60 ELEMENTARY FORESTRY. 



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 50 

 cubic feet or 500 feet board measure per acre. In some cases 

 this rate of increase has been more than doubled. 



The Thickness of the Annual Kings on trees varies with the 

 conditions under which the trees make their errowth 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 

 thick annual rings throughout their lives 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 connection 

 with the environment of the tree. The Division of Forestry 

 of the Minnesota Experiment Station has made several 

 studies of this kind, among which are the following: 



Figure 9 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. Over- 

 coming 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-ffve years, when 

 owing to the crowding of surrounding trees it began to again 

 increase rapidly in height. When eighty-four years old fire 

 killed the surrounding trees and set this one entirely free, in 

 which condition it remained until it was cut eighteen years 

 later. When cut it measured fifty-five feet high, thirteen 

 inches through at the base and contained 29.95 cubic feet of 

 timber. During the last ten years it hade mad an average 

 annual increase of 1.5 cubic feet. 



This study brought out the following facts: (1 ) While 



