18 



MEASUREMENT OF A STAND OF TREES OR GROWING STOCK 



OF A FOREST.* 



On first thought it appears to be a very simple problem to measure 

 the contents of a stand of trees or a forest, since a forest is an aggre- 

 gate of single trees whose volume we already know how to determine. 

 It appears as though we should need only to measure each tree and add 

 the results. But this would be an expensive operation, and since 

 absolute accuracy is neither necessary nor attainable, a method of 

 averaging is employed in which the trees composing the forest are 

 grouped into classes and only sample trees of each class are measured. 

 The measurements are extended either over the whole forest or over 

 only small typical areas, usually called " sample areas." 



DETERMINATION OF THE GROWING STOCK BY EXTENDING THE 

 MEASUREMENTS OVER THE WHOLE FOREST. 



When the forest is not large its growing stock is usually determined 

 by extending the measurements over the whole forest, i.e., the diameters, 

 breast-high, of all the trees constituting the forest are measured with 

 calipers. Of course the diameter measurements of different species are 

 kept separate. If there exists an interdependence between the height 

 and diameter growth, i. e., if the species grows at a uniform rate and 

 trees of larger or smaller diameters are correspondingly taller or 

 shorter in height, there is no necessity of measuring the heights; the 

 average height is then determined by a sample tree, the selection of 

 which will be discussed later. But if the height is not proportional to 

 the diameter, i. e., if trees of the same species and equal diameters 

 differ considerably in height, then classification by height becomes also 

 necessary, and the scoring of trees is done not only by diameter but 

 also by height classes. Differences in height development usually 

 occur when the same species are found in the same forest under differ- 

 ent soil conditions. A shallow compact clay soil for instance would pro- 

 duce relatively different proportions from a deep, loose, loamy sand. 



* In this country it has so far been customary only to estimate the growing stock. 



The result necessarilv is mostly far from the truth even witli the most expert esti- 

 mator, and as the estimator is usuallj^ employed by a purchaser, the estimate usually 

 comes out from 10 to 30 per cent and more below the actual volume. 



There are various ways in which estimators proceed. One of the most frequently 

 used is to establish by either measurement or estimate for the district to be estimated, 

 the average numb(?r of superficial feet per tree, then the trees are counted and their 

 number multiplied by the figure obtained for the average tree, making allowance 

 at the same time for breakage, defects, etc. 



This method is especially in use where one species uniformly developed is to be 

 estimated. More detailed estimates are made when several species of economic value 

 are to be taken into account. 



