GROWTH RATE OF A VERY LARGE CRUSTOSE LICHEN (RHIZOCARPON SUBGENUS) AND ITS IMPLICATIONS FOR LICHENOMETRY

P>The growth rate of an individual thallus of the Rhizocarpon subgenus with a current maximum diameter of 465 mm was measured directly over a 26-year period at ca 5-year intervals along 10 radii. Mean annual diametral growth rate was 0.48 mm yr-1 (the range between individual radii and time intervals was 0.02-1.54 mm yr-1). The southeasterly aspect of the lichen may have led to a higher growth rate in years that were relatively cool and dry. Combined with the growth rate of 18 smaller lichens at the site near the Haugabreen glacier in southern Norway, the data support a parabolic growth-rate model. The corresponding growth curve, which is sigmoidal, indicates an age of about 1000 years for the very large lichen. Results also suggest that it may be possible to extend the temporal range of traditional (largest lichen) lichenometric dating in southern Norway. However, despite having been pointed out by R.E. Beschel in the 1950s, there is still an urgent need for further direct lichenometric data, for the monitoring of relatively large lichens and for a better understanding of the factors causing variability in growth rates.