The Vegetation: Pattern and Succession 217 



movement within the soil is dependent upon water movement, the hydro- 

 phobic nature of oil-contaminated soil may decrease nutrient as well as 

 water availability to plants. 



SUMMARY 



Within the prevailing coastal tundra macroclimate, the topographic 

 position of a site causes the variation in environmental factors which in 

 turn control the growth and distribution over the tundra surface of dif- 

 ferent plant species and growth forms. The complexes of controlling en- 

 vironmental factors were identified by indirect ordination. They are, in 

 order of importance in explaining the overall variation of the vegetation, 

 the complexes of soil moisture, soil anaerobicity, soil phosphate, and 

 snow cover. The distribution of vegetation types, plant growth forms, 

 and various vegetation and growth form attributes such as standing crop 

 of above- and belowground material, productivity, foliage area, turn- 

 over rates, and diversity are described in terms of the controlling en- 

 vironmental complexes. 



The principal growth forms recognized at Barrow are single gramin- 

 oids, erect forbs, deciduous shrubs, and bryophytes. Others that may be 

 locally abundant are caespitose graminoids; cushion, mat and rosette 

 forbs; evergreen shrubs; and lichens. 



Dry, exposed sites with little snow cover usually contain a pre- 

 ponderance of caespitose graminoids and lichens. They may also have 

 evergreen and deciduous shrubs and cushion forbs. Mesic sites tend to 

 have the most forbs but have an abundance of bryophytes and deciduous 

 shrubs. Single graminoids are abundant over much of the tundra and are 

 dominant in moist and wet sites along with bryophytes. 



The distribution of plant growth forms within the tundra is related 

 to structural and functional characteristics, such as leaf longevity, timing 

 of leaf growth, location of perennating organs, location of stored nutri- 

 ents, leaf resistance to water loss, photosynthetic rates, carbon and nutri- 

 ent costs of making new leaf material, and the location of absorbing 

 roots; to the availability or abundance of wind, water, light energy, heat, 

 and inorganic nutrients; and to the influence of grazing animals. 



A hypothetical scheme is presented for the major paths of plant suc- 

 cession in the Biome research area. The scheme is based on the thaw lake 

 cycle, and the major vegetation types are seen as phases in the cycle. 

 Plant colonization of stable surfaces may take only a few years, but the 

 major vegetation types are generally long-lasting and change primarily in 

 response to physical changes, such as microrelief and drainage, rather 

 than in response to biological changes, as a result of competition. 



Most human impacts upon tundra are within the range of natural 



