and habitat changes that may be called a cyclic system, prob- 

 ably first described in detail by Watt.^^^ He was particularly in- 

 terested in determining how the community maintains and re- 

 generates itself. In each of the seven communities that he 



Calluna vulgaris 



Arctostaphylos uva-ursi 



Cladonia sylvatica 



Bare soil 



Chart 6. A cyclic system on the dwarf Calluna vulgaris community in the 

 Cairngorms, Scotland. (After Watt, A. S. (197), "Pattern and Process in the 

 Plant Community," Figure 2, Jour. Ecol, 35, 1-22 (1947).) 



described, the different kinds of patches are related to one an- 

 other, and there is orderliness in the changes in an upgrade series 

 and in a downgrade series. One of these is the dwarf Callunetum 

 community in the Cairngorms (see Chart 6). Calluna vulgaris is at 

 the peak of the upgrade series, and after its death in a patch 

 Cladonia silvatica becomes dominant on the Calluna stems if pro- 

 tected, otherwise Cladonia is present but not dominant. Then 

 Cladonia disintegrates, and bare soil is exposed, with some re- 

 maining Calluna stems, terminating the downgrade series. Arcto- 

 staphylos uva-ursi initiates the upgrade series by invading the bare 

 area and completely occupying it. Then Calluna invades the 



Habitat Patterxis, Chazmges, ax&dl Climax 



143 



