280 GRAZING INDICATORS. 



of years, and in actual practice may be regarded as permanent. The great 

 majority of them result from disturbance, however, and last for a period of a 

 few years, or at most for a decade or two, unless the disturbance is continuous 

 or recurrent. In addition, they show rapid changes of population from year 

 to year. Such communities are usually local and of small extent and have 

 resulted from fire, overgrazing, or cultivation. They belong to secondary 

 successions or subseres in contrast to the larger and more permanent com- 

 munities which constitute stages in the primary succession or prisere. These 

 distinctions apparently disappear in the case of great stretches which are kept 

 more or less permanently in the lodgepole or aspen community as a consequence 

 of repeated fires, or in the Aristida or Gutierrezia stage as a result of continued 

 overgrazing. Even here, however, the differences in the kind and rate of 

 development are of great practical value in determining the proper manage- 

 ment. As a consequence, it is desirable to distinguish serai communities as 

 indicators upon the basis of primary and secondary succession, and then to 

 deal with the indicator value of the respective dominants. Each of these is 

 known as a consocies when it is controlling, and corresponds with the con- 

 sociation among climax types. Two or more consocies regularly occur to- 

 gether to constitute a particular stage or associes, while their subdominants 

 are known as socies, which correspond with the societies of climax communi- 

 ties. A complete treatment of serai indicators is neither possible nor desirable 

 at present, but the following account will serve to illustrate all the important 

 types. 



Prisere communities as indicators. — The four great types of primary suc- 

 cession are those which start in initial bare areas of water, rock, dune-sand, or 

 saline lake or basin respectively. The initial communities and some of the 

 medial ones may be used as negative indicators, denoting that conditions 

 have not reached the point where they can support a plant cover of such 

 density or quality as to furnish grazing. The later communities, and espe- 

 cially the subclimax one that immediately precedes the climax, form a more 

 or less complete cover in which grasses or shrubs are usually in control. The 

 density of the cover and the quality of the grazing increase more or less 

 regularly from the medial stages to the climax, and the position of a particular 

 community in the sere indicates its value in a general way. 



The most important serai indicators of grazing are the later stages of the 

 priseres in dunes and sandhills, in bad lands and in salt basins. These often 

 cover many thousand square miles and frequently occur in agricultural 

 regions, where the indicator distinction between grazing and farming land is 

 especially important. In addition, there are the sedge and grass meadows 

 which are stages of the hydrosere, and are often characteristic of mountain 

 parks in the montane and subalpine zones. Grassland and scrub also develop 

 in rock fields and on talus slopes where the formation of soil is not too slow. 

 While such parks and gravel-slide areas often afford excellent grazing, they 

 are usually both local and relatively small and serve chiefly to increase the 

 grazing value of the forest areas in which they occur. 



Of all the prisere communities, those of sandhills and dunes are probably 

 the most widely distributed and most important. They have been found and 

 studied in each of the 16 Western States, where they may occur as sandhill 

 regions of large extent, as river dunes or ocean dunes. The most extensive 





