88 KINDS OF INDICATORS. 



Indicators of factor-complexes. — While indicators are concerned most imme- 

 diately with direct factors, they are also definitely related to the indirect ones. 

 Since the water-content is profoundly influenced by the nature of the soil, 

 water indicators often serve as indicators of soil also. In practice, the charac- 

 ter of the soil is more readily recognized than the amount of water in it, and 

 the indicators of good soil represent not merely an adequate water-content 

 and air-content, but a proper supply of nutrients as well. Slope or exposure 

 and altitude are similar factor-complexes, in which the relation of the indicator 

 to the complex is often clearer than it is to any one of the factors in it. In 

 all of these, however, it is understood that the correlation is with one or two 

 limiting factors, which are controlled or modified by soil, exposure, or alti- 

 tude (plate 16, a). 



Soil indicators. — Since the soil is the seat of water-content, salts, oxygen, 

 and acids, as well as of numberless organisms, it may be related to the indi- 

 cators of any of these. This is the case in ordinary practice, and plants are 

 spoken of as indicators of moist soil, alkaline or acid soil, as the case may be. 

 In the stricter sense, indicators refer to the soil as defined by its physical 

 properties, though this necessarily includes water-content. On this basis, 

 plants may be indicators of sand, clay, loam, or humus soils. When their 

 growth and distribution are taken into account, they may serve to indicate 

 even finer divisions of each of these types. In such cases, however, local 

 variations in water-content are often more potent than soil texture, and 

 correlation with one does not necessarily mean correlation with the other. 

 Since the physical character of the soil is of primary importance in determin- 

 ing the echard, soil indicators may be used to distinguish high and low echard. 

 The plants of clay and humus soils are indicators of the one, those of gravelly 

 and sandy soils of the other. In humid regions this distinction is of little 

 importance, except possibly in relation to drainage, but in arid climates or 

 during seasons of drought it is frequently a vital matter. This has been 

 emphasized by Shantz (1911 : 87) in his indicator studies in eastern Colorado: 



" Many of the older settlers in eastern Colorado have moved from short-grass 

 onto wire-grass land, or even bunch-grass land, where they claim there is much 

 less likelihood of crop failure ; but the newcomer in the region or the speculator 

 almost invariably chooses the hard or short-grass land because it is darker in 

 color, and looks more like the soil he has been accustomed to farm successfully 

 in the East." 



Slope-exposure indicators. — While slope and exposure are regarded as 

 distinct topographic features, they are so intimately combined on every 

 hill and mountain that their separation is undesirable, so far as indicators are 

 concerned at least. Both modify the direct factors, water-content, humidity, 

 light, and temperature, and through them nearly all other factors of the 

 habitat. Exposure is of the most immediate importance, as it determines the 

 exposition toward the sun or away from it, but is itself determined in large 

 measure by the angle of slope. Exposure directly affects the temperature and 

 humidity, and through them the water-content, and consequently the nutrients 

 and aeration. A northerly exposure also reduces the amount of direct sunlight, 

 but this is perhaps felt only in transpiration. An increase in the angle of slope 

 has a marked effect in increasing the runoff and correspondingly reducing the 



