Growth Rates of Xanthoparmelia Lichens in North Cemetery, Petersham MA since 2005

Lichens fix carbon dioxide from the air to build biomass. Crustose and foliose lichens grow as nearly flat, circular disks. While smaller lichens grow slowly, but with small, steady increases in radial growth rates, larger lichens grow more quickly, with radial velocities that remain roughly constant over the lifetime of an individual. These basic features follow directly from a novel mathematical model based on the diffusion of carbon dioxide around a lichen, and its absorption by the thallus. Smaller lichens fix carbon dioxide across the entire thallus, but larger lichens fix carbon dioxide disproportionately at edges. Because local edges remain the primary source of carbon as a lichen grows ever larger, radial growth rates remain constant. Predicted growth dynamics depend on four measurable parameters, and tests of the model against data suggest the model provides an accurate, robust, and universal framework for understanding the growth dynamics of lichens in nature.