the same time the lower branches are killed by shade and drop off,
the scars being healed and eventually buried. The pin knots near
the center of a big clear log are the remains of branches which
when living were at the top of the young tree.

This is why, if it is to produce good timber, any forest must be
dense enough to cover the ground throughout the early part of its
life at least. When we see an excellent clear stand of mature Douglas
fir, for example, we may know that it consists of the comparatively
few survivors of a close sapling growth in which the weak were
gradually killed out after serving their office of pruning and
forcing the vigorous. Had only the trees we now see been on the
ground they would be worthless except for firewood. For the same
reason artificial forest planting must be thick, although the fillers
or nurse trees may be of inferior species if not of so rapid growth
as to gain the mastery.

Nature teaches many lessons which we must recognize in artificial
management or fail, but she is no more the best grower of forest
crops than she is of agricultural crops. We have to study natural
methods of forest perpetuation to see how they may be improved upon
as much as to adopt them as models. As a rule the virgin forest is
exceedingly wasteful of ground. The possibilities under intelligent
care are not indicated by nature's average, but by her accidental
best, and usually they far exceed even this. A fair comparison is
that of scientific farming with unsystematic gleaning from wild
and untended fields. The foregoing general principles of forest
growth have been purposely outlined very briefly so as to serve
as a mere introduction to their application or modification in
concrete cases.