30 or 40 years, taking out each time the trees which have passed
the minimum diameter since the last previous cut. It is obvious,
however, that if the process is to continue indefinitely, protection
must be absolute. Destruction of young growth will stop the rotation
at the time the surviving older material is harvested. At each
cut the brush should be disposed of with this end in view. If the
stand is very thin it may not add much to the danger of fire and,
especially if reproduction is difficult and requires shelter, may
best be left spread on the ground at some distance from remaining
trees. Otherwise, and this is the rule, it should be piled and
usually burned. In this process and in logging every effort should
be made to protect existing young growth from injury. Ground fires
should be prevented now and always hereafter.

So far, however, we have been considering how to make the most
of a stand of many ages, due to constant reproduction permitted
by the light supply in a fairly open forest. On the other hand,
yellow pine sometimes produces a mature stand so heavy that there
is little young growth beneath it, or even a thin old stand with
either little reproduction or an invasion of lodge-pole pine. Such
conditions are usually due to fire at some period. In the first
of these cases, usually the dense stand has resulted from a fire
which destroyed its predecessor not so completely as to remove the
seed supply, but sufficiently to afford light for a more uniformly
dense crop of seedlings than would occur in the normal forest.
These have been thinned out as the stand grew old, but never to a
degree which allowed much reproduction beneath them. The natural
cycle will be begun again in time, for toward the end of the life of
this unusually heavy stand, seedlings will begin to appear gradually
as individual old trees die and admit more and more light. The