140 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Shantz (1. c, 62) has traced the succession in sandhills and in secondary- 

 areas, and finds that the depth of water penetration is the decisive factor 

 In. sandy soils, the deep-rooted Andropogons are dominant and with them 

 occur many deep-rooted perennial herbs, such as Psoralea tenuiftora, Arte- 

 misia filifolia, and Ipomoea leptophylla. With the entrance of Aristida pur- 

 purea and the Boutelouas, the water is used chiefly in the 1 to 2 foot layer, 

 and the deeper species gradually die out. As the Bouteloua sod becomes 

 denser, Aristida and its associates disappear, and the short-grass climax is 

 established. When the cover is destroyed by cultivation or overgrazing, the 

 water penetrates more deeply, permitting the entrance of Aristida, Gutier- 

 rezia, Grindelia, Artemisia frigida, and other deeper-rooted species. With 

 the return of the gramas, the depth of penetration decreases and the invaders 

 are displaced. Shantz (1917 : 19) has lately shown that the secondary suc- 

 cession in abandoned roads is due to the same cause, though Bulbilis largely 

 takes the place of Bouteloua in effecting the return to the climax (plate 26). 



Range. — The short-grass association ranges from southwestern Nebraska 

 and the western half of Kansas through eastern Colorado into northwestern 

 Texas, northern New Mexico, and Arizona. It is also developed to some 

 extent in southeastern Utah and southwestern Colorado. The chief dominant 

 throughout is Bouteloua gracilis. In the eastern part its usual associate is 

 Bulbilis dactyloides; in the western part it is Muhlenbergia gracillima or 

 Hilaria jamesii. In sandhill and foothill areas, it is often associated with 

 Bouteloua hirsuta. The chief contacts of the short-grass plains are with the 

 other associations of the grassland formation. On the north, in Colorado and 

 Nebraska, they meet the mixed prairies, in Kansas the subclimax prairie, 

 and in west-central Texas, central New Mexico, and Arizona the desert 

 plains. On the west this community comes in contact with the sagebrush 

 association of the Great Basin, while throughout the Southwest generally 

 it is frequent in park-like savannahs of pine and pifion-cedar. 



As already indicated, nearly pure communities of Bouteloua gracilis occur 

 well outside the area outlined above. All of these appear to have resulted 

 from the elimination of the tall-grasses by overgrazing. It is an open question 

 what part grazing has played in the short-grass association proper. There is 

 considerable evidence to show that Stipa and Agropyrum were more abundant 

 formerly, but whether they were sufficiently so to rank as codominants is 

 uncertain. It is possible that a detailed survey of the short-grass region will 

 settle this point, but it is more probable that an adequate idea of the original 

 vegetation will be obtained only from the fenced quadrats established in con- 

 nection with the grazing investigations. At any event, it is practically certain 

 that grazing, especially in connection with the former annual movement of 

 cattle from the south to the north, has played an effective part in maintain- 

 ing the characteristic short-grass cover. 



CONSOCIATIONS. 

 Bouteloua gracilis. Muhlenbergia gracillima. 



Bulbilis dactyloides. Hilaria jamesii. 



Bouteloua hirsuta. 



Bouteloua gracilis is regarded as the chief consociation. It is almost uni- 

 versally present throughout the association, though it varies considerably 

 in abundance. It not only occurs mixed with each of the others and some- 



