Science in Christian Perspective

Ecologic Concepts in Forest Management
LAURENCE C. WALKER
School of Forestry
Stephen F. Austin State University
Nacogdoches, Texas 75962

From: JASA 32 (Dec1980): 207-214
The author, Hunt Professor and formerly Dean, is a fellow of the Society of American Foresters and of the American Association for the Advancement of Science.
The author is indebted to George K. Stephenson, editor (ret.) of the School ofForestry, Stephen F. Austin State University, for his helpfulness.

Concurrent environmental, energy, and raw material crises call for understanding how and why American foresters manage the Nation's timberlands as they do. For close to a century, roughly until the 1950s, criticism centered on the lumber industry as it converted a domain of virgin timber to usable products and profits. Foresters were among the critics. Now a new wave of objectors, this time in the name of ecology, focus on harvesting methods considered essential by foresters for meeting the country's requirements for timber products.¹  As the energy shortage threatens to curtail petroleum for plastic (wood's chief synthetic competitor), and cheap fuels are not available for smelting competing metals, the demand for wood products is expected to continue its upward trend. More intensive siIvicultural practices are therefore inevitable.

Much has been made of the derivation of the term "ecology" from the Greek work oikos, meaning house. Thus the ecologist is concerned with his habitat and its care. In contrast, little has been made of the translation of oikos into Latin and the word derived therefrom. It is i(e)conaea, and, of course, the English "economics" is its offspring. Little, too, is made of the translation, by King James' scholars for instance, of both oikos and iconaea into Anglo-Saxon as "steward" and "stewardship."

Ecology and economics are inseparable in stewardship. Principles of both disciplines concern the ecologist as the manager of the household or the keeper of the accounts. As a faithful steward he acts accountably to the one who has entrusted the resources to his supervision. To the Christian professional conservationist, that one is God.

Perhaps for no other natural resource is the relevance of stewardship more appropriate than for forestry. Most people in this profession take seriously the Genesis directive of Hebrew-Christian persuasion to "replenish and subdue the earth." To subdue is to take charge, but to replenish implies the restoration of exploited sites as well as the care of those being used to provide for the needs of men. Foresters as the original professional ecologists are responsible for the care of the wildland estate for sustained yields of goods and services therefrom.

Christians and the Church are often faulted for abusing nature because of the command to subdue the earth.³ Yet, many resource managers consider it a God-given responsibility to wisely manage natural resources. That man is believed by Christians to be the "head of creation" and the "apex of the system of livings" encourages the accusation. Those who join in the allegation-and some popular Christians are included-have their responsibilities too. These include knowing and understanding the facts, for, in regard to the subject here discussed, the availability of economic goods to serve those born a century hence depends on decisions now being made. So it seems pertinent to inquire into the scientific basis for current management practices for a resource that occupies nearly a third of this Nation's land area.

America's 754 million acres of forest' constitute more than a major physical asset: they are intricately involved in the national economy and culture. Forests provide a fifth of the nation's industrial raw material, protect and regulate its most productive watersheds, graze a sizable portion of its livestock, produce most of its game and much of its nonsport wildlife, and attract millions of recreation visitors annually. The federally owned national forests and some other public lands are, by law, managed for all of these purposes. The public may have some right also to expect these benefits from the 400 or more million acres in private ownership. The obligation, legal or implied, to provide these amenities subjects forest managers to multifaceted public scrutiny.

Management of forests involves accommodations of widely varying viewpoints. In some instances these are essentially irreconcilable. Thus, the preservation of wilderness, though compatible with many watershed and wildlife objectives, requires abstinence from timber harvest and domestic cattle grazing. It even may necessitate limited recreation use. Yet recreation use is often given as the motivating reason for wilderness designation. How much potentially productive land the nation can devote to a use in which only a few may be able to participate-say, in the year 2020-requires a decision now. For decisions adverse to preservation tend to be irreversible: what we do today may set permanent limits on the acreage available for wilderness tomorrow.

