MESA AND FOOTHILL VEGETATION 



39 



Altitude, slope, exposure and soil texture influence the distribution only 

 in so far as they affect the water content of the soil. 



The accompanying maps indicate the distribution of eighteen of the 

 more important shrubs and deciduous trees growing on Long Mesa. 



Fig. 3. — Skunk Bush. Rhus trilobata Nutt. — The most extensive growth of this 

 shrub occurs in the gulches outside the Crataegus zone. Almost every depression and 

 amphitheater contains a well-developed mass of skunk bush, growing just below the rock 

 cap. Skunk bush requires a moderate supply of water to support a thick growth; in the 

 depressions and amphitheaters it receives a sufficient quantity by seepage to form quite 

 a dense growth. Isolated bushes are frequent over both slopes growing even in dry 

 places. They show, however, a characteristic response in numbers; the discontinuous 

 distribution is due to the water supply, the shrub being most abundant in moist situations. 



The distribution of the less important ones (twenty species) is described 

 but it was not considered worth while to map them. A very similar 

 distribution occurs on Horse Mesa. 



