tions usually indicate the degree of stabilization. Thus there is a 

 mosaic of climax types which corresponds to the mosaic of habitats 

 or environmental gradients. ^^^ 



An example of a mosaic of climax types is the dwarf shrub- 

 heath in Sikilsdalen, Norway, which was divided into two 

 alliances differing in relative species composition and abundance 

 chiefly in response to the amount and duration of snow cover in 

 the habitat.^5' The more xeric of the alliances has 7 associations, 

 of which one, the Empetreto-Betuletum nanae, is an example of 

 a pattern in relation to environmental gradients. This association 

 is separated into 4 sociations, chiefly on the basis of differences in 

 the proportion of several species of lichens; Betula nana, the chief 

 vascular dominant, is found in about equal abundance in each 

 sociation. The Betula nana-Alectona ochroleuca sociation occurs in 

 sites that have the least snow cover, dry most rapidly in the spring, 

 and are so exposed to the wind that pieces of the lichen mass are 

 torn loose. It is characterized by high frequency and cover of the 

 lichen, Alectoria ochroleuca. There are also other wind-resistant 

 lichens. The second community, the Betula nana-Empetrum herma- 

 phroditum-Cetraria nivalis sociation, often alternating with the 



Figure 5-8A. Roots of blue 

 gramagrass are very numer- 

 ous here, some extending to 

 a depth of 4 ft. This short- 

 grass is well adapted to 

 drought and grazing, and is 

 a component of a number 

 of climax communities. 

 Willcox, Arizona. (U.S.D.A. 

 Soil Conservation Service.) 



