Lichens as A Measure of the Age of Recent Moraines

Abstract

Epipetric lichens grow extremely slowly. They are capable of establishing themselves immediately after the exposure of a rock surface in crevices and hair-cracks or on irregularities of the rock surface; they can achieve this either by means of spores or by means of fragments of older stock. Hence at the maximum, lichens may be just as old as the rock surface itself. At first there is a considerable period before the young lichen thallus becomes even macroscopically visible. Then a relative acceleration of the growth rate occurs, which continues up to a certain diameter-lichen thalli generally have a circular outline-whereupon the diameter abruptly begins to grow very much more slowly, yet at a constant rate. The period of relatively rapid growth will be termed the great period. According to the latest information, it ends within a few decades. The constant continued increase in diameter, on the other hand, lasts for many centuries in the case of many crustose lichens; in the case of some individual lichen species, an absolute limit has not been set for this increase. The growth rate of a particular lichen species depends primarily on the climatic environment. A prime factor here is that of moisture. Lichens are capable of assimilating and growing only when in a thoroughly saturated condition, while, in fact, they spend the greater part of their life either in a state of xeric rest or else in a cold-influenced torpor. Snowmelt, dew, and, above all, rain provide lichens with the possibility of growth, assuming