744 
Journal of Agricultural Research 
Vol. XXVIII, No. 8 
Summary of Physical Conditions 
A good uniform growth of creosote bush indicates (1) at least 4 feet of light soils 
of coarse texture, very permeable and well drained, with a low run-off; (2) an 
abundant supply of available water in the upper 4 feet of soil after the winter 
rains, but none late in the summer; (3) a nonsaline soil to the depth of 4 feet. A 
poor growth of creosote bush indicates a coarse stoney infertile soil or a shallow 
soil having a hardpan or rocky layer within 2 or 3 feet of the surface or, more 
rarely, a saline subsoil. 
ADAPTATION TO PHYSICAL CONDITIONS 
In the warmer valleys, as the Coachella Valley, the growth of the creosote 
bush begins in February or sometimes earlier and ends before the hot summer 
months. During the summer and fall the plants are dormant, for by this time 
practically all the available soil moisture has been exhausted. The moisture in 
the surface foot of soil is exhausted by the large number of annuals that cover the 
spaces between the creosote bushes even up to the stems of the bushes themselves. 
The moisture below the surface foot is exhausted by the creosote bush. The 
creosote bush has no well-defined taproot but several almost equal branches part¬ 
ing near the surface of the ground (PI. 4, B). These roots penetrate to consider¬ 
able depths since the light texture of the soil and the deep penetration of the 
water favor a deep root system. The soil moisture here is exhausted before that 
of the desert-sage land and other areas in the lower parts of the valley, for the 
creosote bush covers the warmest slopes in the valley and therefore the annuals 
and the creosote bush plants start growth earlier in the spring. By May or 
June the annuals have matured and died while the creosote bush plants are dor¬ 
mant. The creosote bush lives through the long dry summer mainly because the 
small leathery leaves covered with a sticky resin favor a very low transpiration 
rate. When in a drought rest condition they become brownish and protect them¬ 
selves by dropping many of the lower leaves. 
EFFECTS OF DISTURBING FACTORS (SUCCESSIONS) 
Creosote bush areas are not appreciably affected by grazing. The bushes are 
not browsed and the mechanical injury is slight. The herbaceous flora is, of 
course, greatly modified. Fires in the creosote bush areas are rare so that few 
opportunities have been offered for studying successions. Where the creosote 
bush has been cut Franseria probably precedes its return as a dominant plant. 
VARIATIONS FROM THE TYPICAL ASSOCIATION 
Creosote Bush and Bur-sage 
Between the pure creosote bush areas and the foothills there is often a narrow 
tract occupied by a mixture of creosote bush and bur-sage (Franseria dumosa 
A. Gray) (PI. 5, A). This mixture indicates a still lighter, coarser and very 
stony soil. The salt content is negligible, even less than in the typical creosote 
bush land. Bur-sage also occurs mixed with the creosote bush on the sandy 
knolls in the creosote bush areas. 
Creosote Bush with Desert-sage 
Creosote bush mixes with desert-sage (Atriplex polycarpa (Torr.) S. Wats.) 
(PI. 5, B) near the line between these two associations. These mixed areas 
are very small in Coachella Valley, and indicate a higher salt content and heavier 
soil type than that of the typical creosote bush land, but less salt than that of 