Wood in the Economy

Wood-along with products of agriculture and fisheries-is a renewable raw material. Its production requires only soil nutrients, rainfall, and sunlight. Forest regeneration has not been particularly significant in the world of abundance from which Americans are emerging. But in a world of diminishing resources, wood is one of the few materials that can be produced in perpetuity at modest energy costs.⁴

Wood is versatile. It is readily converted into products like wall-boards, newsprint, and rayon, and to substitutes for petrochemicals. Once our chief fuel, wood is being reconsidered as an energy source: between 5 and 20% of our forest area could supply our electrical demands,⁵ and gas and liquid fuels like ethane and ethanol alcohol can be made from wood.

Wood's chief competitors for structural material are steel and aluminum. But steel requires 6 times as much energy to produce as wood of equivalent strength, and aluminum 38 times. Both are increasingly dependent on imported ores or on low-grade ores refinable at greater energy costs.⁴

Wood products are practically indispensible. The amount used in construction or for furniture can be lessened only by substituting more expensive or less satisfactory materials. Suitable substitutes for wooden railroad ties or paper-board shipping containers are unavailable at any cost. As the 1980 demand for wood will be 8 to 34% above the 1970 level 3 prompt intensification of forest management on a decreasing land base is essential.

Demand for softwoods (conifers) and their generally faster growth give them priority over hardwoods in forestry operations. Fortunately, they grow profitably on sites where better quality hardwood species often do not.

Five species groups make up nearly two-thirds of the cut-the southern pines lead, followed by Douglas-fir, oak, western hemlock, and ponderosa pine (Table 1). Essentially all the harvest of southern pines and oaks is from east of the great plains; the other three species are western.

Concentration of a large portion of the cut among a relatively few species suggests the emphasis for future forestry. It tends, however, to obscure the complexity of forest resources and their management. In addition to groups listed in Table 1, some 15 softwood and four hardwood species are important in the West and 15 softwoods and 36 hardwoods are commercially important in the East. Dozens of other species are important for specialized uses or for their ecologic influence. Hundreds of trees, shrubs, and herbaceous plants which appear in ecological succession after timber cutting, but which have no economic value, present the forester with some baffling problems.

Over 30%, 234 million acres, of the forested lands in the United States do not produce merchantable timber and more than 20 million acres are reserved for wilderness and parks. Currently available for wood production are 500 million acres, 3/4 of which are privately owned. Meeting national needs for wood is thus largely a matter of managing private lands.

Important timber species normally occur in ecological associations which foresters catalog as forest cover types. Some species are important in several types.⁸ While certain ones grow successfully when introduced outside their native range, the use of exotics may be ecologically disruptive.

Large trees are old by human standards. One should not, however, assume that they are necessarily permanent or invulnerable to death and decay. All timber stands reflect stages in an ordered sequence of ecologic succession which may be interrupted at any point by natural catastrophes such as windstorm, insect epidemic, and fire, or by logging. To produce the stable, or climax, forest, ecologic succession may require centuries without natural or human disturbance.⁶ Most virgin forests of lumbering history were at sub-climax or even earlier stages when harvests for building materials were begun. For example, white and red pines of the North, the yellow pines of the South and the Douglas-fir of the West seed in abundantly only on denuded sites. Yet these species were ready for harvest in the United States in the 18th, 19th, and 20th centuries because of earlier interruptions in ecologic succession. Indians burned the forest, and lightning fires and hurricanes laid bare the land.

TABLE I

Timber Cut from U. S. Forests, 1971

Species                                                      Growth                                  Removal
                                                                            (Million Cubic Meters)      

Southern pines                                               146                                         111
Douglas-fir                                                     38                                            54
Oaks (select plus other)                                  75                                            46  
Western hemlock                                           11                                             21

Ponderosa & Jeffrey pines                              16                                            21
True firs                                                          14                                            14

Sweetgum                                                      11                                            11
Spruce, balsam fir                                           17                                             6
Hickory                                                            7                                             5
White & Sugar pines                                         2                                             5
All others                                                       186                                         198

Total                                                              525                                          391

   
The key to forest succession is the relative tolerance of trees to competing demands for light, moisture, and nutrients by their neighbors. Species like sugar maple, beech, and the hemlocks persist under dense shade. Regenerating under their own canopies, such species dominate the climax forest.

