162 



PRINCIPLES OF AMERICAN FORESTRY. 



times have annual rings three-fourths of an inch wide, 

 showing the diameter growth to have been one and a 

 half inches per year. 



The Life History of a Mature Tree in virgin forest may 

 often be determined by a study of the annual rings, in 

 connection with the environment of the tree. The Divi- 

 sion of Forestry of the Minnesota Experiment Station 



Fig. 58. — Cross-section of White Pine crowded and then open grown. 



has made several studies of this kind, among which are 

 the following: 



Fig. 58 shows a section of a White Pine which made 

 its growth under varying conditions. This tree started 

 into growth under Birch and Aspen, and when from 

 twenty to twenty-five years old was nearly suppressed 

 by them. Overcoming them when thirty years old it 



