THE DESERT PLAINS. 147 



region generally belongs to the short-grass association. The desert plains of 

 southern and southwestern New Mexico are characterized by Hilaria and 

 Scleropogon in the subclimax stage, and Bouteloua eriopoda in the climax. In 

 the Jornada del Muerto the latter is usually associated with Sporobolus 

 flexuosus, and with S. cryptandrus where the soil is somewhat more sandy. 

 B. racemosa is not infrequent, but it is rarely dominant at this level. On the 

 slopes of the Organ and San Andreas Mountains at 5,000 to 6,000 feet, the 

 dominants are Bouteloua hirsuta, B. gracilis, and B. racemosa, with consider- 

 able Aristida divaricata and little B. eriopoda. 



All of the 12 dominants occur abundantly between elevations of 3,500 to 

 5,500 feet in southern Arizona, and as a consequence the grouping is more 

 varied and complex than in any other part of the association. Since all the 

 dominants are present in northern and central Mexico as well, it is practically 

 certain that the same groupings will be found there. The lowermost com- 

 munity is typically Bouteloua rothrockii and Aristida divaricata, often with 

 some A. purpurea and Muhlenbergia porteri. Aristida calif ornica or A. 

 arizonica may occur as a dominant with either B. rothrockii or B. eriopoda 

 alone or together. Bouteloua eriopoda and Aristida divaricata are frequently 

 associated also. Bouteloua bromoides tends to become controlling at 4,000 

 feet and is often the major dominant for the next 1,000 feet, where its usual 

 associates are B. racemosa and A. divaricata, with more or less B. hirsuta and 

 B. eriopoda. This is the typical condition on the upper levels of the Santa 

 Rita Range Reserve near Tucson. Hilaria cenchroides, Bouteloua hirsuta, 

 and B. gracilis become abundant at about 4,500 feet and with B. bromoides, 

 B. racemosa, and more or less Aristida divaricata, constitute the grassland 

 until it gives way to the Andropogon community of the oak savannah at 

 5,500 to 6,000 feet. Altogether, nore than 30 different groupings of the 

 dominants have been found in Arizona; 9 of these consist of two dominants, 

 12 of three, 8 of four, and 7 of five. These varying combinations furnish 

 invaluable material for the determination of equivalences. 



Sequence of dominants. — No study has yet been made of the habitat rela- 

 tions of the desert plains grassland, and these must be inferred from the 

 groupings and the topographic position, as well as the behavior under dis- 

 turbance. No definite sequence can be suggested without factor measure- 

 ments for such a large number of closely equivalent dominants, but certain 

 general relations will serve as a helpful basis for future work. These have to 

 do with altitude, topography, range, grazing, and succession. The chief 

 dominants of lower altitudes are naturally those of the greater range northward 

 in the association. These are Aristida purpurea, Bouteloua eriopoda, A. 

 divaricata, and B. rothrockii. The most xerophytic of these is B. eriopoda, 

 which finds its best development in southern New Mexico in a rainfall of 10 

 to 15 inches, and the least xerophytic, A. purpurea, with a rainfall of 15 to 20 

 inches in western Texas. At higher altitudes, B. hirsuta, B. bromoides, Hilaria 

 cenchroides, B. racemosa, and B. gracilis are the dominant species. The first 

 three mix intimately and probably are to be regarded as the most nearly 

 equivalent of the many dominants. B. hirsuta is the only one of the three 

 which ranges far to the northward, where it is a regular associate of B. gracilis 

 in sandy soils. In the Empire Valley, and probably in the heart of the asso- 



