Dating Exposed Rock Surfaces in the Arctic by Lichenometry: The Problem of Thallus Circularity and Its Effect on Measurement Errors

Lichenometry represents an extremely useful dating technique in the Arctic. It is most appropriate for exposed rock surfaces, which are abundant in most arctic environments, and on occasion it represents the only suitable technique for estimating the age of a surface. As a result, lichenometry is being used in an increasing number of arctic studies. Despite this, controversy still surrounds the manner in which individual thalli are measured. The majority of workers measure either the longest axis or the diameter of the largest inscribed circle. The measurement error involved in the latter is significantly greater than that for longest axis measurements. The proportion of circular thalli on a surface decreases through time, and studies that use circular thalli only or inscribed circle diameters will underestimate the maximum growth rate of lichens on older surfaces.