238 THE STUDY OF PLANT COMMUNITIES • Chapter X 



high proportion of grasses and sedges. Even the woody plants, in- 

 cluding willows and birches, are prostrate. The herbs are mostly 

 perennial and of a rosette type, producing relatively large flowers, 

 often with conspicuous colors. A4osses and lichens may grow any- 

 where and in favorable habitats form a thick carpet with the low 

 herbs. The number of species is small compared with floras of 

 temperate climates, and, even within the tundra, the number de- 

 creases northward. Most of the genera and numerous species are 

 to be found throughout the Northern Hemisphere wherever 

 tundra occurs. 



The uniformity of the flora is undoubtedly related to the pe- 

 culiarities of environment. The growing season is short and its 

 temperatures are relatively low. The depth to which soil thaws 

 in summer is of great importance. Light is continuous throughout 

 the growing season in the arctic, and is intense and high in ultra- 

 violet rays in alpine habitats. Precipitation is largely in the form 

 of snow and varies greatly. Drying summer winds, which are 

 characteristic, produce high rates of evaporation and transpira- 

 tion. As a result, water is often a critical factor, especially inland 

 away from moist coasts. Local marked differences in vegetation 

 are commonly related to minor variations in topography and the 

 differences they produce in drainage and retention of snow. The 

 poor, haphazard drainage associated with new topography is ap- 

 parent everywhere. 



Arctic Tundra— Although the flora of the tundra is fairly well 

 known, its communities and their successional relationships have 

 not been sufficiently studied. 20 * In contrast with temperate vege- 

 tation, many species may occur in any type of habitat, and several 

 that appear to be climax may also be pioneers in the newest of 

 habitats. Even climax is not agreed upon, possibly because observa- 

 tions have been made at widely separated points. Interpreted in 

 terms of Greenland vegetation, Cassiope heath appears to be cli- 

 max, and a Sedge-Dryas dominated community, of equal extent 

 but on drier sites, is preclimax. 185 Two subclimaxes are frequent. 

 Any habitat with sufficient moisture, whether it be pond margin, 

 seepage area, or boggy ground, eventually is covered with a thick 

 moss mat supporting several herbs of which cotton grass (Eri- 

 ophorum spp.) is most conspicuous. Xerarch succession on rock 



