MESA AND FOOTHILL VEGETATION 33 



seedlings. This seems to be the explanation of the three different 

 sizes of trees on the mesas. 1 



By observing the distribution of the pines as shown in Fig. i, p. 34, 

 it will be noticed that they are more thickly scattered on the north slopes, 

 especially along the north crest. Conditions are more favorable for 

 the growth of pines on the crests than on the other parts of the mesa 

 slope. This is because of the rocky nature of the soil and the consequent 

 scarcity of grass and other vegetation. This enables the seed to reach 

 the bare soil easily. 



It is quite noticeable that young seedlings are often found growing at 

 the side of a large rock. This is probably because there is more moisture 

 around the rock than a little distance away. The seedling in such a 

 location receives the run-off from the rock and the shade of the rock 

 prevents rapid evaporation from the ground in which the seedling is 

 growing. 



It is a common observance that seedlings are found growing along old 

 roads (See Fig. 4, Plate II) . The soil there is bare and free from grass 

 and the seed finds ready access to the soil. Shantz 2 has observed the 

 same in the Pike's Peak mesa region. 



There is a greater growth of pines on the north slopes than on the 

 south slopes. This is largely a matter of moisture, especially in its 

 relation to the seedling. The older pine tree has a very extensive root 

 system which renders available the moisture of the deeper layers of soil; 

 the seedling, on the other hand, with its small root system is dependent 

 upon the moisture in the surface layers. On the south slopes, which are 

 exposed to the direct rays of the sun, the surface soil dries out quickly 

 after a rain. Consequently the young seedling growing there does not 

 find a constant supply of water as does the seedling on the north 

 slope. 



The question may now be asked why the lower altitudinal limit of 

 distribution of the Rock Pine is on the mesa instead of somewhere else. 

 Why do not the Rock Pines extend farther out on to the plains ? The 



1 The fact that the pines are gradually extending outward on the mesas is mentioned by Young, Bot. 

 Gaz., 44:321-52, 1907, who discusses briefly their distribution on the mesas and foothills. 



3 Shantz, H. L., "A Study of the Vegetation of the Mesa Region East of Pike's Peak," Bot. Gaz., 42: 

 16-47, an d 179-207. July and October, 1906. 



