126 ELEMENTARY FORESTRY. 
second growth, and is generally marketed about as soon as it 
attains sufficient size to be salable, without regard to the fact that 
itis 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 Life 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, im which condition 