At the other extreme are species intolerant of shade, including most pines and many hardwoods. Aspen, cottonwood, willow, and yellow-poplar, even though prolific seed-producers, compete poorly with established vegetation and are rarely found in understories. Their seeds germinate readily on exposed mineral soil, and their seedlings grow rapidly, if fight is abundant, to "capture" growing space and exclude competitors. Stands of these species usually result from logging, abandonment of cultivated fields, or deposition of new river alluvim.

Ecology and economics are inseparable in stewardship. Principles of both disciplines concern the ecologist as the manager of the household or the keeper of the accounts.

Species' success in ecologic competition is also affected by morphologic and physiologic traits. Trees of some species may sprout from roots or stumps, using an established root system for nourishment to outgrow adjacent seedlings. Others produce many small seeds which are scattered widely; while species that produce few but large seeds, with much food stored therein, are able to outgrow competing plants.

Seeds of most plants appearing in ecologic succession are usually available. Thus, a disturbed site may be dominated the first season by annual weeds or grasses, followed successively by biennial and perennial herbs and shrubs. Eventually trees encroach. On fertile sites, some of the initial annual invaders may grow so rank as to inhibit establishment of the pioneer tree species, delaying the development of a tree cover until a new catastrophe exposes the site to full sunlight.

Stands of tolerant species are usually climax types, represented by beech, sugar maple, spruce, and hemlock of the Northeast, oaks and hickories of the Central States and the South, and redcedar, spruces, and firs of the Northwest. Harvesting individual trees in these forests by the selection system provides space for new stems and perpetuation of the type. While the single-tree selection system is often effective, in complex types such as those of the southern river botton-lands, encroachment of weed species in the openings may preclude the method.

Intolerant species typical of those found in the pioneer stage of succession include the southern pines, ponderosa pine, sugar pine, eastern and western white pines, aspen, cherry, ash, cottonwood, willow, yellow-poplar and Douglas-fir. Because seedlings and saplings of these species do not compete well in shade, reproduction after selection harvests is undependable. To regenerate the stand, foresters clear sites, using a legitimate early European silvicultural system called clear-cutting.

Five major associations⁹ illustrate key commercial forest types and silvicultural systems necessary to perpetuate them.

An extensive forest cover type in the North consists mainly of sugar maple, American beech, and yellow birch. Its simplistic composition contrasts sharply with the multiplicity of species-perhaps over 50-in the mixed mesophytic forests of the South.

These northern hardwoods are especially valuable for furniture. Small, misshapen trees can be sold for pulpwood. Thus markets are generally good. Secondgrowth, much now of merchantable size, naturally replaced the virgin stands following the cut-out-and-get-out policies of the early loggers.

The maple-beech-birch type is usually climax because all three species are tolerant of shade. On moist sites, however, the more tolerant eastern hemlock may invade and become dominant.' Birch, although the preferred species commercially, is less tolerant than its associates. Its light seeds, widely distributed by wind, and its poor deer browse encourage seedling establishment. Beech, least desirable because of fungus infections that cause rot, is most tolerant and has heavy seeds distributed mainly by animals. Maple is intermediate to its two associates in both tolerance and seed dispersal. ^{10, 11}

Widespread clearcuts were made in the past to regenerate overmature stands. Where such cuttings cover so large an area that the seed supply is limited, pioneer plants (such as blackberries) or light-seeded trees (aspen or paper birch) may invade and capture the site. While this drastic, though usually temporary, disturbance to watersheds and aesthetic values is an adverse consequence of clearcutting, temporary abundance of browse for wildlife usually occurs.

White pine, long the most valuable timber of northeastern America, was found as large, high-quality trees in many of the virgin forests of New England, the Lake States, and the Appalachians. It is not, however, a climax species, being less tolerant than most of its associates and unable to regenerate under their shade. This pioneer tree seeds-in on burned-over, cut-over, or abandoned farm lands. Long-lived, individual stems have stood in undisturbed situations until ecologic succession has surrounded them with climax species such as eastern hemlock.

Because of its intermediate tolerance, white pine can be regenerated by silvicultural systems ranging from individual tree selection to clear-cutting.¹³ The shelterwood system removing the old forests in several harvests-and clearcuttings that simulate nature's hurricanes and lightningcaused fires remove enough tree cover and sufficiently scarify the soil surface for pine seeds to germinate and seedlings to thrive.

