Previous lichenometric dating of young geomorphological forms in the Tatra Mountains has been hindered by differences in the determination of initial colonisation of fresh rock surfaces by the Rhizocarpon geographicum lichen.In order to determine precisely the time needed for the first thalli to appear on newly exposed rock surfaces,two experimental sites were created in 2013.The results from these sites were compared with the measurements taken on boulders in debris formed by a 2013 debris flow.In 2018,the largest thalli measured within the experimental sites and the debris flow area reached a size of approximately 2 mm.Based on this,the time of initial colonisation of rock surfaces was determined to be 5 ± 1 years,which is about half the period previously described in the literature.