FOREST RENEWAL

LEONARD I. BARRETT.

Forestry attempts to perpetuate at the least cost the species that will provide the greatest volume of useful commodities. In forestry, harvesting is followed promptly by a new crop, and maximum productivity is maintained in perpetuity.

The means of establishing new forest crops are few. They include planting small trees or seed, securing a growth of sprouts, and natural seeding from the mature forest. The first two are important locally in several of the forest regions of the United States, but prompt forest renewal, through reproduction by natural seeding, is applicable wherever standing forests exist.

My purpose here is to discuss the basic factors that must be considered in seeking natural forest renewal and the methods that are finding success in the United States.

The methods of renewing forests were born at least 700 years ago, when the feudal lords and communal forest owners of central Europe felt the pinch of short wood supplies and vanishing game habitats and, through edicts and restrictions, sought to perpetuate forest resources. At first, the methods were based on the observations and folklore of huntsmen and did not begin to receive the benefit of systematic and scientific. scrutiny until about the middle of the eighteenth century. From then on, progress was comparatively fast; within 100 years European foresters developed well-defined and effective ways to get continued productivity of forests. The practice of forestry was elevated from a folklore or speculative status to that of applied science.

The beginning of a conservation movement in the United States about 50 years ago saw many attempts in teaching, research, and practice to transplant the European prescriptions to American forests. They did not succeed too well. Gradually, as our own basic knowledge expands, methods more applicable to our varied forests and their requirements are emerging and creating an American science of silviculture. The science is still in its infancy, and many more years of experience and research will be needed before sound solutions are obtained to many important problems of forest renewal. But American foresters believe that modern methods of research will shorten this period of development.

If a farmer, in one operation, could harvest this year's crop of grain and sow the next, using a fraction of the crop as seed, he would accomplish an operation similar to the one the forest grower uses in renewing a timber crop. For farm crops it would not work, because the life processes of the plants require intense culture and care if the yield is to be worth while. The farm manager approaches his job with the viewpoint of comparatively complete control of the crop environment that is needed to meet the demanding requirements of specific plants. He has learned that it pays to modify the weather; he controls moisture by irrigation and frost by smudge pots.

Trees also are demanding in their requirements for germination, early survival, and best growth. The cultural measures necessary to meet these requirements, however, are generally quite different from those needed to meet the requirements of farm crops. The intensity of culture used in farming would be wasteful and sometimes inimical to successful forest renewal. Another article in this book discusses the biology of the forest, and shows how natural trends over long periods change the species in a forest. It indicates also that forests respond to the natural variations in the factors that affect tree growth by a change in species or in rate of growth and thrift.

The biological basis of successful forest regeneration is a knowledge of these long-time trends, of the natural factors and their variations that affect tree growth, and of how the variations meet the basic requirements of the trees for best development. Thus the manager of woodlands must have as fully developed a knowledge of plants and the specific environments with which he is dealing as the farmer. But the woodland manager necessarily seeks his objective by guiding and modifying these natural trends and factors, rather than by attempting such complete environmental control as the farmer. His methods are less obvious therefore than those of farming and often may not be apparent to the untrained eye.

THE BASIC REQUIREMENTS of trees are light, heat, moisture, and wind particularly in the early stages of seed production and dissemination, germination, and survival. When a tree is beyond its first stages, the texture and chemical composition of the soil must be added. Because a man cannot change the requirements of trees, success in securing natural regeneration depends upon how well he can change and modify the natural factors to meet the requirements of the tree.

Reactions of tree species to changes in these factors vary widely. I. T. Haig learned from experiments in Montana that only 8 percent of lowland white fir seedlings on mineral soil survived in full sunlight, whereas about 90 percent survived under intensities of 24 percent and less of full sunlight. At the same locality, less than 15 percent of western larch seedlings survived under either full sunlight or almost complete shade, while more than 80 percent survived under one-fourth of full sunlight.

George P. Burns at the Vermont Agricultural Experiment Station found that sugar maple seedlings required only about 2 percent of full sunlight.

