TREES AND THEIR METHODS 95 



Some wild trees, like many wild animals, do not 

 thrive in captivity. Spruces, particularly, seem un- 

 able to adjust their business methods to fit the 

 conditions of man-raked lawns. Individuals which 

 will thrive in the rocky fissures of some desolate 

 glen will droop and die in a well-kept backyard. 



The annual temperature and the annual rainfall 

 are the primary things which affect tree growth 

 and therefore sylvan business methods. Other in- 

 fluences which the trees have to take into considera- 

 tion are humidity of air, wind exposure, slope 

 exposure, degree of slope, and soil depth. Many 

 trees overcome variations in moisture by sending 

 roots far down into the sub-soil. This makes them 

 unaffected by ordinary droughts provided the 

 year's precipitation is normal. The yellow pine 

 flourishes equally well on the slopes of the Sierra 

 Mountains, where most of the rainfall occurs in 

 winter, and on the Colorado Plateau of Arizona, 

 where summer is the wet season. 



Most trees adjust themselves to winds of all ve- 

 locities. With extra root braces they face the most 

 trying storms. The altitude line of the yellow pine 

 is much the same all over the United States, indi- 

 cating that places of exceptional exposure do not 

 become untenable to the trees. 



When the wind or the cold becomes very severe 



