96 KINDS OF INDICATORS. 



movement and reaction of the succession in swamps and bogs, and the later 

 serai stages clearly indicate when the successive points have been reached 

 at Which the area can be used for grazing, forestation, or crop production. 

 However, in extensive drainage operations, the areas concerned are put into 

 commission so rapidly that the natural communities are destroyed. 



Construction indicators.— Practically all engineering and other construction 

 operations in nature disturb the soil, often in a most striking fashion. The 

 most common and important are the building of roads and the construction 

 of railways and canals. The construction of buildings and similar operations 

 belong in the same category, but the effects are usually masked by the sub- 

 sequent activities of man. The general relation of engineering operations to 

 succession and hence to indicators is best exemplified in the case of a railway 

 cut and fill. In addition to the cut and the corresponding fill, there is often a 

 dump of new earth on each side of the cut. These three secondary areas for 

 succession have much in common, but the loose soil of the dump and the fill 

 is invaded much more rapidly than the firm soil of the sloping sides of the 

 cut. This difference is even more striking when the track runs through a 

 level stretch and the bed is built up from soil scraped out from both sides. 

 The moist depressions are readily invaded by the more mobile or vigorous 

 species of the original community. The bed is not only more xerophytic, but 

 also is disturbed from time to time. Moreover, invasion proceeds along it 

 more readily than into it across the depressions which separate it from the 

 native community. As a consequence, the bed remains more or less permanently 

 in the early stages of succession, which consist of annual and perennial weeds, 

 some of which are derived from the native population. The depressions, on 

 the other hand, pass more or less rapidly through the usual stages to the climax, 

 unless the sere is kept in the subclimax by burning or cutting. Their indicators 

 are often of the most exceptional value in regions where the native vegetation 

 has been greatly modified or largely destroyed (plate 20, b). 



Roads resemble railways in their general relation to succession and indi- 

 cators. This is particularly true of highways in which cutting and filling, 

 though less extensive, are as frequent as in the case of railroads. Roadsides 

 usually show a typical zonation from the bare trackway to the natural com- 

 munity on either side (Clements, 1897 : 968). The sequence of zones sum- 

 marizes the successional movement, and the latter is shown in especial detail 

 when there are many parallel roads of different ages (Shantz, 1917 : 19). 

 In addition to indicating the disturbance caused by roads, plants may be used 

 as indicators in connection with road-building and even in traveling. The 

 correlation between certain communities and good roads is as striking as it is 

 gratifying, and in actual travel it is often a matter of much importance to be 

 able to determine the character of the road from the vegetation which stretches 

 for many miles ahead. During the constant field travel of the past five sum- 

 mers, many communities have been recognized to have some value for road 

 construction as well as travel, but there are a few of the greatest importance 

 and the widest extent. Throughout the mixed prairies, Stipa generally indi- 

 cates good upland roads, Agropyrum, poor lowland ones, while the presence 

 of short-grass on the hills and ridges usually means a road made rough by the 

 matted roots of Carex. In the sagebrush climax, sagebrush, Artemisia tri- 

 dentata, indicates excellent natural roads, Atriplex confertifolia, much poorer 



