if the transition is broad and not well defined, the environmental 

 gradient must be of low magnitude. 



The snow-field pattern of vegetation in relation to environ- 

 mental gradients is common in arctic and alpine areas. The en- 

 vironmental gradients are formed by variations in the depth of 

 snow, duration of snow cover, exposure to wind during winter 

 and summer, length of the growing season, amount and duration 

 of soil moisture, and winter temperatures in the soil, air, and 

 snow cover (see Charts 1 and 2, pp. 18 and 19). The common 

 zonation on lime-poor soil in Norway along the gradients from 

 the exposed top of a ridge to the base of a slope is as follows: 

 Empetrum hermaphroditum-Louiseleuria procumbens community, snow 

 cover very thin or not present, exposed to strong winds; Betula 

 nana community, snow cover thin, less wind; Vaccinium myrtillus 

 community, complete snow cover which melts relatively early; 

 Deschampsia Jiexuosa community, deep snow cover that melts late; 

 and Salix herbacea community, very deep snow cover that melts 

 very late or not at all during some years. ^^•^'^ 



There may also be spatial changes within a community. These 

 changes are likewise caused by significant environmental gra- 

 dients. The gradients or microgradients within the community 

 are in some cases as great or greater in magnitude than the gra- 

 dient between two communities. The differences in the Calluna, 

 Cladoma, bare soil, and Arctostaphylos phases of the dwarf Callune- 

 tum community described by Watt^^^ may indeed be greater 

 than the difference between this community and the adjacent 

 community. The magnitude of microgradients within each com- 

 munity is probably distinctive for the respective community. 



If the gradients remain unchanged for long periods of time, the 

 mosaic of communities remains unchanged and each community 

 thereon is in the steady-state. However, if the environment of 

 one sector or of all the gradient is changing rather rapidly, then 

 there will be directional change taking place in one or more 

 communities on such areas, so that the mosaic is changing. 



During a long period of time the actual spatial position of the 

 environmental gradients may change very slowly. This may be 

 due to slow-acting geological processes, among which are normal 



Habitat Patterns, Changres, and Climax • 157 



