Urban lichenometry: growth of Circinaria contorta on city sidewalks

Lichenometry, a dating method that relies on the largest lichen to infer the age of a surface, is widely used due to its simplicity and affordability. However, several of the processes underlying the basic assumptions are assumed to be constant, while in reality they are climate-dependent, particularly those concerning colonisation, mortality, and thallus growth rates. This limits the comparability of lichenometric studies in different climates. Therefore, a better understanding of the environmental influence on lichen growth is required. Urban environments, with many homogeneous dated surfaces, can offer a unique opportunity to study lichen growth. To get a better understanding of urban lichen growth patterns, we investigated the largest Circinaria contorta thalli on sidewalks of varying ages in three temperate cities. Results suggest a sigmoidal growth pattern, reaching a peak radial growth rate of 1.68 mm year–1, similar to other temperate crustose lichens. Variation within and between cities is substantial and only partially explained by shade and sidewalk area. However, a rough estimation of concrete pavement age can be inferred from the largest thallus diameter.