The ecological basis for using the mean of the five largest lichens in lichenometry has been examined by investigating the Rhizocarpon section Rhizocarpon species growing on gravestones in Highland Scotland. The ratio of the mean of the five largest thalli to the single largest thallus on a substrate provides an index of the lichen performance on each stone which can then be related to environmental and temporal factors. Ratios appear to be unaffected by substrate lithology and orientation. but respond to variations in substrate age. There is a marked spatial pattern of mean' ratios, which can be explained in terms of onshore winds and the distribution of air pollution. In most arctic-alpine areas, these factors are unimportant, and the use of the mean of the five largest lichens in Iichenometry can therefore be justified on ecological as well as statistical grounds