13] VEGETATIONAL TYPES OF POLAR LANDS 387 



while \ery locally the effect of frost action may be paramount. 

 'I'hus differences in substratum, as between limestone and acid- 

 weathering rock, can introduce vegetational differences due to 

 particular plants' preferences quite apart from water-relations, while, 

 as an example of the entry of another factor, heavy pasturing can 

 lead to increased grassiness as in temperate regions. In addition, 

 polygon-formation and solifluction may cause persistent disturbance. 

 It may be noted that, whereas the dominants are usually at least 

 specifically distinct in different types of low-arctic tundra, some of 

 the less exacting, more tolerant associates may be present in a wide 

 range of habitat types. This again is comparable wath the situation 

 in cool-temperate regions and, it often seems, obtains still more 

 forcibly to the north. Thus in the Far North some of the hardier 

 plants, such as Viviparous Knotweed and some of the Saxifrages, 

 grow in an extraordinarily w'ide variety of habitats, ranging from wet 

 to dry, exposed to sheltered, and open-soil to vegetationally ' closed '. 



The middle-arctic belt is characterized by tundras of a generally 

 poorer type, both in the matter of flora and luxuriance of develop- 

 ment, than the low-arctic ones. Thus some of the plants which 

 were important in low-arctic tundras are absent, though all of the 

 dominants, etc., mentioned above for low-arctic tundras can, and 

 frequently do, occupy a similar posi*:ion also in middle-arctic regions. 

 Moreover the range of types is much the same, damp, mesophytic, 

 and drier ones being distinguishable. An example of the second, 

 dominated by mixed Grasses and Sedges, in northernmost Alaska 

 overlooking the Arctic Ocean, is shown in Fig. 112, from which it 

 may be seen that growth tends to be lower and poorer than in low- 

 arctic regions, though this particular area is only just middle-arctic 

 in type. 



In high-arctic regions still further depauperation is general, and 

 indeed only limited and relatively few areas are sufficiently vegetated 

 to be designated as tundra. These areas are chiefly marshy ones 

 and may be still dominated by Sedges, Cotton-grasses, and Grasses 

 — often of the same species as in the South, and including similar 

 associated forbs, though woody plants apart from prostrate Willows 

 are usually absent. Mosses commonly consolidate the whole, and 

 in some places appear to dominate. Fig. 113 shows an unusually 

 extensive area of marshy tundra in Spitsbergen, characterized by 

 peaty hummocks up to 25 cm. high, and of a type commonly termed 

 ' hillock tundra '. While the main dominants in such areas are 

 commonly Sedges and Grasses growing on the sides or tops of the 



