GRAZING RANGES IN SOUTHERN ARIZONA. a) 
area and extending into the needle-grass area, Texas curly-mesquite 
grass (Lilaria cenchroides) and Heteropogon contortus are com- 
mon, forming almost’ pure stands of small extent (Pl. III, fig. 
2). The wire grama (Bouteloua 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 vards 
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 affect the growth of the grass. These add an 
amount of feed of which we have no measurements, because they 
were not obtainable with any degree of accuracy. Prickly pears 
and chollas 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- 
eate 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. What 
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 were 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 two areasis as nearly where the crowfoot grama ceases to 
be the most important grass and the needle grasses assume that i1m- 