Regeneration of white pine by selection, favored by some environmentalists because it is least disturbing to soil, watersheds, and aesthetic value, is feasible primarily where associated species bear sparse foliage and are generally less tolerant than these pines.¹⁴ Such intolerant associates include paper birch, red maple, jack pine, aspen, and pin cherry. Costly manual or chemical release from this competition is often essential, making selection cutting uneconomic. Single-tree selection also tends toward site domination by species with more tolerance but less economic utility than white pine.

Seed-tree, like shelterwood, harvests, remove substantial overstory timber, leaving perhaps 10 choice stems per acre as a source of seed. For successful regeneration, a good seed crop, usually occurring at 3- to 5-year intervals, must fall during the first year or two after cutting.¹⁵

Shelterwood cuttings are designed to achieve regeneration gradually, over periods of 10 to 30 years. A first cut stimulates seed production on about 60076 of the trees left standing. A decade or so later, a second harvest removes 60% or more of the remaining trees, the soil is scarified to enable seed germination, but part of the parent stand is left for shade to protect the seedlings from sunscald. A final cut, after regeneration is established, removes the rest of the old stand.¹⁶

Clearcutting in strips or blocks combines the ecologic characteristics of the species with economic considerations for least-cost continuation of the type. Other advantages include its adaptability to heavy logging machinery, a timber-edge beneficial to wildlife, and ease for replanting by machine if natural regeneration fails. Size of harvest areas is limited by the distance to which adequate seed can be supplied by the adjacent stand. This should prevent openings so large as to be unacceptable for watershed protection and aesthetic appreciation.

Other constraints upon managers of white pine are a weevil which kills terminal shoots, a lethal rust controllable only by elimination of shrubs of the genus Ribes, and root and heart rots.¹⁷ To rejuvenate forests with these problems may require clearcutting prior to regeneration.

The four principal species occur in pure stands, in mixtures with hardwoods, and with each other. None is a true climax species, for on most sites their seedlings are unable to survive under their own shade or that of invading competitors.

The ecologic climax for most southern pine sites is a hardwood type, with occasional relict pines.²¹ Such forests occurred widely in the Georgia-and Carolina Piedmont in pioneer days .²² Many dry sandy sites now growing pine would, without disturbance, succeed to a forest of scrubby hardwoods. Windstorms and forest fires, under natural conditions, interrupted the ecological cycle to initiate the extensive pine stands of the South. Despite wasteful cutting and woods-burning, most existing pine stands originated by natural reseeding of cutover forest or abandoned farmland.

Management of southern pines requires maintenance of a sub-climax vegetative association.²³ The alternative, allowing stands to revert to predominantly hardwood types, is incompatible with optimum production of structural lumber, plywood, and long-fiber pulp. Only when managed primarily for pine can the South's 100 million acres of potential pine land meet the Nation's projected needs for such products.

Loblolly pine can, on favorable sites, be managed by a modified selection system, harvesting trees in small groups instead of singly.²⁴ Elsewhere,²⁵ similar cuttings have produced little pine regeneration and much brush. On most sites, expensive hardwood control, and thus higher woodproducts prices, would be needed to grow continuous pine crops by selection harvests.

Managed southern pine is currently harvested under systems that remove most or all of the existing stand, permitting control of established hardwoods by fire,²⁶ mechanical means, or both. Harvests may leave 10 seedtrees per acre, be limited to strips or patches that will reseed frorn, adjacent timber, or depend on artificial planting or seeding.²⁷

Clearcutting, followed by burning of excessive debris, mechanical site preparation, and mechanized planting or direct seeding, is compatible with efficient logging, thus minimizing harvesting expense. The land is promptly returned to full production. The method has been criticized because it results in monocultures conducive to catastrophic disease or insect attack and for impairment of watershed, wildlife habitat, and aesthetic values.²⁸

Efficient, intensive, evenagedforestry can be compatible with watershed protection, wildlife production, and outdoor recreation.

Monocultures can be vulnerable, although it is uncertain whether any of the known enemies of the southern pines-littleleaf disease, annosus root rot, or the southern pine beetle-are thereby fostered. Evenaged forestry, however, need not be the culprit, as ownership and land use patterns confine "monocultural" forests to units smaller than those prevalent in virgin stands. Hardwoods occurring along streams also reduce the size of evenaged, single-species blocks.

