326 PRINCIPLES UNDERLYING THE STUDY OF GROWTH 



false rings may be formed, giving two rings per season (§ 256). Pro- 

 vided age can be determined, the study of diameter, height and volume 

 growth of individual trees is entirely uimecessary for even-aged stands, 

 as a means of determining the yields per acre. 



But where stands are composed of trees of different ages on the 

 same area, it becomes practically impossible to determine the average 

 age of the stand by any such direct method. Within certain limits, 

 that is, if the ages of the trees composing the stand do not vary too 

 greatly, it is possible to determine an age which may be accepted as 

 the average period required to produce the present volume. Where 

 the diversit}^ of age is so great that this is impossible, it is necessary 

 to shift the basis of age determination from the mere counting of the 

 rings to a determination of the age of trees of a given size or diameter. 

 To determine ages, trees must be cut down or the center reached by 

 borings or choppings. While possible on one or two trees, it becomes 

 out of the question to test every tree in this manner without cutting 

 down the stand. Diameter, on the other hand, can be readily measured. 

 For stands of mixed ages, therefore, two methods are possible. By 

 the first, the average diameter of the trees in the stand is found, and the 

 age of a tree of this size is determined and is assumed to indicate the 

 average age of the stand. By the second, no attempt is made to 

 determine the age of the stand, but instead the growth may be studied 

 for trees of given diameters, and for a short current period, past and 

 future. Either method requires the measurement of the diameter 

 growth of trees to determine the number of years or period which is 

 required to produce trees of given sizes or to grow 1 inch in diameter. 



254. The Tree or Stem Analysis and the Limitations of its Use. 

 The volume growth of an individual tree may be analyzed with almost 

 absolute accuracy by cross-sectioning the bole and measuring the width 

 of the annual rings at different sections by decades. This is termed 

 stem analysis, or tree analysis. The accuracy of these results for a single 

 tree is apt to create a false impression in the minds of investigators 

 as to the value of the figures thus obtained. To what use will volume 

 or total tree analyses of growth of trees be put? What question will 

 they answer? Will they predict the growth per acre of stands or the 

 rate of growth per year on an acre of land? The cost of a tree analysis 

 is excessive compared with the direct measurements of yields and 

 total age or even the measurement of diameter growth on the stump. 

 The number of trees which may be analyzed is therefore limited. How 

 shall these trees be selected? It has been seen in the study of volume 

 tables that trees vary quite extensively in form. To get average 

 growth we must be sure of obtaining average form. Average form is 

 best obtained by averaging hundreds of trees as is done in the prepa- 



