The development of the tree crowns at different ages in the life of a southern yellow pine stand may be regulated by controlling the number of trees to the acre and their distribution over the area. To bring this about, young southern yellow pine stands should be fully stocked from the beginning. Afterwards, the number of trees and the spacing can be regulated as desired by making thinnings. Thus, within certain limits, it was found that the specific gravity of the wood from the pine trees could be controlled. In farm woodlands the thinnings can be used for materials needed on the farm or for sale as posts or pulpwood.

Although a certain amount of crowding or crown restriction increased the specific gravity of the wood in southern yellow pine, overcrowding had an opposite effect, because with very keen competition among the trees the summerwood portion of the growth ring was reduced. Wood resulting under such conditions was of slow growth and low specific gravity. Such wood is found principally in the outer portion of very old pine trees in virgin forests. It is soft, easy to work, has low shrinkage, and is preferred for many uses where high strength is not required. It was not found in second-growth forests.

Wood of southern yellow pine may be grouped into three principal types, depending upon the condition of growth : First, wood of light weight from rapidly growing young trees having large, wide-spreading crowns; second, heavy, dense wood of a medium growth rate from trees in fairly close stands where the crown development has been somewhat restricted; and third, wood of light weight and extremely slow growth from the outer portion of old trees in virgin-growth stands. It is not unusual to find the three types of wood in the same old-growth tree. The first type, however, is more common in second-growth stands; the second type may be found in either second-growth or old-growth; and the third type only in old-growth stands.

Sometimes growth rings of wood of the first type are as much as an inch in width. Because this wood is of low specific gravity and strength, it is not favored for uses where high mechanical strength is required. As the trees increase in size and the forests become more fully stocked, the growth in diameter takes place more slowly. The slowing of the diameter growth of trees results in formation of wood of the second type, so that two quite distinct kinds of wood occur in the cross section of a tree. The first is wide-ringed and light in weight, and the second has narrower rings and is usually considerably heavier. When trees containing these two kinds of wood are sawed into lumber, many boards or timbers may contain one type of wood on one edge and the other type on another edge. The wide-ringed type of wood has been found to shrink much more in length while drying than the wood of less rapid growth, as shown on page 458.

Unfortunately, there are many areas in the South and elsewhere where such partial and uneven stocking has resulted in the growth of low-quality timber. Average differences in the specific gravity of wood from adjacent understocked and fully stocked stands may amount to as much as 22 percent in southern yellow pine.

A well-stocked stand of southern yellow pine will be productive over a long period and supply high-quality timber of uniform growth rate and relatively high strength for poles, piling, and structural timbers. The production of these better products also allows intermittent revenue from the stand during the time that these products are growing. Periodic thinnings, the yield of which may be considerable, are needed to maintain uniform growth. A gradual removal of the mature trees will provide an opportunity for a new crop to become established by natural seeding.

In existing understocked stands of young trees without sufficient seed trees to fill in the openings, improvement of quality in the growing timber can be made by interplanting to provide the desired density of stocking and by pruning lateral branches to prevent the growth of large limbs low down on the trees, thus providing at least one high-quality log 16 feet in length from each tree.

THE AUTHOR

Benson H. Paul is a silviculturist at the Forest Products Laboratory, maintained at Madison, Wis., by the U. S. Forest Service in cooperation with the University of Wisconsin. Before his present position he served as a forester in the New York State Department of Conservation in charge of reforestation; he supervised the planting of some 50 million seedlings during that time. At the Forest Products Laboratory Mr. Paul investigates the effects on wood properties of such forestry practices as pruning, thinning of tree stands, special growth-control measures, importance of soil moisture to growth, and relation of growing space to lumber quality.