LICHENOLOGY OF ICELAND 217 



(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 algae gain the upper hand). The heaths of mountain 

 heights are sometimes rather rich in lichens. 



(6) Conditions concerning the nivean 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 chamrephytes 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. Jons- 

 son the most common ground-vegetations are: heat her- moor (of 

 Empetrum nig rum, Arctostaphylos iwa ursi and Vaccininm uliginosiim), 

 grassland (of Agrostis uulgaris, Aira flexuosa, Anthoxanthum, Festuca 

 rnbra), herb-flat (of Angelica siluestris, Spircea Ulmaria. etc.) and 



