NOTES ON THE FORESTS OF AUVERGNE. 33 



(b) By Volume. — At each working-plan revision an inventory 

 of the growing stock is made under the three age-classes of 



(i.) Old wood; trees more than two-thirds the exploitable 



diameter, 

 (ii.) Middle-aged wood ) trees less than two-thirds but more 



than one-third the exploitable diameter, 

 (iii.) Young wood; less than one-third the exploitable 



diameter — this need not be callipered. 



The method is based on the conception that a Selection 

 forest, normally constituted, is just like an even-aged forest 

 except that the various ages are intermingled. In the even- 

 aged forest a regular yield is got by cutting each year equal 

 areas, but in Selection, only ripe trees can be removed through- 

 out the wood without any reference being made to area. 

 Assuming that the ratio of old to middle-age-class is as 

 5 to 3, it can be taken for granted that these two classes are 

 similar to the first two periodic blocks of a regular forest 

 of three periodic blocks. For a rotation of 150 years, each 

 of these three age-classes would have a mean age of 25, 75, 

 and 125. This ratio of i, 3 and 5 for the three age-classes 

 presumes that the future growth will be equal to the average 

 growth, which is probably inaccurate, but the French prefer 

 approximate formulae and frequent stock revisions to the German 

 methods of detailed formulae built up on past growth figures. 



The total volume of the old-age-class, which is thus presumed 

 to have been built up in a third of the rotation, is divided 

 by the number of years in a third of the rotation, to give the 

 annual yield. If the ratio of old to middle-aged is not 5 to 3 

 the two alternatives are:— (i) An excess of old wood, in 

 which case the surplus can be spread out over a fixed number 

 of years and removed. (2) An excess of the middle-age- class, 

 in which case the minimum diameter for the old tree-class can 

 be temporarily lowered, so that a larger percentage of middle- 

 aged stems can be included in the immediate cutting-plan. 



In many woods, such as that of Boisgrand in the Libardois, 

 the increment is not taken into consideration in calculating the 

 possibility but is saved up as a reserve. Actually this sounds 

 a very conservative policy, but the accumulation only goes on 

 between one working-plan revision and the next, for at each 

 revision the accumulated growing stock is redistributed in the 



VOL. XXXVII. PART I C 



