94 KINDS OF INDICATORS. 



the two elements varies greatly, but the introduced species decrease rapidly 

 in number toward the interior as well as upward into mountain ranges. For 

 a number of reasons, the prairies and plains exhibit the largest number of 

 cultivation indicators, but they occur in all cUmaxes with the exception of the 

 alpine meadow. 



Especial attention has been paid to the subsere originating in fallow and 

 abandoned fields, and on timber claims throughout the grassland climax. 

 In the more arid portions of this vast region, there have been several waves 

 of settlement, coinciding more or less closely with the wet phases of the sun- 

 spot cycle. These waves have receded during the drought phases of the 

 early seventies, the early nineties, and of 1916-1918. However, the recession 

 has been less each time, owing largely to better methods of tillage and to the 

 diversification of crops. In the drought of 1893-1895, the Niobrara region of 

 northeastern Nebraska was nearly depopulated, where to-day there exists 

 an assured agricultural practice. As a consequence, also, the belt of abandoned 

 fields and farms has moved westward, and the indicators have changed to 

 correspond. Many of them occur over much of the region, however, and these 

 are still those of greatest importance and almost universal occurrence. A large 

 number are annuals, and the pioneers are all annual or biennial. As is typical 

 of weeds and subruderals, they occur in dense stands of a single dominant, or 

 a mixture of but two or three major dominants (plate 19, b). 



The widespread dominants of the fallow fields of the prairies and plains are 

 Salsola and Helianthus, the latter represented by H. annuiLS in the eastern 

 portion, and H. petiolaris in the western. Both genera occur from Montana 

 to Texas, but are more abundant southward. Erigeron canadensis is perhaps 

 next in importance in fields, while Grindelia, Gutierrezia, and Artemisia 

 frigida, though abundant, are of still greater importance in pastures. Core- 

 opaia tinctoria and Polygonum pennsylvanicum are typical of moister fields in 

 the eastern half, while Anogra aUncaulis, Oenothera rhombipetala, Eriogonum 

 annuum, and Cycloloma platyphyllum characterize fallow areas with more or 

 less sandy soil, especially in the West. Other indicators of common occurrence 

 are Euphorbia marginata, E. geyeri, Ambrosia artemisifolia, Iva xanthifolia, 

 Chenopodium aUmm, Panicum capillare, Era^rostis pedinacea, Cenchrus 

 tribuloides, etc. A similar wealth of indicators of fallow or abandoned fields 

 is found in Cahfornia. Eschscholtzia calif ornica is by far the most striking of 

 these, though it is less widely distributed than Amsinckia intermedia, Ere- 

 tnocarpus setigerus, Sisymbrium altissimum, Rhaphanus sativus, Brassica 

 nigra, Bromus maximus, etc. 



Grazing indicators. Like the species which indicate cultivation, grazing 

 Indicators mark disturbance in varying degree. It is likewise necessary to 

 distinguish such indicators or indexes from those which denote the kind of 

 grazing possible or desirable, and the carrying capacity as measured by number 

 of animals. The latter are among the most direct of practice indicators, and 

 might well be taken for granted, if their value did not change critically from 

 one community to another, or in different portions of the same community. 

 There is much less difference in the nutritive value of the ordinary grass 

 dominants, for example, than in their palatability, but the latter varies 

 greatly with the choice possible. 