Prompt establishment of herbaceous cover on regeneration areas limits accelerated erosion to a period of 4 to 6 months after each cutting. Prompt tree regeneration limits aesthetic insults to about two growing seasons. Since intervals between harvests are about 20 and 40 years for pulpwood and sawlogs, respectively, such exposure disturbance is not excessive for the moderate slopes of Coastal Plain and Piedmont sites. On mountain sites of Arkansas, less intensive clearing, more limited cutting areas, or special erosion control measures may be desirable.

Artificial regeneration, being more dependable than any form of natural seeding (because seed crops are often inadequate), is widely practiced. As genetically improved stock becomes available, planting occurs on larger acreages.

Pure pine stands dense enough to exclude understory plants are poor habitat for game animals. Thinned stands and the edges of openings produce wildlife food in the form of shrubby and herbaceous browse, seeds, and fruits. Regeneration areas, from the time of site preparation until crown closure of young trees, affords excellent deer and quail habitat. With small, well-distributed cutting areas, intensive pine management is compatible with large populations of deer, quail, doves, and turkeys.²⁹ Nearly as much game is produced as would be under intensive wildlife management, where fiber production is of little concern.

Managed forests in the South are aesthetically monotonous. Because terrain is gentle to flat, only where broken by farmlands and cutting do the forests not obscure everything more than a few rods distant from roads.

Douglas-fir

One of the major timber species of the world,³⁰ Douglas fir reaches its best development in the virgin forests west of the Cascade and Sierra Nevada ranges from northern California to southern British Columbia. Most current harvesting and reserves of the species are concentrated in

Users of forest resources-those who live in houses, read books, wear clothes, and use energy-may be unaware that they force environmental insults upon the land. All Americans face decisions now for which they must eventually pay a price.

Compared with other western conifers, Douglas-fir is intermediate in tolerance, but in comparison with its associates in the same forest, it is intolerant. It cannot regenerate under the shade of the more tolerant redwood, red cedar, hemlock, spruces, firs, and hardwoods with which it competes. Ecologically a fire subclimax species, Douglas-fir owes its dominance to fires and wind-throw and to its longevity. Disturbance provides bare soil for seed germination and openings where seedlings can outgrow competitors; a head-start and fast growth may enable Douglas-fir crowns to retain dominance for 350 years.

Faster growing and intrinsically more valuable than its associates, Douglas-fir best utilizes the sites when grown in pure stands. Clearing of some kind is essential if regeneration is to be accomplished by natural means.³² Other factors that make clearcutting almost unavoidable are the heights and weights of the trees, necessitating large logging equipment which is difficult to maneuver among uncut ones without injuring them. Partial cuttings also expose residual stems to wind, precluding shelterwood and seedtree harvests in dense virgin and second-growth stands.³³

Most foresters of the Douglas-fir region consider clearcutting in relatively small patches the best approach to regeneration of existing old-growth stands. Prompt burning of logging debris after harvest reduces wildfire hazard and enhances seed germination.³⁴ Small blocks permit adequate seed production and dispersal from adjacent stands, a marked contrast with earlier extensive logging practices where the dearth of seed precluded adequate regeneration.³⁵ Small blocks also reduce adverse hydrologic effects by limiting the proportion of a watershed which is disturbed.

Good Douglas-fir seed crops are infrequent, occurring on the average at 5- to 7-year intervals.⁷ Thus only 15 to 20% of the harvests coincide with heavy seed crops. Lessvaluable trees, herbs, and shrubs may then capture the site. The uncertainties of natural regeneration cause many managers to regenerate harvest areas by site-preparation with bulldozers and planting.

As the Douglas-fir region shifts from virgin to managed forests, changes in harvesting methods are likely to occur. Trees will be cut at younger ages-50 to 75 years vs. 150 to 350. To promote high-value growth, fewer stems will be grown per acre.³⁶ As these trees will be smaller and more wind-firm than those in virgin stands now being harvested, partial cutting systems may be feasible, especially where wildlife,³⁷ watershed, and scenic values are paramount. For most commercial sites, however, the trend seems to be toward small block clearcutting followed by planting or direct seeding to insure prompt and adequate restocking.

