152 COLONIAL REPORTS MISCELLANEOUS. 



The area counted over should not be less than 5 per cent, of 

 the total area of the forest. 



It will often be found that the particular species that is being 

 counted occurs in patches at very long- intervals of a mile or 

 more in such cases, and when calculating the area of the survey, 

 the total length of distance chained over should be reduced by 

 the number of chains travelled over in these blanks. 



The results obtained from the valuation surveys should be 

 utilized, not for the purpose of estimating the actual quantity of 

 growing stock in the forest, but with a view to ascertaining the 

 proportion that exists between the numbers of trees in the 

 different girth classes. These proportions or ratios will furnish 

 the most reliable guide as to the condition of the age gradations 

 and their approach to or divergence from that existing in a 

 normal forest. 



Once the rate of growth is known and the condition of the age 

 gradations, it will be possible to frame a simple prescription for 

 the fellings. 



For example, suppose the age gradations are fairly well 

 represented and it has been found that it requires a period of 

 forty-five years for a tree of the middle girth class to attain the 

 dimensions of those in the mature or exploitable class, then if 

 G be the total number of trees in the latter class, the annual 

 yield, Y, of the forest (in trees) will be 



Y = G 



45 



Now supposing the forest is leased for a period say of fifteen 

 years, then the prescription to cover such a period would be 

 somewhat as follows : 



' The lessee shall not fell during the period of his lease more 

 than -| x G mature trees or ^ x G- trees, that is to say, not 

 more than one-third of the total number of mature trees in the 

 forest." This is a simple prescription to apply and check. 



In a similar manner, suppose it is determined to work over 

 the whole area in fifteen years and it is found that the particular 

 species being exploited is found more or less uniformly distributed 

 through the area. Then if the area is divided into fifteen annual 

 coups, one of which is taken in hand each year in succession and 

 one-third of the mature trees in each of these coups are removed, 

 the prescription of felling -"-- trees annually will be satisfied. 



For, the trees being uniformly distributed, on j- of the area 

 there will be rs mature trees, and J of this is equal to -f S , the 

 annual yield. 



If it happens, as is very likely to be the case with the "West. 

 African forests, that the age classes are deficient, especially the 

 middle and younger ones, then the number of years required to 

 replace the surplus stock of mature trees will, in the example 

 given, be much greater than forty -five years ; say it is calculated 

 from the rate of growth of the trees composing the younger classes 

 that another 11 years will be required to replace all the mature 

 trees, then the annual felling prescription will be equal to 



G 

 (45 + n)' 



