390 INTRODUCTION TO PLANT GEOGRAPHY 



So far as regular ecological successions are concerned, these are 

 especially problematical in the Arctic. It has, however, been sug- 

 gested that the marshy and dry tundras may be subclimax and the 

 mesophytic ones climax or preclimax, the scrub and heathlands, 

 which are developed in the most favourable situations (see below), 

 being either postclimax or, perhaps, indicative of a more general 

 climax to be expected ultimately in sufficiently favourable situations, 

 though at present a mixed ' polyclimax ' is commonly found. The 

 significance of difi:erent ' stages ' in the hypothetical successions 

 may, however, vary from place to place. Thus, in the Far North, 

 heathy plants are apt to be so restricted to the most favourable 

 situations as to suggest that without major climatic change they 

 could not become widely dominant in the manner already obtaining 

 in some places in the low-Arctic. Moreover, frost and other dis- 

 turbance is so widespread, inter alia impeding or even preventing 

 the maturation of soils, that it seems as though many areas undergo 

 a kind of perpetual readjustment rather than exhibit the tendency 

 to equilibrium which is implicit in a real climax. 



Arctic Scrub and Heathlands 



A shaggy scrub of Willows and , or Birches is commonly developed 

 on the most favourable slopes, in damp depressions, and especially 

 along watercourses and the margins of lakes in low-arctic regions. 

 It is commonly around 60 cm. (about 2 feet) high, as in the example 

 shown in Fig. 115, but tends to become lower and more restricted 

 northwards until, about the centre of the middle-arctic belt, it 

 becomes usually very limited in extent and stature. However, in 

 the most favourable situations in the extreme south the Willows 

 may be luxuriant (c/. Fig. 116) and even exceed the height of a 

 Man, and especially in southwestern Greenland the scrub is quite 

 extensively developed, in some places including arborescent Birches. 

 These Greenland Birch ' forests ' are of very limited extent, with 

 the trees scattered and scraggy though sometimes nearly 6 metres 

 in height and 25 cm. in stem diameter. Their areas have been 

 termed subarctic but seem too limited to separate on an over-all, 

 world basis ; they are also too fickle, the development of an arbores- 

 cent habit being evidently dependent on local shelter, etc. Apart 

 from these larger Birches, the main dominants in different regions 

 are most often the Dwarf Birch {Betula nana agg.) or Scrub Birch 

 {B. glandulosa agg.), or such shrubby Willows as the Glaucous 



