MESA AND FOOTHILL VEGETATION 



43 



is a mountain form and hence is relatively infrequent in this region. 

 Altitude 5,500 to 10,000 ft. 



Poison Ivy. Rhus rydbergii Small. — Only a few plants of this species 



Fig. 7. — Soapweed. Yucca glauca Nutt. — Yucca is a dry ridge plant. On the 

 mesa its distribution is limited to the ridges on the south and southeast slopes. By com- 

 paring the positions and extent of the societies of Yucca with the outcrops of shale as 

 shown in Fig. 4, p. 17, it is strikingly noticeable that where the underlying shale is 

 exposed, there also is a growth of Yucca. It seems from these observations that the 

 presence of shale soil determines the distribution of Yucca. However, according to the 

 observations of Shantz* in the Pike's Peak mesa region, Yucca is found occupying a 

 coarse, gravelly soil of low water content. Evidently soil texture does not determine 

 entirely its distribution, for the shale in which it grows here is a fine-grained soil. A 

 fine-grained soil can hold more moisture than a coarse, gravelly soil, but the available 

 water is less. In this region the xerophytic ridges, where Yucca commonly grows, shows 

 exposed shale for the reason that the mesa cap has been eroded off into the ravines between. 



1 Shantz, H. L., "A Study of the Vegetation of the Mesa Region Fast of Pike's Peak," Bot. G^z., 42: 

 16-47, an d 179-207. July and October, igo6. 



