PROCESS INDICATORS. 95 



A considerable number of cultivation indicators are also indicators of over- 

 grazing. This is explained by their common relation to disturbance. In the 

 case of cultivation, the disturbance is much greater and usually operates in a 

 shorter time. The disturbance produced by overgrazing is gradual and 

 accumulative, and requires several years or more to attain definite expression. 

 In the case of breaking and tiUing in a new region on the plains, the original 

 vegetation is completely or mostly destroyed, and a distinct subsere beginning 

 with annuals is initiated. On the other hand, overgrazing changes the com- 

 petition relations between the dominants as its primary effect, and the actual 

 disturbance of the soil is usually secondary. The grasses and herbs that are 

 not eaten gradually secure an advantage over the others, and correspondingly 

 increase in dominance or importance. In most cases, they are already present 

 in the community, but where they are not, their invasion from roadsides or 

 other disturbed places into the trampled soil is a simple matter. There are 

 in consequence two general types of indicators of overgrazing, i. e., those due 

 primarily to the fact that they are not eaten, and those which invade because 

 of disturbance. There is naturally no hard-and-fast line between them, as is 

 shown in the detailed discussion in Chapter VI (plat 20, a). 



As a consequence of the difference in the successional process, the indicators 

 of overgrazing resemble those of old fallow fields, and there are instances in 

 which careful scrutiny is needed to distinguish the initial cause. However, 

 when trampling has destroyed the control of the dominants and greatly dis- 

 turbed the surface soil, as happens frequently in sandy areas, a subsere begin- 

 ning with annuals results. Throughout the grassland climax, there occur 

 three overgrazing indicators which outrank all others in importance. These 

 are Gutierrezia sarothrae, Aristida purpurea, and Artemisia frigida. There are 

 many others of great significance, especially among the species of Grindelia, 

 Opuntia, Psoralea, Petalostemon, Verbena, Vernonia, Euphorbia, Carduus, 

 Solidago, etc., which are discussed in Chapter VI. 



Indicators of irrigation and drainage. ^These are related in that they are 

 connected primarily with a decisive disturbance in the water relations, though 

 they are more or less opposite in nature. Plants which register the effects of 

 irrigation are numerous, and are to be found along every irrigation ditch and 

 field. Those which indicate the possibiUty or desirability of irrigation are less 

 definite and have received much less attention. Many of them are of great 

 importance in denoting good soils of sufficient depth, e. g., Artemisia, Prosopis, 

 etc., or sufficiently free from alkali, e. g., Artemisia, Atriplex canescens, etc. 

 The disturbance of the soil in constructing irrigation canals and ditches, 

 coupled with the abundant water supply, has permitted the development of 

 a large and varied plant population along them. This is composed largely of 

 the weeds of cultivated fields and roadsides, but it also contains many sub- 

 ruderals developed from the natural communities. Macbride (1916) has made 

 an interesting study of the successional changes which occur under irrigation, 

 and his results serve to indicate the general indicator value of the dominants. 



Plant communities serve as excellent indicators of the need of drainage, 

 as well as of its progress and success. The need for drainage is clearly indi- 

 cated by the presence of any one of the stages of the hydrosere or oxysere. 

 The latter also indicates the necessity of liming the soil, or employing some 

 other method of securing aeration and neutraUzation. Drainage hastens the 



