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 is
afforded by hemlock, either practically pure or mixed with white
fir, but probably the most typical is the ordinary Western yellow
pine under certain conditions. At its best this tree composes a
forest so dense that all young growth is shaded out, but everyone
is familiar with the frequent opener stand containing all ages.
The younger trees are often called blackjack.

EVEN-AGED FORESTS

On the other hand, trees extremely intolerant of shade occur only
in what the forester calls even-aged forests. Being unable to start
in the darkness of an existing stand of any considerable density,
they must seize opportunities to recover openings. The Douglas
fir of the Northwest, more commonly called red or yellow fir, is
an excellent illustration. In the interior states this species
reproduces under cover to some extent, because there is a stronger