do well on the north side require plenty of moisture and cool weather.  Those that prosper on south exposures are equipped to resist late and early frosts as well as very hot sunshine.  The moisture needs of different trees are as remarkable as their likes and dislikes for warmth and cold.  Some trees attain large size in a swampy country.  Trees of the same kind will become stunted in sections where dry weather persists.

In some parts of the United States forestry experts can tell where they are by the local tree growth.  For example, in the extreme northern districts the spruce and the balsam fir are native.  As one travels farther south these give way to little Jack pine and aspen trees.  Next come the stately forests of white and Norway pine.  Sometimes a few slow-growing hemlock trees appear in the colder sections.  If one continues his journey toward the equator he will next pass through forests of broad-leaved trees.  They will include oak, maple, beech, chestnut, hickory, and sycamore.

In Kentucky, which is a centre of the broad-leaved belt, there are several hundred different varieties of trees.  Farther south, the cone-bearing species prevail.  They are followed in the march toward the Gulf of Mexico by the tropical trees of southern Florida.  If one journeys west from the Mississippi River across the Great Plains he finally will come to the Rocky Mountains, where evergreen trees predominate.  If oak, maple, poplar, or other broad-leaved trees grow in that region, they occur in scattered stands.  In the eastern forests the trees are close together.  They form a leafy canopy overhead.  In the forests of the Rockies the evergreens stand some distance apart so that their tops do not touch.  As a result, these Western forests do not shade the ground as well as those in the east.  This causes the soils of these forests to be much drier, and also increases the danger from fire.

The forests of western Washington and Oregon, unlike most timberlands of the Rocky Mountain Region, are as dense as any forests in the world.  Even at midday it is as dark as twilight in these forests.  The trees are gigantic.  They tower 150 to 300 feet above the ground.  Their trunks often are 6 feet or larger in diameter.  They make the trees of the eastern forests look stunted.  They are excelled in size only by the mammoth redwood trees of northern California and the giant Sequoias of the southern Sierras.

[Illustration:  The Sequoias of California]

Differences of climate have largely influenced tree growth and types in this country.  The distribution of tree families is changing all the time.  It shifts just as the climate and other conditions change.  Trees constantly strive among themselves for control of different localities.  For a time one species will predominate.  Then other varieties will appear and displace the ones already established.  The distribution of trees changes very remarkably from one century to another.  For example, in some sections, the red and black oaks are replacing the white oaks.  Some trees are light-lovers.  They require much more sunlight than others that do well under heavy shade.  Oak trees require plenty of light; maples or beeches thrive on little light.