SPBUCE AND SILVER FIB (PICEA MOBINDA, ABIES PINDBOW) 131 



reproduction, at the same time preventing the growth of noxious 

 weeds. No precise method of executing the regeneration felling 

 is laid down ; it will be enough to prescribe that regeneration fell- 

 ings will be made, leaving it to the knowledge and experience of 

 the local staff to decide in each and every case the amount of 

 illumination desirable and most suitable manner in which the 

 felling should be carried out." 



A periodic block is the part of a forest set aside to be treated Division 

 during a period or number' of years into which the rotation is ^ nto . 

 divided. Each periodic block will be regenerated in its turn 

 during the lapse of one period so that the length of each period 

 should correspond with the time necessary to regenerate a given 

 area of forest under the principles of the silvicultural system with 

 which we are dealing. For instance, a sal working circle such as 

 the Conversion to Uniform Working Circle of Collier's Hald- 

 wani Working Plan with a rotation of 120 years and a regeneration 

 period of 20 years is in theory divided into six periods, e.g., V^ ) = 6 : 

 each of 20 years' duration. With a normal distribution of the 

 age classes the ages of the crops in the different periods would 

 be as follows immediately before commencing work under a new 

 working plan : 



I Period Age of crop 101 120 years. 

 II 81-100 



HI 61- 80 



IV 41- 60 



V 21 40 



VI 1 20 



Such a theoretical distribution of the age classes hardly ever 

 exists in practice. Collier only allotted periodic blocks I, II, and 

 VI, pr scribing special treatment in each case and lumping blocks 

 II, IV, and V together for treatment. Subsequently Collier revised 

 this a.rangement, reducing the rotation to 90 years and the 

 number of the periodic blocks to three. No useful purpose is served 

 by an attempt to make a complete allotment to six periods as was 



