CONCEPT. 31 



exact meaning of any indicator is secured. For the future it is clear that 

 much time will be saved by a method of investigation which replaces more or 

 less vague experience by actual investigation. 



Direct and indirect sequences. — As is shown later, plants may indicate con- 

 ditions, processes, or uses. The simplest of these is the first, the most prac- 

 tical is the last. The plant may indicate a particular soil or climate, or some 

 limiting or controlling factor in either. This would seem to be axiomatic, but 

 it is well known that grassland, which is typically a climatic indicator, often 

 occupies extensive areas in forest climates. Thus, the presence of a plant, 

 even when dominant, is only suggestive of its meaning. It is necessary to 

 correlate it with the existing factors and, better still, to check this correlation 

 by experimental planting, or an actual tracing of the successional development. 



Indicators of processes usually require a double correlation, namely, that of 

 the plant with the controlling factor, and that of the factor with the causal 

 process, such as erosion, disturbance, fire, etc. Thus, in the Red Desert of 

 Wyoming, roads through the sagebrush are marked by vigorous growths of 

 Agropyrum. The latter is here a clear indicator of disturbance. From its 

 usual position in adjacent lowlands, it is presumably an indicator of increased 

 water-content as well. Actual instrumental study alone can determine the 

 exact relation between the disturbance and the water-content, and between 

 the water-content and the presence of Agropyrum. The indicator sequence 

 is further complicated by the question whether the increased water-content is 

 due to disturbance directly, to the elimination of competition, or to both. As 

 a matter of fact, however, the field study of Agropyrum and Artemisia under 

 a wide variety of conditions and in different successional relations indicates that 

 disturbance acts through competition upon water-content (plate 5, b; plate 

 a; cf. Weaver, 1919, for plate 26 a). 



In the case of use or practice indicators, the sequence differs in accordance 

 with the nature of the crop. When the crop is a natural one as in grazing, 

 the sequence is simple and direct. This is especially true of grazing in which 

 the value of the range is determined directly by actual experiential or experi- 

 mental grazing tests, which establish the indicator value of each species. With 

 overgrazing, the sequence is similar to that found in process indicators. 

 Trampling disturbs the soil and destroys the less resistant plants. Both 

 effects tend to increase the water-content of the soil and to give the advantage 

 to such plants as Gutierrezia and Artemisia frigida (Clements, 1897 : 968; 

 Shantz, 1911 : 65). This relation is clearly recognizable in the field from the 

 fact that Gutierrezia, for example, is characteristic of depressions, alluvial fans, 

 roadways and other disturbed areas. In the case of forests, plants may serve 

 directly as indicators of water or light values, or indirectly of disturbance such 

 as lumbering or fire, and of such practices as reforestation and afforestation. 

 In these processes the crop is partly or wholly artificial, and the indicator 

 sequence is essentially the same as for crop plants. This involves the corre- 

 lation of indicator and crop plants with their respective habitats, and the close 

 correspondence of the controlling factors in the latter. With forage and grain 

 crops, the sequence is more complex, partly because the species concerned ar 

 not native, but largely because the physical conditions are unnatural as well 

 as controlled. As a consequence, while factor correlation and indicator corre- 

 spondence are still important, the chief part must be taken by experiment and 



