GEAZIInG EANGES in SOUTHEEN" ARIZONA. 13 



area and extending into the needle-grass area, Texas cnrly-mesquite 

 grass {HUaria cenchroides) and Heteropogon contortus are com- 

 mon, forming almost pure stands of small extent (PL III, fig. 

 2). The wire grama {Boutelo%ia eriopoda) also contributes con- 

 siderable to the forage crop of this association, but has its own 

 distribution limits; it frequently covers areas of a few square yards 

 to the complete exclusion of everything else. Besides the grasses, 

 there are various other herbaceous annuals and perennials that ap- 

 pear either in the spring or summer and add to the total crop. 

 There is a scattering growth of shrubs like mesquite, cat's-claw, desert 

 willow, etc., over most of the crowfoot grama area, thickest along 

 the arroyos and toward the west and north, but usually not heavy 

 enough to in any way aifect the growth of the grass. These add an 

 amount of feed of wdiich we have no measurements, because they 

 were not obtainable with any degree of accuracy. Prickly pears 

 and choUas are quite abundant in places, but a heavy crop of grass 

 tends to kill them out, probably because of occasional fires which 

 sweep the grassed area. 



Earlier reports have shown the rate at which this association took 

 possession of the upper part of the reserve, and photographs show very 

 clearly how well the grass has grown. Pictures recently taken indi- 

 cate that the grass is even thicker and larger now, and observations 

 show very definitely that within five years the boundary of the 

 crowfoot-grama area has moved westward more than a mile at the 

 south end of the reserve, and about 2 miles to the northwest along 

 the Tucson road. In the north-central part of the reserve the char- 

 acteristic plants of this association are now more numerous than 

 those of the six-weeks-grass association clear to the north fence, 

 though much black grama, six-weeks grass, bushes, and cacti occur 

 here, and there is also considerable bare ground in the region. ~\Vhat 

 will ultimately dominate does not yet appear, but the important 

 factor is the aggressiveness of both the black-grama and the crow- 

 foot-grama grasses. It has taken a long time for this improvement 

 and spread to show, because there were few seed plants to start 

 with, and germination conditions are so severe that only a few new 

 plants w^ere established each year, or at even longer intervals. 



THE NEEDLE-GRASS ASSOCIATION. 



The needle-grass association is the assemblage of plants which 

 forms the grass belt along the foothills, covering approximately 

 nine sections of the area under fence (No. 4 in fig. 3). It is not 

 clearly marked off from the crowfoot-grama association, there being 

 more or less overlapping both ways. The line on the map which 

 separates the tw^o areas is as nearly where the crowfoot grama ceases to 

 be the most important grass and the needle grasses assume that ira- 



