The dependence of lichen size on factors other than time often poses a major difficulty for lichenometry. In this study the influence of intraspecific competition on the structure and dynamics of populations of Rhizocarpon sub-genus Cern. is investigated. Size-frequency and size- density data from populations growing on dated moraine sequences in southern Norway are used to test three models describing the process of spatial competition in lichen communities. Spatial competition appears to be a factor influencing the structure and dynamics of these lichen com- munities within about 100 years of deglaciation. During the pre-competi- tive phase population density and mean thallus diameter are positively related. Once a closed lichen community is formed further growth occurs through a corresponding reduction in density in the lichen mosaic. These data are used as an aid to the interpretation of size-frequency data col- lected from the populations. The 1-in-1000 thallus is found to be larger in low density situations compared to high density locations. The factors influencing the density of the lichen mosaic are considered. Both the mean and maximum thallus diameter and the 1-in-1000 lichen appear to be independent of initial colonization density. It is concluded that despite the influence of spatial competition parameters such as the mean or maximum thallus diameter can be used to characterize the structure of the lichen population for lichenometric purposes.