392 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Willow {Salix glauca s.l.), the Broad-leafed Willow [S. cordifoUa s.l.), 

 the Feltleaf Willow {S. alaxensis agg.), or Richardson's Willow {S. 

 richardsonii agg.). Often two or more of these shrubs will dominate 

 a mixed association. In some places bushes of Green Alder [Alnus 

 crispa agg.) are present and may be locally dominant. 



Such scrub at its best is so thickly tangled and produces so much 

 litter that few associated plants occur, apart from tall Grasses such 

 as Bluejoint [Calamagrostis canadensis agg.) and occasional straggling 

 forbs. But where the dominants are less luxuriant, an extensive 

 flora is often found, including a considerable variety of herbs and 

 Mosses, or, in dry situations, subdominant heathy plants such as 

 Crowberry [Empetnim nigrum s.l.). Also characteristic of dry scrub 

 are patches of tall Cladonias, Stereocaulons, and other Lichens, with 

 or without Polytricha or other coarse Mosses. To the north such 

 scrub thins out gradually, its most northerly expression about the 

 northern limit of the middle-arctic belt being usually in the form 

 of single or scarcely confluent bushes that rarely exceed 50 cm. in 

 height and are usually much lower, though often quite wide. 



Heathlands are more widespread and various in the Arctic than 

 is scrub, though still commonly occupying only a very small propor- 

 tion of the total area. They are usually characterized by being 

 dominated by members of the Heath family (Ericaceae) or by heath- 

 like plants such as, particularly, Crowberry. Sometimes, however, 

 broad-leafed plants such as Avens (Dryas), or Sedges such as the 

 Nard Sedge {Car ex nardina s.l). or Bellard's Kobresia {Kobresia 

 myosuroides), may dominate dry and usually exposed, lichen-rich 

 areas that are often classed as heathlands rather than among the 

 drier tundras with which they seem more properly to belong {see 

 p. 386). Leaving aside such cases it may be said that heathlands 

 in the Arctic tend to be confined to the more favourable, sheltered 

 situations that are snow-covered in winter — provided they are not 

 too moist in summer. In many regions they characterize coarse- 

 grained rather than clayey soils, as pointed out by Professor Thorvald 

 Sorensen {in litt.). 



In the low-arctic belt the heathlands are usually covered by a 

 continuous thick sward of mixed woody and herbaceous plants, the 

 main dominants being typically 8-15 cm. high. These commonly 

 mclude Crowberry, Arctic Blueberry {Vaccinium u/iginosum subsp. 

 alpimmi). Mountain Cranberry {V. vitis-idaea agg.), Arctic Bell- 

 heather {Cassiope tetragona), Narrow-leafed Labrador-tea {Ledum 

 palustre agg.), Dwarf Birch, and various diminutixe Willows. Often 



