146 CLIMAX FORMATIONS OF WESTERN NORTH AMERICA. 



form pure communities more than a few acres or at least a few miles in extent. 

 Bouteloua rothrockii might be regarded as an exception to this, for while usually 

 mixed with Aristida, it covers areas of several to many miles as a practically 

 pure dominant. The most frequent groupings are those in which Bouteloua 

 and Aristida occur together, probably because grazing favors the latter at the, 

 expense of the former. 



Rank of dominants — The general rank of the dominants and some of their 

 subclimax associates is indicated by the following table of occurrences in 

 trans-Pecos Texas, southern New Mexico, and Arizona: 



Bouteloua eriopoda 56 



Bouteloua gracilis 40 



Bouteloua racemosa 35 



Bouteloua hirsuta 23 



Bouteloua rothrockii 16 



Bouteloua bromoides 8 



Aristida divaricata 28 



Aristida purpurea 20 



Aristida californica 4 



Aristida arizonica 2 



Hilaria cenchroides 10 



Muhlenbergia porteri 9 



Hilaria mutica 33 



Scleropogon brevifolius 23 



Andropogon saccharoides ... 15 



Sporobolus flexuosus 11 



The abundance of a dominant is not necessarily in accord with its frequence. 

 As a result, Bouteloua racemosa is less important than its occurrence indicates, 

 while B. rothrockii, B. bromoides, and Aristida californica are much more 

 frequent. It is these species, moreover, which appear to be among the most 

 characteristic of the grasslands of northern Mexico. The last four dominants 

 of the list are all really subclimax, with the probable exception of Sporobolus 

 flexuosus. This is particularly true of Hilaria and Scleropogon, which are 

 typical of "swags" and other valley-like depressions throughout the Larrea- 

 Flourensia scrub. Both occur so frequently with Bouteloua, and especially 

 B. eriopoda and B. hirsuta, that they can not be ignored in a treatment of the 

 desert plains. This Hilaria-Scleropogon subclimax covers thousands of square 

 miles from the Pecos River to central Arizona. 



Grouping of dominants. — While all the dominants range more or less 

 throughout the association, with the exception of Bouteloua rothrockii and 

 Aristida californica, they vary greatly in importance and grouping in the three 

 States. This depends upon the altitude and the distance from the center in 

 Mexico. In Texas Aristida purpurea, in various forms, is the chief dominant 

 at the lower levels; toward its northern limit it is much mixed with Bulbilis 

 and Bouteloua gracilis as a lower layer. Hilaria cenchroides, B. eriopoda, A. 

 divaricata, and Muhlenbergia porteri occur more or less frequently with it. 

 In the mountain ranges of western Texas, from the Davis and Guadalupe 

 Mountains to the Sierra Blanca, Bouteloua gracilis, B. eriopoda, and B. 

 racemosa are the climax dominants, with which Aristida and Muhlenbergia 

 occur more or less abundantly. Between these ranges He extensive bolsons 

 or bolson-like valleys, characterized in the center by Hilaria-Scleropogon 

 swags in a more or less open scrub desert. The grama grasses extend far down 

 the gradual slopes of the bolson, and mix with the subclimax grasses over a 

 wide zone (plate 29). 



The chief dominant in New Mexico is Bouteloua eriopoda, often much mixed 

 with Aristida purpurea, as in the valley of the Pecos. They occur abundantly 

 on the marl hills north of Albuquerque with B. gracilis and Muhlenbergia 

 gracillima, and to a smaller degree in northern Arizona. All of the evidence 

 available indicates that this is the northern edge of the ecotone and that the 