Paul J. Kramer at Duke University discovered that loblolly pine seedlings required nearly full sunlight for best development. At the same time he discovered that the life processes of eastern red oak could be fully satisfied under about one-third of full sunlight.

Working in the Lake States, Hardy L. Shirley showed that, under unmodified conditions with only the amount of light varied, the dry weight of 2-year-old jack pines in 80-percent light was four times that of those in 23 percent light. Within those variations of light, white spruce showed no significant difference in dry weight.

Similar variations in requirements between species could be cited for the other factors that affect tree growth. The important point to keep in mind is that trees vary widely in the conditions under which they develop well, and the creation of those conditions is vital to successful forest renewal.

Because these factors are all interrelated, a modification of one affects another. For example, light is one of the most easily controlled. That is accomplished by cutting that changes the density of the forest canopy and allows light to enter the stand in proportion to intensity of the cut. A change in the amount of light reaching the forest floor affects soil temperature. Soil moisture also is affected, because the trees that are removed no longer draw upon it.

Light is so important and (more than any other single factor) is so closely correlated with other factors that species of forest trees are often classified on the basis of their apparent tolerance or intolerance to shade. This concept of tolerance is really an expression not only of the shade-enduring capacity of a species but also of its ability to develop well under the complex of all factors associated with various degrees of light. The concept is imperfect in several respects and is unsatisfactory if it is universally applied to the exclusion of other considerations, but it provides a useful working principle in devising methods of forest renewal.

Under this concept, tolerant species are those that can become established and develop well as an understory in a well-stocked stand of larger trees, while intolerant trees are those that cannot survive such a subordinate position.

From the examples I have cited, it is apparent that species such as sugar maple, white spruce, and the lowland white fir are very tolerant of shade, jack pine and loblolly pine are rather intolerant, and western larch and eastern red oak have intermediate ratings in the scale of tolerance.

SEEDING CHARACTERISTICS of trees are also important in arriving at workable methods of forest renewal.

The means of natural dissemination of seed are key factors and can be divided into two major groups. The largest group is the light-seeded species whose seed can be disseminated by the wind. Seed of these species are attached to wings, downy material, or other structures that aid in distribution by the wind. In this group are the pines, spruces, and firs, and many important broadleaved species such as the yellow-poplar, the ashes, maples, birches, elms, poplars, and others. The second group consists of heavy-seeded species whose seed is distributed only by gravity, with some rather ineffective aid by birds and animals. This class includes the oaks, walnut, hickories. Obviously with these there can be little lateral distribution from the parent tree by wind. Hence, with this group, the trees chosen to reseed an area must be more closely spaced than with the group whose seed is wind-borne.

Seed-producing capacity is another important characteristic to be considered. It may be poor because the intervals between the good seed years may amount to as much as 6 or 7 years (with species such as red pine, longleaf pine, and beech) or because not much seed is produced, as is the case with chestnut oak. Other species (such as Virginia pine and scarlet oak) bear good crops every year or two.

Many other seeding characteristics are of importance. One is the time over which seed is dispersed. Some species, like white pine and the firs, spread their seed within a few days or weeks. Others, such as loblolly pine, spread a considerable portion within a few weeks, but continue to shed significant quantities of seed for several months after the cones open. Still others, such as jack pine, lodgepole pine, and pond pine, retain seed in persistent cones for several years, shedding few or none until opened either by the heat of a fire or by exposure, after felling, to the high temperatures that exist near the soil surface in midsummer.

As in basic requirements, seeding characteristics vary widely between the species, and knowledge of them is needed by anyone who wants to accomplish forest renewal.

AS TO CUTTING: I mentioned earlier that forest renewal is an integral part of the harvesting process and how readily light and associated factors can be modified by cutting. Seeding characteristics, too, can be taken advantage of by cutting, because relatively few trees are needed for regeneration purposes where species produce good crops of wind-borne seed.

The knowledge of a species and its requirements, coupled with the tools of logging, are the basic equipment for successful forest renewal.