Aspen and gray birch in the Lake States prefer welldrained sites. Yellow-poplar in the Southeast prefers mesic conditions. All three grow rapidly, appearing as pioneer species following clearcutting or other denudation. Aspen and gray birch, being short-lived, are managed primarily for pulpwood. Old growth yellow-poplar has been prized since colonial times as a soft, easily worked wood for cabinets, pattern-making and interior finish. On fertile, permeable soil with high nitrogen content, it outgrows other hardwoods and most conifers.³⁹ For natural regeneration, openings of at least one-half acre are required for seedling survival. Harvests of up to 50 acres are effective if seedfall is adequate and the soil is scarified. Prescribed burning also exposes mineral soil to enhance seed germination. In areas where deer populations are high, large seed-tree or shelterwood blocks may be appropriate as deer tend to congregate in small openings to feed on palatable yellow-poplar seedlings.

Despite major acreage losses to agricultural clearings, water control impoundments, and urban sprawl, the nation must look to management of the mixed forests of its bottomlands and drainage-ways for the bulk of its quality hardwoods. Such stands tend to include trees of all sizes, if not all ages, and lend themselves to the selection system. This is not inconsistent with customary commercial logging, which has long selectively removed trees on the basis of value. The tolerance of most lowland species permits them to occupy small openings, even those created by removal of single trees. The selection system, however, is handicapped by the appetite of deer, which destroy the palatable seedlings of the preferred tree species.

Clearcutting systems are feasible for bottomland hardwoods, as seedlings of tolerant species are usually present in the understory before harvest begins. These broadleaf trees also are regenerated by fast-growing sprouts that arise from roots or stumps. Light-seeded species, like the quality ash and maples, have advantages over heavy-seeded mast producers, like the oaks and hickories, because of the ability of the former to disseminate winged seeds great distances. Even where preferred species are retained as seed trees, species composition of a new bottomland forest is uncertain. Competition among the young trees and vigorous growth of brush and vines further alter the mixture, making hand labor necessary to release selected crop trees.

Dwindling reserves of non-renewable raw materials heighten the need for increased forest productivity to meet current and prospective demands of the American people. The time lag between initiation of forestry measures and the marketability of wood products resulting from those measures call for action now to assure meeting requirements for the years 2000 and beyond.

Many Americans have voiced objections to the manner in which forests are being managed to meet the expected demand. Clearcutting methods of harvesting are particularly criticized, as destruction of watersheds, wildlife, and scenery is alleged.

Because the Nation's fastest growing and most versatile timber trees are pioneer species, requiring bare soil and fun sunlight for regeneration and growth, they cannot be economically regenerated by systems which remove only a small part of the stand. While such selection silviculture would minimize disturbance and disruption of non-timber resources, it would either convert stands to less valuable tree species or require costly means-by chemicals, fire, or machinery-to eliminate competition. Greatly increased consumer prices for housing, paper, and other wood products would result.

Fortunately there is room for compromise. Productive sites revegetate promptly and erode but little during the temporary denudation between harvest and establishment of regeneration. Clearcutting in small blocks benefits wildlife by increasing the length of timber margins. With prompt regeneration, naturally or artificially, scenic values are quickly restored. Thus, efficient, intensive, evenaged forestry can be compatible with watershed protection, wildlife production, and outdoor recreation; while in some sensitive situations, timber harvest may be inappropriate.

Users of forest resources-those who live in houses, read books, wear clothes, and use energy-may be unaware that they force environmental insults upon the land. So all Americans face decisions now for which they must eventually pay a price. One choice is to endorse forest policies which will insure increased and efficient wood production from the best forest sites while accepting minimal reductions in other amenities. Alternatives which divert commercial forest from efficient wood production will be costly in dollars and in energy resources spent for substitutes from non-renewable raw materials.

