insects are always present in the forest. However, it is only
occasionally that they concentrate and work great injury and
damage in any one section. At rare intervals, some very
destructive insects may centre their work in one district. They
will kill a large number of trees in a short time. They continue
their destruction until some natural agency puts them to flight.
The fungi, on the other hand, develop slowly and work over long
periods. Sudden outbreaks of fungous diseases are unusual.

Heavy snows, lightning and wind storms also lay low many of
the tree giants of the forest. Heavy falls of snow may weigh
down the young, tall trees to such an extent that they break.
Lightning--it is worst in the hills and mountains of the western
states--may strike and damage a number of trees in the same
vicinity. If these trees are not killed outright, they are
usually damaged so badly that forest insects and fungi complete
their destruction.

Big trees are sometimes uprooted during forest storms so that
they fall on younger trees and cripple and deform them. Winds
benefit the forests in that they blow down old trees that are no
longer of much use and provide space for younger and healthier
trees to grow. Usually the trees that are blown down have shallow
roots or else are situated in marshy, wet spots so that their
root-hold in the soil is not secure. Trees that have been exposed
to fire are often weakened and blown down easily.

Where excessive livestock grazing is permitted in young forests
considerable damage may result. Goats, cattle and sheep injure
young seedlings by browsing. They eat the tender shoots of the