98 KINDS OF INDICATORS. 



their general values as indicators are pointed out in the succeeding chapter. 

 In addition to this, plants and communities have striking significance as 

 indicators of climatic cycles and hence may become of great value in deter- 

 mining the proper practices in production for the arid and semi-arid regions of 

 the West. The existence of such cycles has been demonstrated beyond a 

 doubt by the work of Douglass (1909, 1914), Arctowski (1912), Huntington 

 (1914), Kapteyn (1914), and Clements (1916). The relation of climatic cycles 

 to succession and hence to indicators has been discussed at some length in 

 "Plant Succession," and an extensive study of the relation of the 11-year 

 cycle to grazing and dry-farming has been made during the drought of 1916-1918 

 (Clements, 1917, 1918). A complete summary of the relations between cycles 

 of rainfall, sun-spots, and tree-growth has recently been made by Douglass 

 (1919). 



Trees and shrubs are the best indicators of minor climatic cycles by virtue 

 of the annual record of growth in rings. It is also probable that height-growth 

 furnishes a correlated record, but little study has as yet been made of the 

 cyclic nature of the latter. It has been found that the height-growth and the 

 reproduction of dominant grasses and halfshrubs, such as Bouteloua, Agro- 

 pyrum, Gutierrezia, and Isocoma, show a close correspondence with the rain- 

 fall of the dry and wet phases of the sun-spot cycle. It is, moreover, a matter 

 of general experience that the carrying capacity of the western ranges varies 

 100 per cent or more from wet periods to times of drought. Even more 

 striking variations in the yield of field crops are shown for similar periods 

 (Ball and Rothgeb, 1918 : 49). Since wet phases usually offer the best con- 

 ditions for germination and growth, and drought periods the poorest, the 

 ecesis of dominants often affords striking indications of climatic phases. This 

 is especially well seen in the ecotone between two adjacent communities such 

 as grassland and scrub, woodland and sagebrush, or forest and grassland. In 

 the majority of cases so far investigated in which a woody dominant is extend- 

 ing into another community of smaller water requirements, the annual rings 

 indicate its establishment during the wet phase of the cycle. 



The general significance of climatic cycles and of cycle indicators in practice 

 is discussed in the next section. Their fundamental value in paleo-ecology 

 is dealt with under paleic indicators at the close of the chapter. 



PRACTICE INDICATORS. 



Nature. — Practice indicators are those plants or communities which point 

 out the possibility or desirability of a particular practice. This is the original 

 as well as the general use of the word "indicator," and there are good reasons 

 for restricting it to this sense, and designating the so-called indicators of 

 factors and processes as "indexes." However, two cogent reasons have 

 caused the word indicator to be retained in the general as well as the special 

 sense. The first of these is the impossibility of drawing a line between actual 

 practices, as in agriculture, and the combination of human practice and 

 natural process in forestry and grazing. The second is that the value of an 

 indicator for practice rests upon the factor or process which it denotes. 

 Furthermore, the term indicator has become so generally understood that it 

 would be unfortunate to restrict its meaning, though it has been found con- 

 venient to employ "index" as a partial synonym. 



