242 SUCCESSION IN EURASIA. 



of Alnus incana; (3) on higher areas flooded but occasionally, sedges and 

 rushes yield to herbs, and especially to dense stands of Salix; (4) Alnus incana 

 replaces the wUlows, and builds a closed consocies; (5) the spruce replaces the 

 alder, or more rarely, mixed stands of pine and spruce, or pine alone. 



Henning (1895) has described the reproduction of forest trees in Sweden 

 with especial reference to their succession: 



The pine reproduces itself as a rule in nearly all soils except in swamp 

 meadows and similar moist areas. Reproduction is, moreover, hindered by 

 too dense a stand. In all habitats, except the moor, the pine is replaced by 

 spruce. The reproduction of the spruce takes place with difficulty in dense 

 communities of Polypodium alpestre, of Empetrum or in dense grass communi- 

 ties, as well as in dense stands of the tree itself. The birch, which is the 

 common invader on denuded areas, reproduces readily as a rule. After a 

 while reproduction is hindered by increasing shade and the lower branches 

 die and disappear. This permits reproduction again unless the spruce invades 

 and produces a mixed forest. 



In burns the birch is commonly the first invader, occasionally the aspen, 

 and rarely the gray alder. In one case a mixed forest of coniferous trees 

 appears directly without an intervening deciduous forest stage. In some 

 burns, even after a long period, there was no development of a climax forest. 

 The groxmd vegetation is very complex, but Epilobium angustifolium, Des- 

 champsia flexuosa, D. caespitosa, Agrostis vulgaris, and Vaccinium vitis-ddaea 

 are the most characteristic. Treeless areas ivithin the forest are often due to 

 snow-drifts. In the development of moors, Carex ampullacea, C. limosa, and 

 Sphagnum are often the first invaders at the margin of stagnant water. Scir- 

 pus caespitosus and Eriophorum vaginatum follow soon and begin the develop- 

 ment of himamocks. Upon these are later found dwarf shrubs, Hypnum, and 

 Cladonia, while the intervening hollows are taken by species of Carex. The 

 cyperaceous communities are commonly suppressed by the increasing thick- 

 ness of the Sphagnum hillocks. The moors are often sooner or later clothed 

 with pines, which establish themselves only on the hillocks if the intervals 

 are periodically covered with water. 



Nilsson (1895), in a study of the forests of northern Sweden, has empha- 

 sized the fact that too Uttle attention has been paid in the past to the changes 

 in the soil brought about by plants during the course of succession. He points 

 out, moreover, that complete equilibrium between the different members of a 

 conununity never occurs, and that in consequence no formation can be called 

 absolutely closed. He has distinguished the following types of coniferous for- 

 ests: 



Pine heath (Pineta cladinosa) has an undergrowth chiefly of Cladonia rangif- 

 erina and Calluna vulgaris. In certain regions, reproduction is hindered by 

 the thickness of the hchen covering, and the pine heath changes gradually 

 into lichen heath {Cladineta ericosa). In the region studied, however, this 

 type is slowly changed into a transition forest (Pineta cladino-hylocomiosa), 

 distinguished chiefly by the almost equal abundance of mosses and Uchens. 

 This second type passes readily into the third, the mossy pine forest {Pineta 

 hylocomiosa), characterized by an almost closed moss cover. Such pine 

 forests have often arisen from birch forests. After two or three generations 

 they are converted into a mixed coniferous forest {Pineta^abiegna hylocomiosa) 

 which is finally changed into the mossy spruce forest (Abiegna hylocomiosa). 

 This type reproduces only with difficulty and it consequently becomes grad- 

 ually more open, favoring the development of grasses and herbs. This leads 

 to the grassy spruce forest {Abiegna graminosa). The reproduction in this 



