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Perhaps you have noticed it yourself -- the manner in which some
trees have their trunks entirely free of limbs for great portions of
their length while others have branches growing from the trunk almost to
the ground. On one of our Nature Trails in the Park this summer there
was a clump of trees that illustrated this feature, which is called
"self pruning", to good advantage and a small sign was placed there to
call attention to it.
Wherever conditions are such that trees grow close together so that
their branches interlace above to form a sort of foliage canopy the
sun's rays are retarded - and often prevented from penetrating to the
ground. Trees need sunlight just as we do and so the leaves, or
needles, of the lower branches on trees of the deep forest do not get
their share of sun and the food making processes there gradually slow
down and finally cease altogether and the needles die. Soon after the
branch follows suite and is sluffed off. In this grove of trees on the
Nature Trail were the healthy branches with their green needles; lower
down were others that were just beginning to show the result of lack of
sunlight; still lower were the dead branches that had lost their needles
and the ground was strewn with dead twigs that had been discarded in
years past. If these trees had been growing in such a place that
permitted an abundance of direct sunlight they would not have lost many
of their lower branches and the trunks, instead of being free from limbs
for the greater portion of their length, would have been possessed of
limbs nearly to the ground. Thus nature aids in the manufacture of
"clear length" lumber -- called thus because the wood is comparatively
free from knots and other blemishes.
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