248 SUCCESSION IN EURASIA. 



called retrogressive development; and alpine mat. The subalpine communi- 

 ties are essentially similar, though mixed for the most part with deformed 

 birch. The same author (1905) has made the following comparisons between 

 Bavarian and Lapland moors: (1) Bavarian moors show no regressive develop- 

 ment, their surface is much more convex and even, without hummocks and 

 pools, and they are much drier during the vegetative period; (2) Calluna is 

 the dominant in the Bavarian moors, but nowhere in those of Lapland, while 

 Eriophorum vaginatum, Molinia, and Rhynchospora are much more abundant 

 in the former. The further conclusion is reached that regressive development 

 (secondary succession) increases with the latitude, and usually with the 

 altitude also. 



Birger (1906 : 212) investigated the development of vegetation on the islands 

 of Lake Hjalmar, which had been produced by a fall of 1.2 m. in the water- 

 level in 1882 and of 0.7 m. in 1886: 



The vegetation of these islands had been studied by Callm6 in 1886, and by 

 Grevillius in 1892, with the result that the development had been described at 

 4, 10, and 22 years after the origin of the new areas. A detailed comparison 

 is made of the plant population of the islands at the three periods, and migra- 

 tion and ecesis are dealt with thoroughly. The changes in vegetation have 

 had to do chiefly with the disappearance through shading of the Salix zone 

 described by GreAollius, and the extension of the forests. Birger applies the 

 term pioneer to a species which invades bare soil, but does not buUd sharply 

 limited communities, and colonist to those which enter later, replace the 

 pioneers, and form characteristic and relatively constant communities. Such 

 a distinction, however, does not seem a fundamental one, as many pioneers 

 are also colonists. 



Jonsson (1905) has described the formations of southern Iceland and has 

 studied the development of vegetation on lava-fields produced by the erup- 

 tions of 1783 and 1878. The succession is shown in figure 11. 



Scrub Grassland Grassland 



Dwarf shrub heath Herb field 



t / 



Grimmia-heatb 



Mosses and Lichens 



Bare lava 



Fig. 11. — Lithosere on lava in Iceland. After Jonsson. 



Noren (1906) has foimd three zones of vegetation on the sandy shore of the 

 inland Vanemsee in Sweden: 



The wet, sandy shore is frequently overflowed, and is characterized by 

 swamp plants such as Sdrpus and Phragmites. The dry sandy shore has an 

 outer sparse zone of Salix repens, Sagina modesta (both of which give rise to 

 small dimes), Carex oederi, Viola canina, Spergvla arvensis, etc. Beyond the 

 action of the waves the vegetation is denser, consisting of so-called formations 

 of Calamagrostis neglecta, Equisetum palvstre, Polytrichum juniperinum, etc. 

 The next zone is characterized by stable dunes, 20 to 30 feet high. The chief 

 sand-binders are Carex arenaria, Calamagrostis neglecta, and C. epigaea, while 

 Epildbium angustifolium and Rumex acetosella are regarded as important in 

 this connection. 



