LICHENOLOGY OF ICELAND D7 
(5) The snow-covering in some localities has a not unfavourable 
influence provided it disappears for a few weeks every summer with- 
out leaving too great masses of water behind it (in which case 
mosses and alge gain the upper hand). The heaths of mountain 
heights are sometimes rather rich in lichens. 
(6) Conditions concerning the niveau of the ground, appear to 
be of fairly great importance, inasmuch as knolly ground, in most 
cases, bears lichens on the sides of the knolls, whilst the horizontal 
surfaces of the knolls are covered with lichens in damp heaths only. 
The depressions between the knolls frequently bear mosses and no 
lichens at all or only a minute quantity. Here, it is most probably, 
the conditions of moisture that make themselves felt. 
(7) The plant-covering (the competitors) plays essentially the 
part of contending against the lichens by covering them with de- 
caying leaves (see above) or by overshadowing them. Both these 
drawbacks occur on Icelandic as well as on Danish heaths, where 
the higher plant-growth is more luxuriant. But experience shows 
that the growth and luxuriance of the chamephytes themselves is 
not great enough on all heaths to exclude lichens. 
f. Coppices. 
These, the only phanerophytic birch-vegetation of Iceland, are, 
as elsewhere mentioned in this work (see vol. I, p. 312 et seq.), 
widely distributed over the whole of the island, — but may, how- 
ever, possibly be absent from a narrow strip of North Iceland. 
They do not extend upwards on the mountains beyond a height of 
about 550 metres, and the majority of them are situated at lower 
levels. Everywhere the coppices consist, to a certain extent, of rather 
poorly developed individuals, the height of which ranges from that 
of a low-growing shrub to a height of several metres (8—9). (The 
most frequent height is 1—2 metres). The density of the tree-trunks 
varies considerably, which consequently results in a fairly varying 
ground-vegetation. 
The soil is often knolly clay, and rests on gravel or also on 
rock, but sometimes there is a stony bottom, and sometimes the 
bottom is boggy soil (Thoroddsen, p. 342). According to H. Jöns- 
son the most common ground-vegetations are: heather-moor (of 
Empetrum nigrum, Arctostaphylos uva ursi and Vaccinium uliginosum), 
grassland (of Agrostis vulgaris, Aira flexuosa, Anthoxanthum, Festuca 
rubra), herb-flat (of Angelica silvestris, Spirea Ulmaria, ete.) and 
