112 CUMAX FORMATIONS OF WESTERN NORTH AMERICA. 



and by a long series of stages leading very gradually to the climax. Such 

 priaeres are found in lakes, ponds, and streams, and on rock cliffs, ridges, lava 

 flows, and cinder cones. They usually occur also in salt marshes and basins, 

 and in shifting dune-sand, both of which regularly afford extreme conditions 

 for colonization, in spite of the presence of a soil. A secondary area is one in 

 which an existing vegetation has been destroyed or removed without destroy- 

 ing the soil. Its water relations are never extreme, and a large number of 

 herbs or shrubs can invade in the first few years, often indeed during the first 

 year. The secondary succession or subsere which results is short, consists of 

 relatively few stages, and passes rapidly into a climax. Subseres are the most 

 conspicuous and easily understood of all successions. Since they are largely 

 due to human disturbance, they are most abundant in settled regions and 

 hence are of the most immediate practical importance. 



The nature of the succession in both priseres and subseres is further deter- 

 mined by the water relations of the bare areas. This is best illustrated by the 

 prisere, which may begin in water or on rock. In the first case, the reactions 

 of the successive communities are chiefly concerned with reducing the amount 

 of water and increasing the amount of solid material. In the second case just 

 the reverse is true. The amount of water is increased and the rock is broken 

 down into actual soil. The one begins with submerged aquatics of the highest 

 water requirements, the other with the rock lichens of the lowest water 

 requirements. The former is called a hydrosere, the latter a xerosere. Sub- 

 seres are similarly divided, since they regularly begin in conditions wetter or 

 drier than the final climax. It is further desirable, especially for indicator 

 purposes, to recognize hydroseres in which the lack of oxygen is a critical 

 factor, and xeroseres in which the abundance of alkali or the instability of the 

 sand is decisive. For the sake of convenience, these are called respectively 

 oxysere, halosere, and psammosere (Clements, 1916 : 182). 



Indicator value of disturbed areas. As has already been suggested, the most 

 usable of all successions are subseres, which occur typically in areas disturbed 

 by man or as a result of his activities. Even a relatively new country, such 

 as ours, has been the seat of widespread and almost universal disturbance. 

 Arable lands have been cleared, broken, cultivated, permitted to lie fallow or 

 to "go back." Forests have been lumbered, burned, or grazed, while grass- 

 lands and deserts have been constantly grazed and burned. Even in the most 

 inaccessible parts of the West it is difficult to find wholly primitive conditions, 

 even though by comparison most of the vegetation may fairly be called natural. 

 As a consequence, practically all regions show many areas of disturbance 

 marked by secondary successions. These furnish an enormous amount of 

 indicator material, which only needs interpretation in the light of successional 

 knowledge to be of the greatest practical importance. Every burn, every 

 clearing, every pasture or open range, each fallow field, irrigation ditch, 

 roadside, or railway fill or cut, in fact every place of whatever size from a 

 square foot to a township, in which the soil has been disturbed or removed, 

 has indicator evidence of value to offer. Indeed, the problem is often to find 

 primitive areas for determining the original conditions of the vegetation and 

 thus permitting a proper correlation of the subsere. As long ago as 1898, a 

 systematic search was made in several counties of eastern Nebraska for 

 prairie that had never been pastured or mowed. Only an insignificant rocky 



