14 BULLETIN 367, U. S. DEPARTMENT OF AGRICULTURE. 
portance as the writer was able to locate it. It may be undesirable 
from some standpoints to try to separate these associations, but to 
do so has seemed to give a httle clearer conception of the condi- 
tions existing on the reserve, even though the crowfoot grama will 
go higher and the needle grasses do go considerably lower and not- 
withstanding the fact that other grasses occur freely in both asso- 
clations and seem to link the two. 
The needle-grass association consists of a number of important 
perennial grasses, of which Aristida divaricata and what is probably 
A. scabra are the most abundant (Pl. IV, fig. 1), hence the name 
here suggested. The next most important grass is Bouteloua fili- 
formis, Which frequently makes up from one-fourth to one-third 
of the assemblage. Toward the upper limit of the belt this grass is 
apt to be replaced by B. chondrosioides. Hairy grama (Bouteloua 
hirsuta) also occurs on the rockier hills, and Texas curly mesquite 
({Tilaria cenchroides) is not uncommon at the lower side of the 
zone. — 
Wherever the needle-grass association is entirely lilled out the 
six-weeks grasses and annuals first take the ground, and then the 
short-lived perennial gramas appear in abundance before the longer 
lived perennial Aristidas become established. As the greater part 
of this belt that is inclosed is grazed by cattle and horses, the various 
conditions mentioned may be found at different places in the differ- 
ent pastures. Wherever the stock congregate most the six-weeks 
grasses and annuals abound. Where this condition of local over- 
_ grazing is relieved some step in the sequence of complete replace- 
ment of the association occurs. 
Additional perennial grasses in this association are Texan timothy 
(Lycurus phleoides), tall, or side-oats, grama (Bouteloua curtipen- 
dula), Eragrostis lugens, Elionurus barbiculmis, and Trachypogon 
montufari, while numerous spring and summer herbaceous annuals 
and perennials add considerable to the forage crop. The lower 
limit of these needle grasses is not the limit of the association, since 
they are common in patches in the crowfoot-grama association and 
follow down the dry watercourses, or arroyos, to the very lowest 
parts of the inclosed area. In many places in the crowfoot-grama 
association they may constitute as much as 25 per cent of the forage 
present on the ground. Whether or not they will gradually crowd 
downhill and finally replace the crowfoot-grama association remains 
to be seen, but at present the writer believes they require a little 
more water than the crowfoot grama and will hardly be able to en- 
tirely replace that association as it now exists on the reserve, no 
matter how long the area may be protected. No data are available 
relative to the crop of spring feed upon this area, but it is doubtless 
of no great importance except where overgrazing has occurred. 
