GROWING BETTER TIMBER

ARTHUR KOEHLER.

The man who grows trees for timber will do well to remember that as the twig is bent the tree is inclined. He will find that he can guide natural processes and improve on them. With a purpose like the one watchful parents and teachers have with young people, he can straighten out deficiencies in tree growth by his proper management of young stands of timber, which, if left to follow their bent, make inferior wood.

He knows less about the possibilities of improving on nature in growing timber than he does about agricultural crops. But because many of the present second-growth stands still are in the formative stage and all future stands will be so, his opportunities for improving the quality of the wood in such stands are many. As a rule, second-growth forests (that is, young forests that develop after the old, virgin growth is removed) are smaller when they are cut, have more taper, produce a smaller proportion of knot-free wood, furnish little quarter-sawed lumber, and their individual boards vary more in width and density than old-growth timber.

Furthermore, although intensive cultural methods to improve the quality of the crop may not be so well justified for forest as for agricultural products, the difference in value between timber of poor quality and timber of good quality is so large that net profit and the usefulness of forest products can be enhanced by judicious timber-growing practices.

The first question is: What quality of timber are we going to want when the crop is mature in 25 to 100 years?

Sawlogs and veneer logs generally will be the chief products, in volume and value, of commercial forests for generations to come, because timber, lumber, and veneer have certain outstanding characteristics unequaled by other materials comparatively low cost of manufacture, ease of working, ease of fastening with nails, screws, and glues, light weight coupled with adequate strength in appropriate sizes for many uses. It is likely that coarse-fiber products (insulating boards, sheathing boards, hard boards, and papers for fiberboard-box manufacture) will find a wider future use than now; for them, however, we should be able to get a large part of the raw material from thinnings, forest residues, low-quality wood, secondary species, and offal from the major wood-utilization processes. Timber, ties, poles, and most lumber and veneer products will still require natural wood of good quality.

What kind of trees do we want for timbers, lumber, and veneer?

IN THE FIRST PLACE, they must have adequate size in order to be converted and used profitably. In the future, that size probably will be somewhere between 12 and 24 inches in diameter. It may not be profitable to grow trees 4 feet in diameter on a commercial basis because it takes too long. But size is only one consideration. Fully as important are the form of the tree trunk and the defects and quality of the clear wood that it contains.

A valuable quality in trees for saw-logs and veneer is straightness and uprightness of the trunk. Crookedness in logs reduces the amount of lumber and the maximum size of timbers that can be cut from them and also causes warping of sawed products in drying, difficulty in getting a smooth surface, and, because of the cross grain that accompanies crookedness low strength.

Leaning tree trunks usually are curved up or down. They also produce abnormal wood in softwoods, on the lower side, where it is known as compression wood; in hardwoods, on the upper side, where it is known as tension wood. M. Y. Pillow, in his investigations at the Forest Products Laboratory, found that both types of abnormal wood shrink excessively and unevenly along the grain in drying, so that large and small pieces alike are crooked, and that they have unreliable strength properties. Compression wood becomes more pronounced the farther the tree trunk leans and the faster it grows. In rapidly growing, second-growth softwood stands it is especially important to eliminate trees that lean 5 or more. Less is known about tension wood in hardwoods.

It is not practical to straighten small trees that are crooked or leaning. Deformed and inclined trees should be removed while young; they will not produce high-grade wood.

Excessive taper also is objectionable, for obvious reasons, in logs for veneer, electric-wire poles, piling, railway ties, and fence posts.

Taper is governed by the ratio of diameter to height growth.

ANNUAL GROWTH in height is determined principally by the quality of the site, that is, climatic and soil conditions. The density of the stand influences the height growth only slightly.

Growth in diameter is determined by the quality of the site and the density of the stand. On a given site the ratio of diameter to height growth, or the amount of taper, is determined by the growing space of a tree. The faster trees grow in diameter, the more taper they will have. Open-grown trees have too much taper for many uses. As will be seen later, growing space also influences the size and persistence of the lower limbs, hence taper also is an index of the character of the hidden knots in a tree trunk; that is, the greater the taper, the larger the knots.

