126 ELEMENTARY EORESTRY. 



second growth, and is generally marketed about as soon as it 

 attains sufficient size to be salable, without regard to the fact that 

 it is then making its most rapid growth. 



From careful observation, the Experiment Station of the 

 University of Minnesota estimates that on land adapted to the 

 White Pine, with a thick growth of this kind of trees eight inches 

 in diameter, the annual increase should be about fifty cubic feet, 

 or 500 feet board measure, per acre. In some cases this rate of 

 increase has been more than doubled, but under ordinary good 

 conditions not over one-third as much increase need be ex- 

 pected. 



The Thickness of the Annual Rings on trees varies with 

 the conditions under which the trees make their growth, and is 

 therefore a good index to these conditions. Trees that are 

 crowded so that they make a very rapid upward growth form 

 very thin rings, and when this upward growth ceases owing to 

 the removal or suppression of surrounding trees much thicker 

 rings are formed. Trees that are grown in the open produce 

 throughout their lives thick annual rings, which vary in thick- 

 ness according to varying climatic conditions. Those of the 

 White Pine vary in thickness from one-sixteenth of an inch or 

 less in trees that are severely crowded to one-third of an inch 

 in open-grown trees in good soil. Willows sometimes have 

 annual rings three-fourths of an inch wide. 



The I/ife History of a Mature Tree in virgin forest 

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

 nection with the environment of the tree. The Division of For- 

 estry of the Minnesota Experiment Station has made several 

 studies of this kind, among which are the following: 



Figure 33 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 pushed upward rapidly, until about its 

 fiftieth year. It was then set free by fire, which checked its 

 upward growth for about twenty-five years, when, owing to the 

 crowding of surrounding trees, it began to again increase rap- 

 idly in height. When eighty-four years old fire killed the sur- 

 rounding trees and set this one entirely free, in which condition 



