yield of a given species under given natural conditions will be
the same in the future that it is now. To predict it requires only
study of existing stands without being misled by the influence
of conditions which will not be repeated.

On the other hand, an immense amount of misinformation is circulated
because of superficial observation. Enthusiasts discovering individual
trees which have made prodigious growth, or even fairly extensive
stands on fertile soil with heavy rainfall, will compute sawlog
yields at 40 or 50 years which are much too optimistic for general
application. Others, remembering some stand they have seen in
unfavorable localities, or noting shade-suppressed trees which
will not be paralleled after the virgin forest is removed, are
unduly discouraged. It is most essential that yield tables be made
by trained observers who know how to reach the true average, and
that the figures either actually come from the region to which they
are to be applied or are accompanied by a systematic analysis of
climatic and other conditions which permits intelligent comparison.

In calculating another yield on cut-over land, the system for an
even-aged new growth, such as will follow clean cutting of Douglas
fir, for example, is quite different from that necessary if the
cutting amounts only to selection of the merchantable trees and
leaves a fair stand of smaller ones. In the latter case, yield
tables based on average acreage production are of little use because
so much depends upon the character of the stand which remains on
the tract in question. Here the basis must be the rate of growth
of the average individual tree. An estimate by the number in each
present diameter class may be made of the trees which will escape
logging, showing, let us say for example, about five trees of each