REFERENCES

¹G. Pinchot, Breaking New Ground, U. Wash. Press, 522 pp. (1972, reissue); R. S. Maxwell and J. W. Martin, A Short Hiytory ofForest Conservation in Texas, Stephen F. Austin State Univ. School of Forestry Bul. 20, p. 22-23 (1970); N. Wood, Clearcut, Sierra Club, 151 pp. (197 1); F. C. Brooks, J. Forestry 69, 299-302 (197 1). C. Holden and L. Carter, Science 182, 144 (1973).
²L. White, "The Historical Roots of our Ecologic Crisis," Science 155,
1203-1209 (1967).
³Data on forest resources, including current and projected needs, are from The Outlook for Timber in the United States, U.S.b.A. Forest Service FRR-20. The relationship of timber production and use to the American economy and environment are analyzed in some detail by Edward P. Cliff in Timber: The Renewable Resource, prepared for the National Commission on Materials Policy, 1973. Govt. Print. Office.
⁴H. 0. Fleischer, Wood Science and Technology 5, 247-254 (1971).
⁵T. B. Reed and R. M. Lerner, Science 182, 1299-1304 (1973).
⁶For further discussions of the mechanisms of plant and forest succession, see H. J. Ousting, The Study ofPlant Communities,* an Introduction to Plant Ecology (W. H. Freeman and Co., San Francisco, 388 pp., 1948); and S. H. Spurr, Forest Ecology (The Ronald Press, New York, 352 pp., 1964).
⁷Information on the traits of individual tree species, their ecological impact and silvicultural application may be found in Agricultural Handbook No. 271, Silvics of Forest Trees of the United States (U.S.D.A. Forest Service, Washington, D.C., 1965).
⁸Each of these associati6ns covers several of the forest types recognized in Forest Cover Types ofNorth America (exclusive of Mexico) (Society of American Foresters, Washington, D.C., 1954).
⁹W. B. Leak, Development of Second-growth Northern Hardwoods on Bartlett Experimental Forest - a 25-year record (U.S. Forest Service, Northeast. Forest Exp. Sta. Paper 155, 1961).
¹⁰F. H. Eyre and W. M. Zilgitt, Partial Cuttings in Northern Hardwoods of the Lake States, Tech. Buil. 1076 (U.S. Dept. of Agriculture, Washington, D.C., 1953).
¹¹G. R. Trimble, Jr., and G. Hart, An Appraisal of Early Reproduction After Cutting in Northern Appalachian Hardwood Stands (U.S. Forest Service, Northeast. For. Exp. Sta. Paper 162, 1961).
¹²W. M. Zilgitt, J. Forestry 49, 494-497 (1951).
¹³W. T. Doolittle, Soil Sci. Soc. Amer. Proc. 22, 455458 (1958).
¹⁴K. W. Horton and G. H. D. Bedell, While and Red Pine Ecology, Silviculture, and Management (Canad. Dept. No. Aff. and Natl. Resources, Forestry Branch, Bul. 124, 185 pp., 1960).
¹⁵H. L. Shirley, Amer. Midland Nat. 33, 537-612 (1945).
¹⁶H. J. Lutz and A. C. C line, ResuIts of the First Th irty Years of Experimentation in Silviculture in the Harvard Forest, 1908-1938. Part L The Con version of Stands of Old-field Origin by Various Methods of Cutting and Subsequent Cultural Treatments (Harvard Forest Bul. 23, 182 pp., 1947).
¹⁷R. B. Friend and H. H. Chamberlin, Conn. Agr. Exp. Sta. Bul. 461, 530-537 (1942); H. J. McAloney, The White Pine Weevil (Pinodes strobi Puk.)-Its Biology and Control (N.Y. State Col. Forestry Tech. Pub. 28, 87 pp., 1930); W. R. Haddon, Roy. Canad. Inst. Trans. 22, 21-80; J. .T. Ball, J. Forestry 47, 285-291 (1949).
¹⁸Peter Koch, Utilization of the Southern Pines, U. S. Dept. of Agr., Agr. Handbook 420, 1163 pp. (1972).
¹⁹Slash and longleaf pines share a coastal plain range, subject in historic and probably in prehistoric times to frequent forest fires. Each grows well on a variety of sites, but their physiological adaptions to fire relegate them to separate niches. Slash pine's rapid early growth enables it to survive light fires after about 3 years; longleaf grows little in height for several years, meanwhile growing a large thick bud at the ground line which survives fires after the first year. After several years the seedling grows as much as 6 feet (2m) in a single season. Slash pine thus predominates on moist sites where fires are less frequent, while longleaf occupies drier sites which may burn almost annually. With effective fire control, slash pine is now grown on much land formerly dominated by longleaf pine.
