the forest, the lower branches die and fall off--a process known in
Forestry as "natural pruning," The influence of light on the form of
trees should be well understood by all those who plant trees and by
those designing landscape effects.

[Illustration: FIG. 91.--A Tree in The Forest. Note the tall stem free
from branches and the small, narrow crown.]

(4) Influence of heat: Trees require a certain amount of heat. They
receive it partly from the sun and partly from the soil. Evaporation
prevents the overheating of the crown. The main stem of the tree is
heated by water from the soil; therefore trees in the open begin
growth in the spring earlier than trees in the forest because the
soil in the open is warmer. Shrubs begin their growth earlier than
trees because of the nearness of their crowns to their root systems.
This also explains why a warm rain will start vegetation quickly.
Too much heat will naturally cause excessive drying of the roots or
excessive evaporation from the leaves and therefore more water is
needed by the tree in summer than in winter.

(5) Influence of season and frost: The life processes of a tree are
checked when the temperature sinks below a certain point. The tree
is thus, during the winter, in a period of rest and only a few
chemical changes take place which lead up to the starting of
vegetation. In eastern United States, growth starts in April and
ceases during the latter part of August or in early September. The
different parts of a tree may freeze solid during the winter without
injury, provided the tree is a native one. Exotic trees may suffer
greatly from extreme cold. This is one of the main reasons why it is
always advisable to plant native trees rather than those that are
imported and have not yet been acclimatized. Frosts during
mid-winter are not quite as injurious as early and late frosts and,
therefore, if one is going to protect plants from the winter's cold,
it is well to apply the covering early enough and to keep it on
late enough to overcome this difficulty.

The mechanical injuries from frost are also important. Snow and
sleet will weigh down branches but rarely break them, while frost
will cause them to become brittle and to break easily. Those who
climb and prune trees should be especially cautious on frosty days.

(6) Influence of air