GEAZIISTG EAISTGES IN" SOUTHERN ARIZONA. 9 



CHARACTER AND DISTRIBUTION OF FORAGE. 



Attention has already been directed to the variation in the average 

 annual precipitation that occurs on different parts of the Santa Rita 

 Range Reserve and its relation to the forage crop produced. 



Protection from stock has allowed the plants of the greater part of 

 the range to assume something like a normal adjustment among 

 themselves and to the climatic conditions, and this adjustment has re- 

 sulted in certain tolerably well-marked groupings of species of plants 

 that cover areas of considerable size. Such assemblages of species, 

 which have similar climatic and soil requirements and live together 

 in a given area, are here called plant associations, and five of the 

 principal associations are named. An attempt to map somewhat 

 roughly the area covered by each, in order that the relative impor- 

 tance and productivity of each maj be readily grasped, is shown in 

 figure 3. 



A more detailed description of these associations follows, in which 

 the writer has attempted to give some idea of the character of the 

 forage which they produce. Each association (except one) is desig- 

 nated by the name of its most important and most abundant grass. 

 This species is not always the most noticeable or largest plant of its 

 distribution area, but is the most important forage plant. 



Plant associations selected upon this economic basis can hardly be 

 expected to be the typical ecologic associations of the region, and a 

 map setting forth these ideas is not an ecological map in the generally 

 accepted sense. As a matter of fact, the major part of the attention 

 is directed to the subdivisions of the grass zone of the region, and the 

 areas represented are not of equal rank from an ecological standpoint. 

 However, the economic consideration is of first importance here, and 

 Avith these explanations the descriptions of what had better be called 

 associations of forage plants may be taken up. 



THE SIX-WEEKS-GRASS ASSOCIATION. 



The dominant species of the six-weeks-grass association (No. 1 in 

 fig. 3) are Aristida hroTnoides and Bouteloua arlstidoides, short- 

 season annuals, as is indicated by the common name. (PL II, fig. 1.) 

 A grass that is usually referred to under the first name, but may be 

 another species, makes a growth during the late spring before the 

 early-summer dry spell, if there be sufficient spring rainfall, though 

 tliis growth is often quite scanty. During the summer-growing 

 period these grasses make surprisingly rapid growth and are very 

 numerous on most of the bare ground at nearly any level on the 

 reserve. They, grow as thickly as they can stand, the stronger crowd- 

 ing out the weak, and all mature seed whether the season or the con- 

 ditions be such as will produce a growth of a foot or only a few 

 28465°— Bull. 367—16 2 



