IV. THE CLASSIFICATION OF THE LICHENS 

 INTO ASSOCIATIONS. 



1. BARK-LICHEN ASSOCIATION. 

 EPIPHYTIC-LICHEN ASSOCIATION (BARK-LICHENS). 



IN Iceland there is only one kind of tree which bears a lichen- 

 vegetation worth mentioning, viz. the birch, Betula odorata. 



This, like all other kinds of trees, runs through a fixed deve- 

 lopment as regards its lichen-vegetation, as I have formerly shown 

 in my treatise "Danske Likeners 0kologi" (1908). When quite young 

 it is devoid of lichens, after which crustaceous lichens make their 

 appearance, and later on loliaceous, and eventually fruticose lichens. 



The bark of the birch, as is well-known, is smooth and arranged 

 in layers; it contains plenty of birch-resin which helps to preserve 

 it, so that it is but slightly liable to decomposition and rotting; it 

 is especially for this reason that it is used for covering wooden 

 houses, for soles of boots, etc. 



When the trunks become old, the bark bursts and is thrown 

 off in thin sheets, and at the foot of the trunk more or less dis- 

 tinctly radial cracks are formed in the bark, so that the bark be- 

 comes "scaly" in that part. Moreover, fissures, wounds from fallen- 

 off branches, and cracks due to old lenticels, etc., are abundantly 

 formed on the persistent parts of the bark. 



Generally, the rule holds 'good that the lie hen- vegetation 

 begins on damaged, rough bark, in bark-cracks, lenticels, etc., 

 while the smooth, undamaged bark is devoid of lichens. 



As is well-known birches form coppices of very varying extent 

 in Iceland. The highest are found in HallormstaQskogur (see Thor- 

 oddsen's fig. 36 in Part I of this work) and in South Iceland, while 

 the coppices of North Iceland are of lower growth (Halsskogur, etc.). 

 I myself have unfortunately seen the coppices of North Iceland only. 



The light-conditions in almost all the coppices are favourable 



