Seeds range in weight from the heavy, large black walnut down to specks almost as fine as sand. Some are borne on wings, so remarkably balanced that they ride the winds for miles. Some are sown _only_ by birds and other animals—if not consumed by them first! Many trees produce seeds every year, others less often or irregularly. Of every 100 seeds reaching the ground, only a few may sprout. And the cones of some trees, like jack pine in the Lake States (sometimes called fire pine), and knobcone pine in the West, are opened and their seeds prepared for germination by fire.
The forest manager must reckon with seed behavior and the complex conditions required for seeds to take root and reproduce the forest. Some are best allowed to seed naturally from mature parent trees. Others are best sown by hand or by airplane. Millions of seedlings are grown in National Forest nurseries and are planted in the forest by hand and by machines. Then, once in the ground, the young trees (plantations) may require some shade to survive, freedom from competing brush, and protection from animals, insects, and fire. Providing this is called plantation care.
Forest trees are also classified by their tolerance and intolerance to shade. Sugar maple, for instance, is called a tolerant tree because it will endure as a youngster with only a minimum of sunlight under the cover of taller trees. But Douglas-fir is relatively intolerant to shade, as is black cherry.
Various silvicultural cutting practices are used to obtain the desired degree of light and to encourage seedling establishment. Cutting can alter the density of the forest canopy or _overstory_ to enable more light to reach the forest floor. Or it can open a new bed for seed if located the proper distance from nearby uncut stands. For southern pines, such a distance is usually not over 500 feet; for red spruce, in contrast, it may be as far as 1,500 feet.
In essence, three systems of harvest cutting are applied on the National Forests: Selection cutting, seed tree cutting, and clear cutting, with variations based on specific terrain and other conditions.
Some forests are best managed through selection or partial cutting. In starting harvest of a virgin stand, older trees are cut first. So are the defective and diseased. Younger, healthier trees are encouraged, by this release from competition, to further growth—much like the weeding of a garden. The forest is also opened to stimulate new seedling growth. In seed tree cutting, the entire stand is logged except for a few carefully selected seed trees which are left to regenerate the forest. These, in turn, are harvested after the new stand has been successfully started.