148 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



ciation generally, it is intermediate in requirements between B. gracilis and 

 B. racemosa, which occupy valleys and north slopes, and B. bromoides and 

 Hilaria which take upper slopes and tables. It mixes with the former in the 

 valleys and runs up the slopes to mingle with the latter on equal terms. From 

 a number of similar transects the sequence as to water relation seems to be 

 (1) B. racemosa, (2) B. gracilis, (3) B. hirsuta, (4) B. bromoides, (5) Hilaria 

 cenchroides. This corresponds well with the successional sequence, so far as 

 known, and also with the climatic relations. 



In the secondary succession due to disturbance, and especially to grazing, 

 it is apparent that the Aristidas are subclimax to the Boutelouas as a general 

 rule. Overgrazing and trampling tend to destroy the more xerophytic species, 

 such as B. rothrockii and B. eriopoda, and to permit the entrance of Aristida, 

 especially A. divaricata. Bouteloua rothrockii and Muhlenbergia porteri are 

 particularly susceptible to grazing injury and have consequently disappeared 

 over large areas. Muhlenbergia, in fact, is rarely found at present, except in 

 the protection of a catclaw or mesquite clump. A similar tendency for 

 Aristida to replace Bouteloua occurs at the higher altitudes also, but is much 

 less marked. One would expect disturbance to bring about the replacement 

 of B. gracilis and B. hirsuta by A. purpurea, as is the case in the short-grass 

 plains, but no important areas of this sort have been seen. A. divaricata 

 sometimes plays this role, but much less frequently than at lower levels. 



SOCIETIES. 



The desert plains have two groupings of subdominants. The most charac- 

 teristic consists of those found in the heart of the association in southern 

 Arizona and New Mexico. The second group comprises those found along 

 the north, where the association meets the short-grass plains. The latter are 

 those species which constitute the typical societies of the prairies and plains. 

 While they are largely southwestern in origin, they have had time and oppor- 

 tunity to migrate throughout the formation east of the Rocky Mountains. 

 The more characteristic societies appear to be of relatively recent derivation 

 from the Mexican center, and they are best represented in the region along 

 the boundary. 



The desert plains resemble the short-grass plains in the relatively small 

 number of societies, and especially of mixed societies. This is readily explained 

 by the low rainfall over much of the area and the thoroughness with which 

 the water available is utilized by the associated dominants of slightly different 

 demands. Wherever the rainfall increases materially, as in the Aristida 

 consociation of Texas or toward the mountains, the number and complexity 

 of the societies increase also. 



Vernal Societies: Estival Societies: Estival Societies — continued. 



Antennaria dioeca. Psoralea tenuifiora. Chrysopsis villosa. 



Calliandra eriophylla. Petalostemon purpureus. Eriogonum wrightii. 



Astragalus bigelovii. Petalostemon candidus. Verbesina encelioides. 



Krameria secundiflora. Dalea laxiflora. Haplopappus gracilis. 



Zinnia pumila. Linum rigidum. Yucca radiosa. 



Eschscholtizia mexicana. Meriolix serrulata. Yucca baccata. 



Malacothrix fendleri. Malvastrum coccineum. Eriogonum polycladum. 



Lithospermum linearifolium. Thelesperma gracile. Gaillardia aristata. 



Psilostrophe cooperi. Hymenopappus filifolius. Lepachys columnaris. 



Eriogonum abertianum. Aster tanacetifolius. Plantago elata. 



