[Illustration: Forest suited for selective cutting]
Douglas-fir, however, is one of several species best managed with clear cutting in blocks. Selection cutting of the lusty giant has been tried in the National Forests, but with little success. It proved difficult to remove the tall Douglas-firs, standing over 200 feet, without seriously injuring others as they fell. Those remaining in a stand, shorn of protection from neighboring trees above and interweaving roots below, became victims of blowdown in high winds. Nor would Douglas-fir reproduce itself without benefit of full sunlight. Thus, clear cutting or patch cutting is practiced on blocks of 40 to 100 acres; this enables sunlight to reach the ground and to help the valuable Douglas-fir forest renew itself. Seeding by hand or by airplane and planting nursery-grown seedlings are methods used to reforest these large openings.
Throughout the 182 million acres of the National Forests other important factors are weighed in the balance with the need to produce timber. They may be the reason for heavy cutting, light cutting, or no cutting.
Wildlife, for example, has become increasingly dependent upon the National Forests. Almost everywhere the land available to it has been reduced through the spread of towns, cities, industries, and highways. It is estimated that the native American forest once covered more than 900 million acres, with extensive borders and many open places where grass, plants, and herbs grew and served as food for deer, elk, wild turkey, and other animals and birds. In our modern managed forests, clear cutting and heavy selection cutting make more food available to animals and birds by opening clearings and creating _edges_.
On the other hand, fish are benefited by not harvesting timber along the streamside. The forest canopy provides shade and a favorably cool temperature. Insects fall in the water from overhanging branches, providing food. Roots of trees and shrubs bind the soil, holding banks in place, affording sheltered retreats along the streamside.
A vital factor in balanced management is the need to protect the forest as a natural reservoir of water. Uncontrolled logging and fire can strip the land of its porous cover, leaving it unable to absorb water. The disastrous consequences are soil erosion, flash floods, and muddy streams. Thus, as soil conditions require, the forester may plan light to moderate selection cuttings. Or in other cases, as in the Southwest where virgin timber grows amid a dense understory of young growth and water is in short supply for a growing population, he may consider heavy thinnings to replace deep-rooted trees with grasses and broadleaf herbs, making more water available for human use.