There are large areas, however, in both the United States and Europe, that are treeless. In some instances this condition, without doubt, resulted from the fires that annually burnt the grass. With the cessation of the prairie fires, forest growths have steadily increased.
In other instances these areas are treeless because the seeds of trees have never been planted there. The high plains at the eastern base of the Rocky Mountains are an example. This region is deficient in the moisture required to give young trees the vigorous start that will carry them to maturity. Moreover, the westerly winds and the streams of this region come from localities also deficient in forestry, and there are therefore no seeds to be carried.
As a rule, the distribution of forests is effected by the winds and by moving water. The prevailing westerly winds of the temperate zones have carried many species eastward and have extended the forest areas in that direction. Freshets, floods, and overflows have been even more active in carrying seeds, sprouts, and even trees into new territories. Waves and currents have likewise played a similar part. Wherever the soil of the region into which the species have been carried is moist and nutritious, the forest growth has usually extended.
The Pine Family.—The pine family includes the various species of pine, tamarack, spruce, hemlock, fir, juniper, larch, cypress, and cedar. A few members of the family thrive in the warmer parts of the temperate zone, but for the greater part they flourish between the fortieth and sixtieth parallels. Most of the species found in low latitudes are mountain-trees. They constitute the greater part of the American and Russian forests. The American pine forest is thought to be the largest in the world.
The white pine (Pinus strobus) is the most valuable member of the family. Its value is due in part to the fact that the wood is soft, clear, and easily worked, and in part to the accessibility of the forests. Not much inroad has yet been made upon the great Russian forest, owing to the fact that the timber is too far away from seaports and water transportation. Rough lumber becomes too expensive for use when transported by land, but it will stand the expense of shipment by water many miles.
The Georgia or long-leafed pine (Pinus palustris) is also commonly called pitch pine, turpentine pine, and southern pine; it grows chiefly along the south Atlantic coast and in the northern counties of Georgia. It is harder than white pine and makes excellent flooring.
The sugar pine (Pinus lambertiana) occurs mainly in Oregon and California. The grain is fine and soft and the trees reach a large girth.
The loblolly pine (Pinus tæda) has a considerably larger area than the Georgia pine, extending into Indian Territory. The short-leaf pine (Pinus echinata) occurs in small areas from New York to the Gulf of Mexico, and across to Missouri; it is the Chattahoochee pine of Florida. The pitch pine (Pinus rigida) occurs in various areas mainly north of the Ohio River and west of the prairies. The lumber cut annually from these pines aggregates about thirty billion feet.
The common white cedar (Chamæcyparis thyoides) occurs along the Atlantic and Gulf coasts nearly to the Mississippi. On account of its fine grain it is much used in cabinet work and as a finishing wood. Red cedar, probably a different species, occurs along the Atlantic coast. It is largely used in the manufacture of lead-pencils, and the forests are wellnigh exhausted.