that, since for a period of its life the normal individual tree
increases in wood production and then declines, it is bad economy
to cut it while it is still growing rapidly or to allow it, after
slowing down, to occupy ground which might be used by a tree still
in the vigor of production. For example, if at 100 years old it
contains 500 board feet, it has averaged an addition of 5 feet,
a year throughout its life. If at 125 years old it contains but
560 feet, the average increment will be but 4-1/2 feet a year.
It will not give equal return for the soil, moisture and light
it monopolizes during these 25 years. At the same time, probably
there are young trees nearby which hitherto have averaged below
the maximum, but if released from its competition will forge ahead
for a period at the end of which they will give a greater annual
return than if cut at present. It would be as bad economy to cut
these today as to spare the over-mature tree. In short, the production
of the forest is not only sustained, but actually increased, by
removing the oldest trees at just the proper time; and is decreased
by taking out young trees either not yet at the natural age of
greatest mean annual increment or capable of artificial stimulation
by thinning.

By studying the relation of age to production in the particular
locality, the proportion of different age classes, and also finding
the approximate average diameter which corresponds to the age at
which he desires to cut, the professional forester can make a very
accurate selection of the trees which can be removed to best advantage
at present and also fix the time and yield of the next cutting.
Fortunately, however, commercial and silvicultural considerations
accidentally coincide so nearly under average yellow pine conditions
as to make certain rough rules which can be laid down entirely