Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 333 



many arctic and alpine regions ; (9) discontinuous ' fell-fields ' and 

 sparser ' barrens ', etc., characterizing still more frigid regions ; (10) 

 grassland, of various types dominated by Grasses and grass-like 

 plants such as Sedges, often with scattered trees or shrubs forming 

 a savanna; (11) semi-desert scrub; (12) desert, with scanty but 

 characteristic vegetation ; (13) mangrove ; (14) salt-marsh, which 

 like some mangrove seems capable of persisting in the absence of 

 disturbance ; (15) benthos, of submerged bottom aquatics ; (16) 

 plankton of free-floating Algae, etc., including those of snow and 

 ice ; and (17) the edaphon or soil communities including numerous 

 Algae, Fungi, and Bacteria. Most of these are major, climatically 

 determined vegetation-types {formations) of each of which various 

 different ' aspects ' exist. Several are, however, apt to be serai in 

 some instances — as are, of course, the many recognized stages in 

 successions, such as the moss stage in the lithosere and the reed- 

 swamp and bog or fen stages in the hydrosere. But strictly speaking 

 a formation should represent the local climax. Examples of most 

 of the above types and of some other (usually serai) ones, such as 

 various swamps and marshes, are described (and often illustrated) 

 in the next five chapters, which deal with the outstanding vegetational 

 features of the world. 



Before describing the vegetational types of different regions and 

 media, we must outline, in descending order of ecological status, the 

 main classificatory units (eca) of vegetation which it seems practicable 

 to recognize : 



(i) Formations. These are the great climatic units or regional 

 climaxes such as desert, semi-desert scrub, tvmdra, deciduous forest, 

 coniferous forest, broad-leafed evergreen forest, and some others, 

 such as many heaths and grasslands which are determined par- 

 ticularlv bv edaphic or biotic conditions but are so distinctive as to 

 rank as formations. Each formation usually covers a wide area in- 

 volving various conditions and so consists of more or less numerous 



(2) Associations. These are climax units dominated by normally 

 more than one species having the life-form characterizing the for- 

 mation to which their association belongs. An association exists 

 under broadlv uniform habitat conditions and is uniform in type so 

 far as the general characters of the dominants and main associates 

 are concerned. Such units become aggregated regionally to consti- 

 tute formations. Examples of associations include various of the 

 mixed deciduous forests of Old and New England, such as an 

 Oak-Beech association. The developmental counterpart of the 



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