236 GOVERNMENT REGULATION AND WORKING PLANS 



Then if we designate the volume of the 75-year-old wood as 3 the 125- 

 year-old volume will be 



3 

 S -\- yzX 50 = 3 + 2 or 5. This assumption of an equal mean annual 



growth of course is not exact, but according to French reasoning it is 

 sufficiently accurate for an approximate formula which is being continu- 

 ally revised at working plan revisions, when the standing timber is re- 

 calipered. According to the original circular: 



"One can object to this method of classification (see definition, page 234) because 

 the diameters are not exactly proportional to the ages, that they are not equal for the 

 same species, or same age, inasmuch as the trees of a selection forest are very far from 

 growing under the same conditions. But it is to be supposed that with a large number 

 of trees ... a sufficient compensation will take place in order to even off the 

 inaccuracies and render them negligible. Moreover, it is not essential, nor possible, 

 to arrive at exact mathematical results. . . ." 



The language and the argument of the original French instructions are 

 instructive in considering the method and in applying it. As originally 

 promulgated, so as to be conservative, no increase was made in the cut 

 for the growth which took place on the old wood while it was being 

 harvested. But within recent years it is customary to figure growth. 



The method is simple when the proportion of the old wood to the 

 young wood is as 5 to 3 or nearly so (see definition of method, page 234) 

 but this normal ratio is not usually found. Instead there is (1) an 

 excess of old wood, (2) an excess of average wood. In either (1) or 

 (2) an approximately normal ratio is secured by transferring diameter 

 classes from the old wood to the average wood or vice versa if it is safe 

 silviculturally to hold over some of the older trees or if, where the average 

 wood is too great, the large average wood sizes can be cut without too 

 great a sacrifice. 



An important feature of the application of this method by the best 

 French working plan officers is that they compare the actual growing 

 stock, on the basis of number of trees per acre of different sizes, with 

 an empirical "normal" stand (an adjusted average for the region). 

 This is an essential and important part of the method as best applied 

 but is not mentioned in the official instructions. Fig. 19 illustrates 

 the method used, where the actual forest is progressing toward an em- 

 pirically normal state. At the first stocktaking the stand was open; 

 there was an improvement of the stand at the second measurement, 

 and the curve of the third stocktaking is approaching the normal by 

 a wave movement already referred to on page 215, 



A rough area check can be applied, if desired, by considering that 

 the area cut over should be proportional to the volume removed. The 

 original instructions stipulated that (1) the length of the felling period 



