IV. THE CLASSIFICATION OF THE LICHENS 
INTO ASSOCIATIONS. 
1. BARK-LICHEN ASSOCIATION. 
EPIPHYTIC-LICHEN ASSOCIATION (BARK-LICHENS). 
I" 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 Økologi” (1908). When quite young 
it is devoid of lichens, after which crustaceous lichens make their 
appearance, and later on foliaceous, 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 lichen-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 Hallormstaöskogur (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 (Hälsskogur, etc.). 
I myself have unfortunately seen the coppices of North Iceland only. 
The light-conditions in almost all the coppices are favourable 
