DETERMINING AGE OF AVERAGE TREES AND STAND 339 



the purpose of the investigation is to determine the period which will 

 produce an equal volume of material in an even-aged stand, the product 

 in terms of which this volume is measured actually affects the average 

 age (§260). For board-foot contents which increases more slowly at 

 first and more rapidly later in the life of an individual tree, the average 

 tree wiU be larger and older than for cubic contents, since a portion of 

 the stand will be rejected altogether and fall in a younger age group 

 or else will logically receive a smaller weight in the average for determin- 

 ing the equivalent age of an even-aged stand. 



The first step is therefore to determine the volume of the average 

 tree of the stand or sub-group. It is evident that the inclusion of a 

 large number of trees of the smaller diameters in a large group will 

 pull down the volume of the average tree and tend to unduly lower its 

 age. The plan of subdividing age classes into smaller diameter groups 

 is chiefly useful in avoiding this tendency to error, and is accomplished 

 by throwing together trees varying but little in size, to obtain the 

 average. It is of advantage therefore to make two or more of these 

 sub-groups where possible. 



When volume is measured in cubic feet, basal area may be sub- 

 stituted for volume and the diameter of a tree of average basal area 

 determined. To obtain this, the sum of the basal areas of the trees 

 in the group is divided by the number of trees to obtain average basal 

 area. The diameter of a tree of this area is found in Table LXXVIII, 

 Appendix C, p. 490. 



When measured in board feet, the volume of the average tree is 

 found directly by dividing the total volume of the stand or of the sub- 

 group in board feet by the number of trees. As in case of basal area, 

 the diameter of a tree of this volume is now required if sample trees 

 are to be felled to determine age. For this purpose a local volume 

 table based on diameter is used (§ 142) from which the D.B.H. of 

 a tree of the given volume can be determined to within iV-inch. 



262. Determining the Age of Average Trees and of the Stand. The 

 age of these selected trees can then be obtained by felling trees of this 

 diameter. In stands of variable age from two to three trees are pref- 

 erable to one. As a substitute for this method, where it is extremely 

 uncertain that the tree selected will have the average age, a table of 

 diameter growth showing the ages of trees of different diameters may 

 be prepared from similar stands in the vicinity. If the average rate 

 ot growth thus obtained applies to the stand in question, the age of a 

 tree of the given diameter may be taken from this curve instead of 

 from felled timber. On account of the uncertainty of the correlation 

 between the growth figures obtained in this way and of the age of the 

 stand in question, the method has not been widely used and the felling 



