May 24,1924 Significance of the Southwestern Desert Vegetation 
757 
ADAPTIONS TO PHYSICAL CONDITIONS 
The desert-sage has a well developed tap root besides numerous supplementary 
roots just below the surface of the ground (PL 7, B). Plants of good growth feed 
on at least 3 to 4 feet of soil, but those of poor growth, where the penetration of 
water is less, do not feed below 2 feet. In general, the period of growth of the 
desert-sage corresponds to that of the creosote bush but since the areas covered 
by the former are not so favorably situated with regard to temperature, the 
beginning of the season's growth of the desert-sage lags about two weeks behind 
that of the creosote bush. This difference is reflected in the growth of the an¬ 
nuals. In 1915 in the middle of February annuals were in bloom in the creosote- 
bush areas, but were not blooming up to March 1 in the desert-sage areas. Dur¬ 
ing the summer months the leaves are shed until by fall the plant is bare except 
for the fruits, which, in favorable years, are so heavy that when they have fallen 
they cover the ground. The desert-sage has to resist the most severe drought 
conditions of any of the plant associations in Coachella Valley. The areas 
having the poorest growth were found next to the seepweed, or, in the Gila 
Valley, near narrowleaf saltbush where the salt content is high, especially below 
the surface, foot. The areas of the best growth were those at the upper end of 
the desert-sage belt bordering the sand hills or the creosote bush areas. 
The number of annuals on the desert sage land is very small compared to that 
of the creosote bush land and therefore the moisture of the surface foot is not 
exhausted by them as in the case of the creosote bush land. As there are de¬ 
pressions between the desert-sage bushes where the water accumulates after 
rains, the plants avail themselves of this surface moisture by means of the super¬ 
ficial part of their root systems. 
EFFECTS OF DISTURBING FACTORS (SUCCESSIONS) 
In Coachella Valley, when a typical desert-sage area is destroyed by fire or 
cultivation, Isocoma veneta acradenia (Greene) H. M. Hall takes a prominent 
place in the successions preceded by annuals and biennials and followed by desert- 
sage. This is well illustrated when the vegetation along the roads (PI. 6, A) 
is destroyed for use in “brushing" the roads. The strips where the vegetation 
has been destroyed are then dominated by Isocoma, while the rest remains as 
virgin desert-sage. 
In places having more favorable moisture conditions, that is, land formerly 
cultivated, the seepweed, arrowweed, or saltbush may act as a weed in an early 
stage of the successions until the moisture is somewhat exhausted, which may 
then be followed by Isocoma, and, finally, by desert sage. 
VARIATIONS FROM THE TYPICAL ASSOCIATIONS 
The desert-sage mixes with the creosote bush association above it, and with the 
seepweed association below. The former has been discussed as a variation of the 
creosote bush association; the latter will be discussed under the seepweed associa¬ 
tion. 
MESQUITE THICKET 
TOPOGRAPHICAL RELATIONS 
The mesquite thicket has a wider range than the creosote bush, extending as 
far west as the creosote bush and much farther east into Texas. The mesquite 
thickets occupy the lowest land in the valleys. The largest and densest areas 
occur along the river bottoms where the land is subirrigated, but these give way 
to desert-sage as the land rises gradually toward the mountains. 