Even when trees grow straight and vertical, the grain in them that is, the direction of the fibers often is not parallel with the axis of the stem. Various types of distortions of the fibers, some of them detrimental and some advantageous, may occur. Spiral grain, which is an inclined growth of the fibers that gives the trunks a twisted appearance, may occur in individual trees of any species. The twist may be to the right or to the left; usually it is more pronounced in wood the farther it is from the center of the trunk. This is a point in favor of second-growth, because the trees are smaller when harvested than are old-growth trees and consequently the maximum slope of spiral grain should average less in second-growth timber.

SPIRAL GRAIN is consistently objectionable. It causes poles, timbers, ties, and lumber to twist during drying. It has a weakening effect when the slope is greater than 1 in 20. It causes chipping and roughness when lumber is planed against the grain.

We do not know the cause of spiral grain, but we do know that it is not caused by actual twisting of the tree trunk by the wind or otherwise. Opinions differ as to whether spiral grain is due to heredity or environment. It seems to be more severe in trees that grow slowly under adverse conditions, as at timber line; it may be that slow growth brings out more strongly any hereditary tendencies toward spiral grain that may be present.

TO BE ON THE SAFE SIDE, seed for forest planting should not be collected from trees that have spiral grain. Young trees with spiral grain should be removed from a forest as soon as convenient after they are discovered. In trees with stringy outer bark, such as the cedars, cypress, sequoias, and willows, the direction of the grain in the wood can be gaged by the direction of the fibers in the bark or by bark ridges. Even in such trees as pine, Douglas-fir, white oak, elm, ash, and the basswood, which have scaly bark with pronounced fissures and ridges after they have passed the young stage, spiral grain can be detected by the direction of the ridges in the bark. In many kinds of young trees with smooth bark, unfortunately, spiral grain cannot be detected by any simple means.

INTERLOCKED GRAIN, that is, spiral grain that reverses in direction from right to left and back every few years, is hereditary, because it occurs almost universally in certain species, notably sweetgum, black tupelo, and many of the tropical species. It produces a beautiful ribbon figure in quarter-sawed lumber and quarter-sliced veneer, especially in species in which the wood has a high natural luster, such as mahogany, Philippine lauan, and African sapele. But it also causes lumber, especially plain-sawed boards, t6 warp in drying, and makes planing difficult, because the knives must cut against the grain in part of the board no matter which way it is planed. Wood with interlocked grain is difficult to split, although for driveway planking and large rollers, such as those used for house moving, that is an advantage.

Other types of distorted grain that occur in occasional trees are wavy, curly, and bird's-eye grain, all of which are considered ornamental and increase the value of the trees in which they are found. Unfortunately, they cannot be detected easily without mutilating the young trees, although a limited amount of research indicates that, if the outer bark is removed over a small area of the stem, the pattern of the grain is revealed by the fibers in the inner bark, which follow the same course as the wood fibers. Cutting into but not through the inner bark in spots does not damage the tree.

The profits from growing trees certainly could be increased if wood of desirable types of figure could be produced at will. Apparently successful experiments are being made in Finland in growing figured birch. If, as in the case of walnut, a delicious nut with a thin shell could be produced in addition to figured wood, there need be little question as to whether the financial outcome of growing such timber would be plus or minus. Problems of that kind require a great deal of special study for a long period, but, like all research, it need not be repeated once it is done thoroughly.

KNOTS, the most common defects in lumber, are the bases of live and dead branches imbedded in the growing tree trunk. They affect the appearance, smoothness, strength, tightness, finishing, and other properties of lumber and veneer. Lumber without knots is worth three or four times as much as knotty lumber, except where the knots are such that they are considered ornamental. The parts of knots that are produced by limbs while green, known as intergrown knots, are not so detrimental as those produced by limbs that persist after death, which often are discolored, even partly decayed, and loose.

The development of knots in trees can be reduced in two ways. One way is to maintain stand conditions crowded enough while the trees are young that the lower branches will die and break off while they and the tree trunk are still small in diameter. In such trees the knots in the lower part of the trunk, especially the intergrown parts, as a rule will be shorter.

The dead branches often persist for an extraordinarily long time in some species, notably eastern and western white pines, sugar pine, red pine, ponderosa pine, Douglas-fir, and Engelmann spruce. They may hang on after death for 50 to 150 years or more before they break off, leaving longer dead knots than intergrown knots in the lower portion of tree trunks from stands that are fairly well-stocked. In such species, practically no knot-free lumber can be produced naturally in a commercially reasonable length of time, 75 to 125 years.