²⁰The silvical characteristics and silviculture of the shortleaf and longleaf pines are reviewed by L. C. Walker and H. V. Wiant, Jr., in Stephen F. Austin State University School of Forestry Bul. 9 and 11; characteristics and silviculture of the minor southern pines and slash pine are reviewed by L. C. Walker in Bul. 15 and 16 of the same series.
²¹H. H. Chapman, Ecology 13, 328-334 (1932); L. S. Barrett and A. A. Downs, J. Agr. Res. 67, 111-127 (1943).
²²W. Bartram, Travels, ed. by Francis Harper (Yale Univ. Press, New Haven (1958).
²³H. H. Chapman, Management of Loblolly Pine in the Pine-Hardwood Region in Arkansas and in Louisiana West of the Mississippi River. Yale Univ. School of Forestry Bul. 49, 150 pp. (1942); F. H. Borman, Ecol. Mongr. 23, 339-358 (1953); R. J. Riebold, In La. State Univ. Fourth Ann. Forestry Symp. Proc., 92-99 (1955); K. B. Trousdell, J. Forestry 52, 174-176 (1954).
²⁴R. R. Reynolds, Eighteen Years of Selection Timber Management on the Crossett EXperimental Forest. U.S. Dept. Agr. Tech. Bul. 1206, 63 pp. (1959).
²⁵E. V. Brender, Silviculture of Loblolly Pine in the Georgia Piedmont, Ga. Forest Res. Council Rep. 33, 74 pp. (1973); G. K. Stephenson, J. Forestry 61, 270-272 (1963).
²⁶T. Lotti, J. Forestry 54, 191, 192 (1956).
²⁷Consistently successful regeneration of longleaf pine by skillful shelterwood cutting and timely prescribed burning is reported currently by Forest Service Researchers Thomas C. Croker and William D. Boyer, Reproducing Longleaf Pine Naturally, U.S. For. Serv. Res. Pap. SO-105(1975). Several industrial landowners participated in the 20-year studies, the results of which are probably applicable to other pines, but none has yet adapted this management on a large scale.
²⁸D. M. Smith, J. Forestry 70, 89-92 (1972); A. W. Smith, Nat. Parks and Cons. 46, 111 (1972); W. 0. Douglas, Farewell to Texas (McGraw-Hill, 1967), pp. 1-37; J. W. Farrar and L. Brunett, La. Cons. 23, 22-27.
²⁹J. J. Stransky, Texas Forestry Papers 9 (1971) and 23 (1973), Stephen F. Austin State University, School of Forestry, Nacogdoches, Texas.
³⁰D. Wharton, Am. Forests 80, 31-35, 58-60 (1974).
³¹E. L. Little, Check List of Native and Naturalized Trees of the United States, U. S. Department of Agr. Forest Serv. Agr. Handbook No. 41 (1953).
³²J. Hoffman, Ecology 1, 49-53 (1920).
³³H. J. Gratowski, For. Sci. 2, 60-74 (1956).
³⁴D. N. Dever, Evaluation of Factors Affecting Natural Regeneration of Forest Areas in Central Western Oregon. Oreg. State Bd. Forestry, Res. Bul. 3, 4 pp. (1954); D. C. Hagar, Ecology 41, 116-125 (1960).
³⁵T. T. Munger, Timber Growing and Logging Practice in the Douglas-fw Region (U. S. Dept. Agr. Bul. 1493, 22 pp., 1927)
³⁶E. S. Kotok, Timberman 52 (10), 104, 106, 108-109 (1951).
³⁷ F. Hooven, J. Forestry 71, 211-214 (1973).
³⁸The silvical characteristics and silviculture of the southern hardwoods are reviewed by L. C. Walker in Stephen F. Austin State University School of Forestry Bul. 22, and by L. C. Walker and K. G. Watterston in Bul. 25 of the same series. Detailed presentation of characteristics, harvesting, and silvicultural management of bottomland hardwoods are in J. A. Putnam, Management ofBottomland Hardwoods, U. S. Forest Serv. So. For. Exp. Sta. Occas. Paper 116, 60 pp. (1951); J. A. Putnam and H. Bull, Trees, of the Bottomlands of the Mississippi River Delta Region.
³⁹L. Della-Bianca, For. Sci. 7, 320-329 (1961).