RUSSIA. 271 



Steenstrup, in which aspen was followed successively by pine, oak, alder, and 

 beech, and points out that this series corresponds almost exactly to the 

 increasing ability of the species to grow in the shade. The oak forests of 

 middle Russia have arisen by the invasion of treeless areas. The oak at first 

 invades singly in the steppe scrub, but reproduces readily and gives to the 

 steppe the appearance of an oak woodland with scrub. From these develop 

 small oak forests which spread over extensive areas and form an uninterrupted 

 one between the steppe and the coniferous forest. The oak forests then are 

 to be regarded as a temporary association, which will yield in the competition 

 with the fir and the spruce. A thousand years ago the area of the oak forests 

 was occupied by the steppe. This change in vegetation has taken place with- 

 out any corresponding change in climate, just as will be true of the replacement 

 of the oak forests by forests of spruce and fir. Thus, it is probable that the 

 Russian steppes, like the prairies of North America, furnish a striking example 

 of potential succession. 



Fleroff (1898) has made a detailed comparative study of the plant population 

 in the diverse areas of a morass or swamp in Russia: 



The largest portion of the swamp develops into birch swamp, fir moor, and 

 Sphagnum moor. The latter are relatively recent and their development can 

 be readily followed. Phragmites, Menyanthes, and Equisetum limosum buUd 

 hmnmocks in the Sphagnetum; these are covered with swamp-plants, which 

 continue the accumiilation until firs and birches can enter. Reed-grasses also 

 develop hummocks in fir moors, the Sphagnum disappears, and the fir moor 

 becomes a reed-grass swamp. No development of birch swamps and reed- 

 grass swamps into Sphagnum moor was seen. 



Pohle (1901), in a study of the vegetation of the peninsula of Kanin, con- 

 cludes that there are three formations that can gain a foothold on deforested 

 soil: tundra moors, arctic heath and Saliceta: 



Heath occurs only where there are very dry sandy knolls in exposed situa- 

 tions. The course of the transformation is the following: As soon as an 

 opening occurs, the mosses and typical forest plants, with the exception of 

 Deschampsia flexuosa, disappear. The latter takes the ground and is associ- 

 ated with Carex canescens and Arctostaphylus alpina, plants which retain 

 enough moisture in the soil to permit of good conditions for the existence of 

 swamp plants. In the further course of events, Eriophorum vaginatum and 

 Rvbus chamxiemorus appear, and determine the fate of the former forest 

 portion. These plants form a peaty substratum and enable Sphagna to enter. 

 Thus a normal tundra moor arises. The formation of a Salicetum is possible 

 when tundra moor occurs upon the hilly periphery of an island or on the slope 

 of some elevation. The trickling moor-water causes the destruction of the 

 trees, since good drainage is one of the prime essentials for the existence of 

 trees in the tundra region. Thus it results that forest islands are to be found 

 always on rolling groimd, hills, and smnmits, not in protected valleys and 

 depressions. 



Cajander (1903) has studied in detail the succesions on alluvial soil in the 

 lower valley of the Lena River in Siberia and has established four classes of 

 associations: (1) hydrophytic, (2) woodland, (3) grassland, (4) moss and lichen: 



Each of these is divided into different "series," which are obviously only 

 portions or sections of a complete succession. Cajander (1905) has also 

 described five "series of associations" on the flood-plain of the Onega River: 



