246 SUCCESSION IN EURASIA. 



The coniferous cliffs show stretches of naked rock, together with areas 

 covered with crustaceous Hchens and larger areas with fruticose lichens, e. g., 

 Cladina silvatica. With these are associated mosses, while on the margin of 

 the cliff there is commonly a zone of low shrubs, especially Calluna, with 

 lichens and mosses and scattered pines and spruces. The first colonists on 

 the bare rock are the crustose lichens which are later overgrown by fruticose 

 lichens. Among the latter appear scanty grass areas, upon which birch and 

 pine can establish themselves. More often heath is established on the loose 

 soil in the hollows between the hchens and overgrows the latter, changing 

 finally into coniferous forest. Where the forest has been cleared the crustose 

 lichens persist longer, but foliose lichens appear among them, and a moss 

 carpet is also formed, chiefly of Grimmia hypnoides and Hedwigia albicans. In 

 place of heath, grass areas ofDeschampsiaJlexuosa, Festuca ovina, etc., develop, 

 or areas of Sedum, Saxifraga, Draba, etc. 



Nilsson regards as moor all coimnunities in which the groimd cover consists 

 of Sphagnum. When a lake is shallowed, a sedge-moor develops around it; 

 this changes later into an Eriophorum moor and the latter into a dwarf-shrub 

 moor. The latter may develop further into a forest-moor, chiefly formed by 

 pine, but in the north often by the spruce and occasionally by birch. More 

 frequent than this progressive development is the retrogressive one in which 

 the Sphagnum cushions of the dwarf-shrub moor are invaded by fruticose 

 lichens, as a consequence of which Sphagnum and the shrubs die for the most 

 part. Finally lichens dominate the cover, with scattered areas of Sphagnum, 

 Eriophorum, and Calluna here and there. The cause of this change lies in 

 the fact that the moor dries out as it rises higher. The Sphagnum dies and 

 the young peat weathers. On the other hand, the lichen moor also changes. 

 The weathered Sphagnum permits the appearance of Uttle pools which destroy 

 the lichens. The hillocks of Eriophorum grow again after a while and dwarf- 

 shrubs appear upon them, especially Andromeda polifolia. These finally also 

 die off, and in the accumulating water. Sphagnum and sedges, especially Carex 

 iimosa, Scirpus caespitosus, or Scheuxhzeria, begin to grow. These form a 

 secondary sedge-moor which may again develop progressively into an Erio- 

 phorumr-mooT or dwarf-shrub moor, and then either to forest-moor or again 

 retrogressively to lichen-moor. This alternation repeats itself until finally 

 the progressive development ends in forest. This whole round of develop- 

 ment is rarely found over great areas uniformly, but usually the areas are very 

 diverse, progressive and retrogressive stages of development being everywhere 

 intermingled. After repeated progressive and retrogressive development, the 

 level of the peat rises, the margin of the moor turns into a Carex swamp, this 

 develops into Eriophorum and dwarf-shrub moor, and the latter invades the 

 surrounding forest. 



Ostenfeld (1899) has studied the development of vegetation about the hot 

 springs and on the lava fields of Iceland: 



In streams above the hot springs are found Potamogeton, Carex, Heleocharis, 

 Hippuris, Menyanthes, etc., but this vegetation ceases where the hot water 

 enters the brook. The chief constituent of the vegetation here is Phormidium. 

 Agrostis alba is the first plant to colonize the new soil about the solfataras. It 

 is followed by Sagina procumbens, Cerastium vulgatum, Plantago major, and 

 Stellaria media. A list is also given of species which grow only in the new 

 soils of thermal springs, those which grow rarely elsewhere, and those which 

 are abundant in both places. 



The lava-streams of Iceland represent different ages from preglacial times 

 to the present, and their vegetation is a good example of the way in which new 



