240 GOVERNMENT REGULATION AND WORKING PLANS 



"Each periodic block must be formed of a single contiguous area, naturally de- 

 limited, separated and distinguished in a permanent way from the bordering blocks 

 so as to form a topographic mass in the forest. . . . The yield of chief fellings is 

 calculated by volume, as in Cottas method. The immediate fellings (thinnings) have 

 no fixed yield; it is enough to indicate the annual area they run over during the period. 

 . . . The (exact) location of the principal fellings is not determined (in advance); 

 they take place according to the needs of regeneration, at any point within the periodic 

 block to be realized in turn." 



At the end of each period the yield is recalculated for the periodic 

 block which wiU then be cut over. In theory the areas of periodic 

 blocks should be equal, but owing to varying soil quality they may vary 

 considerably. 



As already stated the period must be long enough so that regeneration 

 can be secured, because "during a period of fixed length an entire periodic 

 block of determined area will be cut." It is considered better to pre- 

 scribe the cutting of 125 acres in 20 years rather than 250 acres in 40 

 years, because in the latter case it leads to irregularity and confusion. 

 For example, with a rotation of 144 years, there would be a choice of 

 (a) eight periods of 18 years, and with oak and a mild climate (a) would 

 be preferred to the (&) or (c) alternatives which follow; (6) six periods 

 of 24 years; (c) four periods of 36 years. The compartments are arranged 

 in the order in which they require cutting, and they put in the first 

 period all compartments most in need of cutting, in the order of 

 urgency. 



If there is a lot of old overmature timber declining in vigor the French 

 use the " precomptage, where they then subtract their volume from the 

 yield of the normal maximum fellings . . . and by the same amount 

 decrease the cut of the block when its turn comes to be cut." 



This is too artificial and often results in confusion. It is really borrow- 

 ing from the future cuts to make a heavy present cut so as to get rid of 

 overmature stands in need of regeneration. 



The main disadvantages of this method if applied too rigidly is that 

 it is impossible to fix the order of cutting in advance even for one period, 

 because the schemes are soon disarranged by nature, and contiguous 

 blocks are impossible. If an ironclad order of cutting is maintained 

 heavy sacrifices must be made, because stands are cut before or after 

 they should be cut silvically. If there are subtractions and transfers, 

 i.e., ''precomptage," then the whole scheme of future management 

 becomes disarranged (see p. 75). Variations in the commonly accepted 

 periodic block method have been suggested by Huffel and others. 

 Changeable periodic blocks which are not formed of contiguous compart- 

 ments are advocated in preference to the fixed periodic block of the 

 older working plans. This means that there is only one periodic block 

 the first which at the end of each period is always being revised. A 



