244 SUCCESSION IN EURASIA. 



The formations, so-called, are partly associes and partly consocies. The 

 communities of dry and moist habitats are also given in such way that the 

 succession may be inferred, but the sequence is not explicitly stated. 



Nilsson (1897) has analyzed the vegetation of the Swedish myrs in Norr- 

 botten into: 



(1) Reed-grass swamps, consocies of Car ex ampullacea, Scirfms caespitosus, 

 Eriophorum angustifolium, E. scheuchzeri, Molinia, etc. ; (2) reed-grass moors, 

 differing from the preceding in the presence of Sphagnum; (3) Eriophorum 

 moors, with Sphagnum; (4) shrub moors of Andromeda polifolia, Betula nana, 

 Myrtillus uliginosa, Oxycoccus palustris, and Rubus chamaemorus. In the 

 great myrs, the first two communities are dominant, Eriophorum moor sub- 

 ordinate, and the shrub moor a marginal zone. Spruce and pine frequently 

 occur in the myr. 



The myrs of Norrbotten have arisen partly through the filling up of lakes, 

 partly through the change of forest into swamp. In the first case, the suc- 

 cession is reed-grass swamp, reed-grass moor, cotton-grass moor, and shrub 

 moor, or the third stage may be lacking. The development is from hydrophilous 

 to increasingly xerophilous communities. The change of forest to swamp is 

 brought about by the spread of Sphagnum and Polytrichum from the myr into 

 the forest. These play a passive r61e in holding water above the forest floor, 

 or the water level of the forest soil may also be raised in consequence of the 

 filUng up of lakes or ponds, or by interference with the drainage of the myr. 

 The author has found a stump layer in a few cases which ran continuously 

 from beneath the peat into the trees of adjacent swamp forests. The active 

 work of the mosses is to hold dew and rain water so tenaciously that Sphagnum 

 hummocks may rise 0.6 m. above the forest floor. The larger myrs are regarded 

 as produced first by the filling of lakes and ponds, after which these areas are 

 connected by the change of woodland to moor. In all such cases it would 

 seem that the accumulation of the run-off in consequence of the filling of the 

 drainage basin or channel is the basic cause of the swamping of the forest. 

 As is obvious from the above, the author does not find Blytt's theory of dry 

 and wet periods necessary to explain the appearance of stiunp layers in peat- 

 beds, though he admits the important influence of climate upon the rate of 

 increase in the peat. Nilsson (1897) has also described the following forma- 

 tions in northern Sweden: 1. Birch region: (1) Betuleta cladinosa, (2) Betuleta 

 hylocomiosa, (3) Betuleta herbida; 2. Coniferous region: (1) Pineta cladinosa, 

 (2) Pineta cladino-hylocomiosa, (3) Abiegna hylocomiosa, (4) Abiegna graminosa, 

 and (5) swamp forest. After fire in spruce forest, Deschampsia flexuosa 

 quickly appears in great abundance, often with many other herbs. This is 

 followed as a rule by birch forest in which the spruce appears only occasionally 

 in the first generation, but becomes again predominant in 200 to 300 years. 

 In other cases the burn is covered with a dense carpet of Polytrichum or of 

 Polytrichum and Sphagnum, which probably marks the beginning of the 

 development of a swamp. 



Warming (1897) has described the following conununities about Lake 

 Skarrid in Denmark: 



(1) Plankton, (2) bottom vegetation, (3) Limnaean, (4) swamp, (5) meadow 

 moor, (6) meadow, (7) alder swamp, (8) beech forest. He has considered in 

 detail the vegetative relations of the members of the different conununities 

 with particular reference to their succession. This is well illustrated by the 

 species of the reed swamp, Typha, Sdrpus, and Phragmites. They stand 0.5 



