48 THE BOOK OF FORESTRY 
soon die from lack of light and food and the next high 
wind or ice storm breaks them off leaving the lower part 
of the trunk bare of limbs. This process is called 
“natural pruning” and explains why trees which grew 
in dense stands in their youth produce such splendid 
clear timber free from knots. Whenever a tree is found 
with thick, wide-spreading branches low down on 
the trunk it is certain that this individual had full 
light upon all sides when it was a sapling else it would 
not have kept these limbs so long. 
During this period of rapid height growth every tree 
is struggling for all the sunlight it can get. Each 
acre of ground has just so much rain and so many 
days of sunlight in the growing season. Naturally if 
the seedlings have been very densely sown by Mother 
Nature, say 10,000 or 15,000 per acre, there is not 
enough food to go around and those that were weak 
or a little slow in getting started fall behind. Once 
they are overtopped, sunlight is cut off which deprives 
the submerged tree of the energy required to assimilate 
its food and then it dies of starvation, or soon falls a 
prey to fungus or insect attacks. 
In this way the number of pine or spruce trees 
standing upon an acre of the old pasture becomes 
greatly diminished during the first forty years of reoc- 
cupation. By that time the weakest have fallen down, 
have rotted and may have vanished completely beneath 
the needle carpet. The leaves and twigs which have 
fallen as well as the trunks, have decomposed to form 
the layer of dark mold or humus lying on top of the 
mineral soil while on top of the humus may be found 
the carpet of needles and twigs which have not yet 
been long enough exposed to fungi and bacteria to have 
lost their structure. 
