144 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



Fretmmid Clana: Eatital Clans: 

 ArtmW* frickla. Cympoterus acaulis. Lathyrus ornatus. 



Outierreiia SMothne. Phellopterua montanus. Aater ericoides 



ftiMdo dousUiU. Vernal Clan,: Asclepias v. puimla. 



Orinddia ,uam>8a. EriReron flagellaris. Cactus viviparua. 



CarduUS UndulatUS. Lanmila t^Tuna v^u vivipaiuo. 



Am^ua dracunculoidcs. irnnarfaToeca. ^t""'" "'^^"''- 



8ohdo misaouriensis. EriReron canus. ^alea aurea. 



Uatna punctata. Pentatemon jameaii. PotenUlla pennaylvamca. 



Aatr multiflorua. Phyaalia'lobata. Allionia linearis. 



Kuhnia glutinoaa. Allium cernuum. Serolinal Clan: 



Vernonia baldwinii. Aatragalua lotiflorua. Eriogonum jamesii. 



THE DESERT PLAINS. 

 ARISTIDA-BOUTEIX)UA ASSOCIATION. 



Nature. The grassland of the Southwest derives its character primarily 

 from Aristida and BouteUma. In general appearance it closely resembles the 

 short-grass plains, but the grasses are taller, more numerous, and the group- 

 ings more varied. The sod-forming habit is much less developed. It is absent 

 in Aristida and in Bouteloua rothrockii. While it is more or less evident in 

 Bouteloua eriopoda, B. hirsuta, and B. bromoides, the sod has no continuity, but 

 is broken into many small mats. Although this condition obtains in some 

 parts of the short-grass plains, the sod is much more complete as a rule. No 

 single species of this association possesses the importance shown by Bouteloua 

 gracilis in the short-grass region . Probably Bouteloua eriopoda is to be regarded 

 as the most dominant species of this genus, and A. purpurea, in its several 

 forms, of Aristida. 



The close relationship between the two associations is shown by the long 

 contact from Texas through New Mexico and Arizona and by their similar 

 appearance. They are also alike in their successional relation to such sub- 

 climax dominants as Andropogon scoparius, A. saccharoides, and Bouteloua 

 racemosa. Their chief relationship, however, lies in the fact that certain 

 dominants occur in both, although usually with different values. These are 

 Bouteloua gracilis, B. hirsuta, Aristida purpurea, and Bulhilis dactyloides. 

 B. gracilis may be more or less subclimax in nature and restricted to mountain 

 valleys or it may be intimately mixed with B. eriopoda, hirsuta or racemosa. 

 B. hirsuta is one of the important dominants, usually with B. bromoides or 

 Hilaria cenchroides on foothills and on mountain slopes. Bulhilis usually 

 occurs only in small scattered patches, except in Texas, where it meets Hilaria 

 cenchroides, Bouteloua eriopoda, or Aristida purpurea on more or less equal 

 terms. Aristida purpurea changes from subclimax to a climax dominant, 

 especially important in Texas and New Mexico. The similarity as to societies 

 and clans is less than that between the prairies and plains, but this is due 

 chiefly to the proximity to the original center of the flora. However, as the 

 lists show, there is much agreement as to the genera concerned (plate 28). 



The desert plains are in close contact with but one other association of the 

 grassland formation, namely, the short-grass plains. It is probable that there 

 was formerly a second contact, with the Stipa bunch-grass prairie of California, 

 but to-day there is a wide gap between, bridged to a certain extent by Hilaria 

 jamesii H. rigida and Boutelona gracilis. The contact mentioned is from 

 Snyder and Big Springs in the Staked Plains of Texas to Roswell and Socorro 



