some of them, frost nips some and excessive moisture and unfavorable
soil conditions prevent others from starting. The few successful
ones soon sprout into a number of young trees that grow thriftily
until their crowns begin to meet. When the trees have thus met, the
struggle is at its height. The side branches encroach upon each
other (Fig. 123), shut out the light without which the branches
cannot live, and finally kill each other off. The upper branches vie
with one another for light, grow unusually fast, and the trees
increase in height with special rapidity. This is nature's method of
producing clear, straight trunks which are so desirable for poles
and large timber. In this struggle for dominance, some survive and
tower above the others, but many become stunted and fail to grow,
while the majority become entirely overtopped and succumb in the
struggle; see Fig. 139.

But in this strife there is also mutual aid. Each tree helps to
protect its neighbors against the danger of being uprooted by the
wind, and against the sun, which is liable to dry up the rich soil
around the roots. This soil is different from the soil on the open
lawn. It consists of an accumulation of decayed leaves mixed with
inorganic matter, forming, together, a rich composition known as
_humus_. The trees also aid each other in forming a close canopy
that prevents the rapid evaporation of water from the ground.

The intensity of these conditions will vary a great deal with the
composition of the forest and the nature and habits of the
individual trees. By composition, or type of forest, is meant the
proportion in which the various species of trees are grouped; i.e.,
whether a certain section of woodland is composed of one species or
of a mixture of species. By habit is meant the requirements of the