GROWTH OF TREES AND FORESTS 197 



trees. With other species it is necessary to cut and measure a 

 few trees of average height. 



4. Volume Growth. 



One of the simplest methods of determining the volume growth 

 of a stand is to obtain the growth per cent of a mean sample 

 tree, that is, a tree of average size, and apply this per cent to the 

 total estimated volume of the stand. Thus, if the sample tree 

 is growing at the rate of 4 per cent and the total volume of the 

 stand per acre is 20 cords, the growth of the stand may be con- 

 sidered as .8 of a cord per acre. 



Often the mean annual growth of a stand is found and used 

 as a basis for predicting the future growth. To secure this the 

 stand is carefully estimated, and the total volume divided by 

 the average age, which may be considered as the age of the 

 average sized tree. For example, a stand which has produced 

 forty cords to the acre in thirty years may be expected to produce 

 about one and one-third cords per acre a year, for the next few 

 years. This method of predicting growth is only approximate, 

 because the growth of a stand is not uniform throughout its 

 life. During the first part of its life the annual growth would 

 exceed the mean annual growth, but in later years it would be 

 less. 



As a basis of forest management to indicate what different 

 types of forest can produce under various conditions and at 

 different ages, so-called yield tables 1 are in use. Compara- 

 tively few of these have as yet been constructed in this country, 

 but their construction is one of the most important lines of 

 forestry research open to the forester. These tables are based 

 on the measurements of many stands of a given type at different 

 ages, and express the average volume per acre that can be ex- 

 pected at different ages. Such tables may be "local," if based 

 on stands in a single community, or " general," when the data 

 is secured over a large area, as a whole forest region. It will be 



1 Several yield tables are included at the back of the book. 



