92 KINDS OF INDICATORS. 



climatic and physiographic cycles, and those of secondary processes such as 

 superficial disturbances which result in denudation merely, whether produced 

 by man or other agencies. The major secondary processes are fire, lumbering, 

 cultivation, grazing, engineering operations which involve cutting or filling, 

 irrigation, drainage, and superficial erosion and deposition due to natural 

 agencies. These are all alike in that they initiate secondary seres, but they 

 differ sufficiently in detail to be characterized by more or less distinctive 

 indicators. This is so true of some that it is possible to distinguish different 

 kinds of cultivation, grazing, etc., by means of their indicators. 



Process indicators serve not only to denote the kind of process, as well as 

 certain variations in it, but they can also be used to approximate the time of 

 origin and the rate of movement. This is the natural outcome of the sequence 

 of stages in sere and habitat which marks succession. Moreover, they possess 

 the further advantage peculiar to all successional dominants of indicating 

 communities and conditions which have preceded, as well as those which are 

 to follow, including the final climax. As already indicated, this is often of the 

 greatest practical value in enabling one to restore an earlier condition or com- 

 munity, to hasten a later one, or to hold the succession in the stage desired. 

 Accurate determinations of the rate of progress can be made only by the use 

 of permanent quadrats, but it can often be closely approximated, in woody 

 communities especially, by ascertaining the age of the dominants in relation 

 to the life-history. 



Fire indicators. — While fire has some points in common with other agencies 

 which cause denudation, it differs especially in its action upon the surface 

 soil and in the more or less complete destruction of plants and germules, as 

 well as in the fact that the soil is not actually disturbed. These differences are 

 reflected in the large number of indicators either peculiar to it or more typical 

 of it than of other processes. Certain vegetation-forms appear to owe their 

 character or at least their dominance to fire. This is particularly true of 

 scrub, where the form and consequently the dominance are due to the root- 

 sprouts produced after fire. This relation is practically universal in the 

 Coastal chaparral, and explains the greater massiveness of this association in 

 comparison with the other scrub communities. It is general in the Petran 

 chaparral and the desert scrub, and is poorly developed only in the Basin 

 sagebrush. The response to fire is typical of the subclimax chaparral in Cali- 

 fornia and Oregon, as well as of that which occurs in the prairies of the Middle 

 West. The bush or scrub type is a characteristic fire indicator in forest cli- 

 maxes the world over, and throughout the northern hemisphere it often con- 

 sists of the same genera and even species (plate 19, a). 



Fire has played a similar role in making certain genera and species of trees 

 almost universal indicators of its action. The best known examples are 

 found in Populus and Betula. Populus tremuloides and Betula papyrifera are 

 the characteristic indicators of fire in forest communities throughout boreal 

 North America, as well as in many mountain regions. They owe this to their 

 ability to form root-sprouts, and the trees often or regularly consist of several 

 stems in consequence. In the Old World, corresponding species of the same 

 genera play a similar part. A second striking group of indicators is found 

 among the conifers, and especially the pines. The latter are characterized 

 by cones which may remain closed upon the branches for many years, but open 



