OF WHOLE WOODS. 95 



explained here, as it forms the basis of calculating the incre- 

 ment of whole woods, to be explained further on. 



Example : 



Find the increment which a tree now 50 years old with a 

 volume of 30 c' is likely to lay on during the next 10 years. 



A tree has been found, now 58 years old, with a volume of 

 40 c', and which had 33 cubic feet when it was 50 years old. 



SECTION II. DETERMINATION OF THE INCREMENT OF WHOLE 



WOODS. 



It has been shown that, in the case of single trees, the 

 accumulation of the volume, as well as of the factors which 

 lead up to it, height, diameter, or basal area increment, can 

 be followed backwards with a considerable degree of accu- 

 racy. This is not the case as regards whole woods, because 

 trees die or are taken away in thinnings. Investigations made 

 on sample trees selected in a wood show only the successive 

 development of the individuals existing at the time of examina- 

 tion, but they throw no light on that of those trees which have 

 disappeared in course of time, since the wood was created. 

 Height growth alone makes an exception. An analysis of a 

 number of sample trees will indicate the mean height of these 

 trees during previous periods, which may be taken as the upper 

 height of the wood at those periods. These would of course 

 not represent the mean heights at the several ages, because it 

 may safely be assumed that the now existing trees were, as a 

 rule, always the leading trees. Investigations have proved that 

 the mean height of woods can be deduced from the upper 

 height. For instance, in the case of the Scotch pine the 

 difference ranges from about 3 per cent, to 5 per cent, according 

 to the age of the wood. But no such relation has as yet been 

 found as regards the basal area or the volume, and to evolve 



