258 AGRICULTURAL INDICATORS. 



value. This is due to a number of causes, chief among which are the use of 

 the same crops and methods, the wide extent of the studies, the large number 

 of stations in a single great climax, the grassland, the more or less gradual 

 decrease in rainfall to the westward, and the consequent readiness and accuracy 

 with which co nparative results can be obtained. The correlations discussed 

 below have been based chiefly upon the results obtained by this Office, supple- 

 mented for the more or less representative central portion by the studies made 

 at the experiment stations of Nebraska and Kansas. In all of them, it should 

 be borne in mind that the correlation and the corresponding indicator com- 

 munity have the greatest accuracy in the region of the particular station or 

 stations, and that this value decreases more or less regularly in the direction 

 of stations with different correlations. However, the practical usefulness of 

 the indicator increases with the remoteness from a particular station, providing 

 always that the plant community remains the same, since the latter indicates 

 that the conditions are essentially unchanged. 



Climatic indicators of the types of crops. — The correlations considered here 

 are based upon the fact that crops, like natural dominants, have an area of 

 maximum production about which they shade out in all directions. This 

 diminution is generally less marked in the case of crops, owing to the modi- 

 fying influence of culture as well as of economic factors. Corn affords the 

 most striking example of a crop grown throughout an extensive region, but 

 with a well-defined area of maximum production. As a crop it extends over 

 the major portion of several climaxes, but its optimum area, the "corn belt," 

 is more or less clearly limited. The limits of this belt fall within the main area 

 of the subclimax and true prairies, which are to be regarded as the indicators 

 of maximum corn production. In this connection, it is at least suggestive 

 that four of the dominants of these communities belong to the genus Andropo- 

 gon, which systematically and ecologically resembles corn more closely than do 

 any of the other grassland dominants. As might be expected, wheat exhibits 

 an even more extensive correlation with the grassland. The region of max- 

 imum production is from Saskatchewan to Oklahoma, with secondary maxima 

 in Indiana and Illinois, and in Washington and Oregon. The maximum falls 

 almost wholly within the region occupied by the true and mixed prairies. 

 Here also it is perhaps significant that Agropijrum, with its close relationship 

 to Triticum, is an important dominant in these communities and is the major 

 dominant in the great wheat region of the Palouse. Oats show a somewhat 

 similar relation to grassland, as does barley, but rye manifests no clear cor- 

 relation. On the whole, however, there is good evidence for regarding grass- 

 land made up of tall-grasses as the primary indicator for the optimum pro- 

 duction of cereals (cf. Smith, Baker and Hainsworth, 1916; Waller, 1918). 



Hay and forage crops generally are more or less evenly distributed through 

 the deciduous forest and grassland climaxes, but there is a clear regional 

 differentiation in the case of alfalfa and sorghums. The chief center for 

 alfalfa is in central Nebraska, Kansas, and Oklahoma, with local centers in 

 the main irrigated sections of the West, practically all of which occur in grass- 

 land or sagebrush. The sorghums, whether grown exclusively for fodder or 

 for grain as well, have their center of maximum production in western Kansas, 

 Oklahoma, Texas, and eastern New Mexico. It corresponds closely with the 

 eastern half of the short-grass association, in which Bulbilis and Bouteloua 



