13] VEGETATIONAL TYPES OF POLAR LANDS 3S3 



arborescent growth — except where shrubs or undershrubs pre- 

 dominate (in scrub and heathlands), or vegetation covers less than 

 half of the area (in ' fell-fields ' and ' barrens '). Although this still 

 includes such special cases as salt-marshes and manured areas, it 

 is customarv to consider these separately, as is done in the present 

 work. On the other hand, it excludes the taiga and at least the 

 forested parts of the mixed ' forest-tundra ' described in the last 

 chapter (p. 348). Generally comparable types occur in antarctic 

 regions where, however, suitable land areas are relatively small. 

 Alpine tundra bears a similar relationship to the timber-line on 

 mountains. Instead of true Grasses which, however, are rarely 

 absent, grass-like plants such as Sedges {Carex spp.), Cotton-grasses 

 {Eriophorum spp.), Rushes {jfuncus spp.), and Wood-rushes {Luziila 

 spp.) commonly afford most of the ' grassiness ' of the tundra, 

 though various perennial forbs are usually associated, as are often 

 a sprinkling of dwarf woody plants. 



Even in this restricted sense the tundra developed in almost any 

 arctic region is usually very variable, different areas supporting 

 widely different types. The variation takes place particularly with 

 differences in exposure and in water and other soil conditions, and, 

 at all events in low- and middle-arctic regions, affords faciations 

 far too numerous even to mention here. We may, however, dis- 

 tinguish and outline, besides a general central type, the tundras of 

 damper depressions on the one hand and of drier exposed areas on 

 the other. 



The general run of tundra which covers a large proportion of the 

 lowland plains and some less extensive upland areas of most low- 

 arctic regions is commonly a rather thin ' grassy ' sward dominated 

 by mesophytic Sedges such as the Rigid Sedge {Carex bigelowii agg.) 

 and Grasses such as the Arctic Meadow-grass {Poa arctica s.l.), with 

 various associated forbs and under-shrubs including dwarf Willows. 

 The whole forms a continuous if often poor sward commonly 15- 

 35 cm. (approximately 6-14 in.) high in which a mixture of various 

 Bryophytes and Lichens usually forms a rather poorly marked second 

 layer a very few centimetres high. 



Commonly the low-arctic tundra is a mosaic made up of faciations 

 having each some lesser number of the total association dominants, 

 and including consociations having only one of these. The areas 

 of the component communities are often small and the variation 

 from spot to spot in the tundra is accordingly usually considerable. 

 In addition there are often local societies dominated by species other 



