y as this determines the moisture
contents. Very little nourishment is actually taken out of the
soil for, as someone has said, wood is nothing but air solidified
by sunshine. A tree's immense and complicated foliage system is
the laboratory with which it effects this transformation.

Since air exists everywhere and the chemical quality of the soil
is comparatively unimportant, the requirements of different species
for light, heat and moisture are what mainly determine their
distribution and habits of growth. And since heat and moisture are
largely climatic factors and fairly uniform in given localities,
it follows that the demand of a species upon light may practically
fix its habits and possibilities in those localities. The very great
variance of species in light requirement accounts to a large extent
for the composition of most primeval forests. It is of peculiar
importance in the management of forests by man because he cannot
control it as he may be able to control some of the other agencies
which affected the primeval forest, such as fire or seed supply.

SELECTION FORESTS

It would be unprofitable to discuss here all the many methods of
forest management which have proved to be best, technically, for
given species and combinations of species. Where market and
transportation facilities are highly favorable, as in Europe, the
timber owner can adopt the method which will bring the best results,
but here he has no such choice. He can but bear in mind certain
fundamental principles, uniformly applicable to large areas for
considerable periods of time. Roughly, however, our Western forests
can be classified by their adaptability to the two directly opposite
systems, known as clean cutting and selection cutting, of which
almost all methods are modifications.

A selection forest is one in which all ages of trees exist, from
seedling to maturity. It is the natural growth of species which
are tolerant of shade. In a natural state, undisturbed by cutting,
it maintains much the same aspect continuously, for as the oldest
trees die, their place is taken by younger ones. Obviously such a
forest must be composed of species, whether one or several, which
can grow beneath its own shade. The understories of varying ages
are as dense as their light requirements and the density of the
overwood permit.

The common hardwood forests of the East illustrate one type of
the natural selection forest. On the Pacific slope an example