44 



the top. If the section shows n annual rings, the portion cut off 

 is n years old, and the poiat must he found at which there are 

 exactly n such rings, the, number immediately below being n + I. 



When the increment for every stage of the existence of the tree 

 is sought, the stem must be divided into equal sections, say of 5 

 feet length. The number of annual rings counted on the upper 

 surface of each section will give the number of years' growth 

 above it, and the difference between the age of the whole tree and 

 this number will give the number of years which the tree has 

 taken. to attain the height at which the rings have been counted. 

 The preceding mode of procedure must be adopted even for conifers 

 when the lower branches have fallen off. 



It is evident that if the number of eoncenlric rings does not 

 correspond in any way with the age of the tree, the determination 

 of the height increment, except for recent years the shoots corre- 

 sponding to which are apparent at the top of the stem, is impos- 

 sible. 



B. Bate of increase in diameter and basal area. 



As it is impossible, or at least very diflScult, to ascertain the 

 girth-increment by direct measurement, it must be deduced from 

 the diameter increment by multiplication by tt = 3' 14. To 

 obtain the diameter increment, measurements are generally made 

 at the ustial height of 4i foot, or breast-height, but they may be 

 made instead at the middle of the tree, or, if great accuracy is 

 required, at several places. 



In the case of trees forming one or a regular number of rings 

 each year, the diameter increment of a given year or period oE 

 years is found by measuring the thickness of the layer of wood 

 put on during that interval. The thickness of this layer must be 

 measured on several pair of radii, each pair belonging to one and 

 the same diameter. Twice the mean of the several measurements 

 will give the increment sought. A single measurement is 

 justifiable only when both the shape of the stem and crown and the 

 branching are extremely regular. If the tree cannot be felled, 

 Pressler's borer should be used.- The increment of the sectional 

 area at any height will be given by the formula 



I iD^-d^) = 0-7854 {D + d) {D-d) 



4 



