108 ECONOMICS OF FORESTRY. 



Moreover, the wood of many species undergoes 

 a chemical change as yet unexplained, but supposed 

 to improve its quality, or, as in the black walnut, 

 its usefulness, the change into heart-wood, which 

 begins earlier or later with different species and 

 progresses more or less slowly, so that, while the 

 useful size and form may have been attained, the 

 useful quality may still have to be waited for. 



As the tree develops, it exhibits in all its parts 

 the various sizes and qualities of all its stages of 

 development, but in varying relative proportion, 

 and as the log timber of the bole begins to pre- 

 ponderate over the branch and brushwood of the 

 crown, naturally the value production increases, and 

 influences the financial result of the production. 



Now, the accumulation of annual layers of wood 

 does not proceed by any means in a regular, even 

 rate of equal proportions for each year. Not only 

 is this rate of accretion varying with every species, 

 and with every difference in soil and climate and 

 other surrounding conditions, and with the seasons, 

 but it differs in the different life periods of the 

 tree. 



The soft, light-wooded trees, like the cottonwood, 

 aspen, silver maple, willow, and others, start out with 

 a rapid growth, making good-sized trees in thirty 

 to forty years, then rapidly decline in the rate of 

 growth, and soon cease almost entirely, being com- 

 paratively short-lived. Others, like many of our 

 important hardwoods and useful conifers, grow 



