162 WEST AFRICAN FORESTS AND FORESTRY 
greater supervision and more stringent forest regulations than in the 
ordinary leased areas, on the whole his prospects of getting out greater 
quantities of timber more expeditiously and cheaply are much greater 
than in the ordinary areas. 
Particularizing some of the great concrete objects for which reserve 
is made: 
Firstly, the permanent supplies of timber of all kinds, with a view 
to turning out approximately the same quantity year after year in 
perpetuity—in fact, if anything, gradually increasing the output year 
after year. This means to say that, although the forest is cut down, 
it is replaced at the same rate at which it is cut down. Putting it in 
another way, there must be as many blocks, or portions. or actual 
“stands ”’ in the forest as represent the number of years which elapse 
between the time a tree is planted and the time it is cut down. This 
period is known to foresters as ‘‘ the rotation,” and in northern temper- 
ate countries, for Coniferous trees the time is usually reckoned at about 
80 years. In the tropics, with mahogany, Iroko and other timbers, 
it may be only necessary to allow 60 years, or perhaps on very poor 
ground 100 years. In the case of teak in Burma, a rotation between 
80 and 100 years is perhaps about sufficient to enable merchantable 
timber to be produced. Putting it in another way, in the forest, 
in order to have the permanent yield, there must be sufficient trees 
of each girth class, so that when the largest, say those over 12 feet in 
girth, are cut down, there must be sufficient of those between 10 and 
12 feet, which will grow during a 10-year period (in which they are 
cut) from 10 feet to 12 feet in girth. At the present time it has been 
found convenient to classify each class as “under 2 feet, 2 to 4 feet, 
4 to 6 feet, 6 to 8 feet, 8 to 10 feet, 10 to 12 feet, and over 12 feet.” 
The object of this is to clear away, or have cut, all those trees over 
12 feet in girth. This makes room for younger, quicker-growing timber. 
Although, under the Nigerian Forest Law, the girth of mahogany has 
been reduced to 11 feet, and in some districts to 10 feet, eventually, 
as the older and more unremunerative slower-growing trees are cut 
down, it will be possible to reduce the girth still further. To put this 
question of the permanent yield in yet another way: All the trees 
from one year old, or from the smallest size to those over 12 feet in 
girth, represent what we may call the forest capital. Now, the object 
of the rotation is only to take the interest on this capital; that is to 
say, the amount which grows on all the trees over the whole area for 
one year. It would, however, not do to clip off little Pieces from each 
tree over the whole area, as they would be quite useless as timber. 
Therefore this amount is calculated out as so many cubic feet per 
annum, or so many trees above a girth of 12 feet or 11 feet, or whatever 
the girth limit may be, and this amount, either in cubic feet or in the 
number of trees, is allowed to be cut down each year. Then we know 
