GROWTH OF TREES AND FORESTS 



175 



rings should be counted and the growth measured on the aver- 

 age radius, in order to secure the average growth on all sides. 

 The results of a study of this kind may be expressed in various 

 ways as, for example: the average diameter of trees of different 

 ages; the average age of trees of different diameters; the number 

 of rings in the last inch of radius for trees of different diameters; 

 the number of years required for each diameter inch class to 

 increase one inch. Frequently confusion arises among amateurs 

 between the first and the second of these expressions. They 

 are apt to infer because the average diameter of a species fifty 

 years old is fifteen inches that conversely the average age of trees 

 of that species fifteen inches in diameter is fifty years. It is im- 

 possible to use the tables in this way, since one is made by aver- 

 aging all trees that are fifteen inches in diameter, and the other is 

 made by averaging a different set of trees that happen to be fifty 

 years old. Among these, many that have been suppressed may 

 be only eight or ten inches in diameter, and others that have 

 grown rapidly may be correspondingly larger. 



3. Height Growth. 



By means of a hypsometer,^ the heights of standing trees can 

 be rapidly and easily obtained. A table made from these results 

 will show the average height of trees of different diameters, and 

 as the method of determining the average age of different diam- 

 eters has already been given, the average height growth can be 

 ascertained by combining the two in the following manner. 



See note, Chapter IX. 



