increasing elevation from Point Barrow to the Brooks Range in 

 Alaska. Some gradients are on a large scale, such as this one in 

 Alaska, and may be divided into sections. The major gradient in 

 Spitzbergen, climatic in nature, v^^as divided into four parts, based 

 on the kinds and relative abundance of species, namely the 

 barren, Dryas, Cassiope, and Empetrum zones. This gradient also is 

 found, on a broader scale, in the Spitzbergen Archipelago, Green- 

 land, arctic Canada, and in the mountains of Scandinavia.'^'' 



Gradients may occur on a medium scale, such as those formed 

 in small mountain valleys in arctic and subarctic regions where 

 snow accumulates during the winter. From the bottom of the 

 valleys to the tops of the ridges changes occur in the depth and 

 time of melting of the snow cover (see Charts 1 and 2, pp. 18 

 and 19), in the exposure to wind, especially in winter, and in 

 insolation, temperature, availability of water, and solifluction.'^' 



Gradients ("microgradients," "intrastand gradients," "micro- 

 community gradients" ) are also present within very small areas. 

 These may be caused by variations in microrelief, in texture or 

 organic content of the soil, in flow of surface water, or in other 

 conditions; and are especially evident in the Arctic and Sub- 

 arctic in frost scars, polygons, peat rings, stone stripes, tundra 

 mounds, and solifluction terraces. An example will be given from 

 Norwegian Lapland where five distinct kinds of habitats and 

 vegetation types or phases occur within an area of a few square 

 meters on dolomite slopes. ^^ fhg f^j-st phase is characterized by 

 barren, shallow mineral soil, occupied by many plants that are 

 intolerant of competition, such as Draba incana, Dryas octopetala, 

 Festuca ovina, and Saxifraga oppositifolia. Peat begins to accumu- 

 late, making possible the invasion of plants requiring a more 

 organic substratum — the second phase. The vegetation then be- 

 comes closed and comprises the dominants Carex rupestris and 

 Dryas octopetala, with Arctostaphylos alpina and ^. uva-ursi promi- 

 nent in the secondary group. The third phase, with deeper peat, 

 has Empetrum hermaphroditum and Vaccimum uliginosum as domi- 

 nants; and in the fourth phase, where considerable decomposed 

 organic matter is present, lichens, especially Cladoma spp., are 

 prominent. Erosion apparently begins and continues through the 



Habitat Patterns, CHanges, and Climax • 139 



