocation in Rhizocarpon are recommended to achieve future improvements in this direction.