daptable
to them. While the one class demands light, the other does not
demand shade. It is merely capable of enduring it. Indeed, except
for the greater susceptibility of some species to extreme heat
and dryness when very young, as a rule shade bearing trees grow
much better if they do have ample light supply. Consequently clean
cutting may be the best system for these also under certain economic
conditions.

Besides its influence upon the occurrence of species in the forest,
light practically governs the physical form of the individual tree.
If grown in an opening and not artificially pruned, a tree will have
a conical trunk and living branches almost down to the ground. The
denser and consequently darker the forest, the more cylindrical the
trunk, the smaller the crown of branches and the greater the clear
length. The individual tree has no object in assuming a desirable
commercial form and does so only when deprived of side light by
numerous neighbors. Then it sacrifices diameter growth to height
growth in reaching for the top light necessary for its life. At
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