IRVINE T. HAIG, Chief, Forest Technology Branch *, Forestry Division, FAO
* Dr. Haig has since rejoined the U. S. Forest Service.
This article is an excerpt from the FAO publication entitled Tropical Silviculture, prepared by I. T. Haig, M. A. Huberman and U Aung Din, which summarizes the highlights of similarity and differences among the major forest formations in the tropics. The physical and silvical features of the wet evergreen (rain), moist deciduous, and dry deciduous forests are compared as a basis for discussing the silvicultural practices which have been developed in Asia, Africa, and Latin America. Future research and field experiments needed to fill gaps in existing knowledge are outlined as a guide to tropical silviculturists.
Tropical forests constitute almost half of the world's forest area. Widely dispersed in three major world regions, Asia, Africa and America, they contain an enormous quantity of renewable raw material in a world of rapidly growing needs. Their proper management on a permanent basis is one of the most complex and exacting tasks confronting professional foresters This study attempts to sum up, from a world standpoint, the information now available on one major phase of this task, the natural regeneration of tropical forests, as a guide to the future development of sound silvicultural practice.
Though the task is a formidable one, it is aided by the fact that each major formation shows striking resemblances in physiognomy, structure, succession and other silvical features in response to climate and other environmental factors in all three world areas in spite of the great dissimilarities in flora and the presence of a multitude of edaphic, floristic and altitudinal variants requiring special and localized consideration. Climate, and particularly rainfall and rainfall pattern, are especially influential and permit grouping most tropical forest vegetation in three broad formations, arranged according to increasing dryness and deciduous character; namely, wet evergreen (rain) forest; moist deciduous forest; and dry deciduous forest, including thorn and scrub forest. This has greatly simplified consideration of formations roughly similar in physical environment and silvical characteristics despite the great areas and general complexity of the forests involved. The Table on page 160 summarizes the broad comparisons that can be made from a world viewpoint on the basis of present knowledge. In addition, three special forest types of great current or potential economic importance are dealt with in some detail. Nevertheless, although the physical environment and major silvical features of each major formation appear reasonably clear, there are obvious gaps in knowledge for all major areas and types.
In general, knowledge and practice have progressed further in Asia than in Africa and America, in botanical fields such as dendrology, as well as in strictly silvical and silvicultural fields. In addition, in both Asia and Africa, well-organized research agencies and active research programs promise rapid advance, with such institutions and programs limited in scope and mostly in initial stages in America. Nevertheless, lack of knowledge is a definite hindrance to the development of fully dependable silvicultural practices, while the growing availability of funds that can be invested in such practices, and the necessity of seeing they are usefully employed, makes the forester's task increasingly difficult.
In the major climatic groups recognized here (wet evergreen, moist deciduous and dry deciduous), the development of silviculture has been dominated in most forests by the problems created by the relative great complexity of tropical forests, especially as regards the large number of species, greatly intermixed, combined with markets limited to a few species of high commercial value. Only with good markets, as in the dry fuel forests of India and Pakistan, gregarious stands of commercial species or areas with large supplies of cheap labor, have adequate natural regeneration practices been applied on substantial areas, areas relatively restricted in terms of world totals. Elsewhere in these major formations, in all three regions, commercial harvesting is still largely logger's selection consisting of the removal of the few valuable stems per unit of area, with little or no provision for future productivity, except the establishment of minimum girth requirements to preserve younger or smaller-growing stock of commercial species, sometimes supplemented by improvement cuttings. The problem is to maintain, and possibly increase, the proportion of such valuable commercial species.
One of the most striking aspects of tropical forestry to date has been the manner in which the problems created by floristic richness have been circumvented, sometimes with impressive and outstanding success, by concentration of forestry attention and effort upon relatively few of the large number of tree species present. Fortunately, in all or almost all important tropical forest formations, one or more species or species groups, sometimes gregarious, have combined commercial values with adequate to excellent silvical features. Indeed, current tropical silviculture can almost be written in its entirety in terms of a small number of such superior species or species groups: teak, sal, limba, okoumé, Spanish cedar, mahoganies and other Meliaceae, the dipterocarps, bamboos, mangrove, etc. Progress, indeed all forestry activity, has depended primarily, in its initial stages, on the discovery and recognition of such species, beginning with their commercial qualities and advancing to concentrated efforts to learn how to use these commercially valuable species to replace or enrich the forest estate. Such efforts have often taken the course of compensatory plantations, or the enrichment or replacement of mixed forests through artificial regeneration, for many of these species possess excellent silvical as well as merchantable qualities.
PHYSICAL, AND SILVICAL FEATURES OF THE MAJOR CLIMATIC FOREST FORMATIONS
|
Item |
Major forest formations |
||
|
Wet (rain) |
Moist |
Dry |
|
|
Physical factors |
|||
|
Temperature |
Frost-free |
Essentially frost-free |
Essentially frost-free |
|
Insolation (radiation) |
Not high: 40-50 percent of possible |
Higher |
Very high |
|
Wind |
Light: 3-7 m.p.h. (6-12 km.) |
Gentle to moderate: 6 m.p.h. (10 to 20 km.) |
Moderate to strong |
|
Humidity |
Very high: 90 percent |
High: 60-80 percent (sal: 46-60 percent in March) |
Medium high: 50-60 percent |
|
Rainfall |
80 + inches (2,030 mm.) |
50-80 inches (1,270-2,030 mm.) |
Under 50 inches (1,270 mm) |
|
Rainfall pattern |
No real drought: up to two months dry; four months at extremes |
Seasonal drought: up to four to six months dry; seven at extremes |
Marked drought: up to six to eleven months dry eleven and a half at extremes |
|
Soils |
Very varied |
Very varied |
Very varied |
|
Silvical factors |
|||
|
Physiognomy |
Evergreen |
Semi-deciduous |
Deciduous |
|
Height |
Trees tall - dominants 140 to 180 feet (43 to 55 meters) |
Trees tall to medium - dominants 80 to 120 feet (24 to 37 meters) |
Trees short - dominants 50 to 75 feet (15 to 23 meters) |
|
Flora |
Very rich in species |
Less rich in species |
Least rich in species |
|
Commercially valuable species |
Present and aggressive |
Present and very aggressive |
Present and very aggressive |
|
Uncontrolled fire and grazing |
Little to none |
Frequent |
Frequent |
|
Succession |
Climax |
Toward wet forest |
Sometimes toward moist forest |
|
Seed production |
Satisfactory to abundant: periodic |
Satisfactory to abundant: periodic |
Profuse: often annual |
|
Dissemination |
Wind and other |
Wind and other |
Wind and other |
|
Germination |
Good to excellent |
Good |
Good |
|
Establishment |
Fair to good |
Negligible to fair |
Negligible |
|
Development |
Light-demanding |
Strongly light-demanding |
Strongly light-demanding |
|
Hardiness |
Hardly |
Hardy |
Fire-resistant |
Insofar as natural regeneration is concerned, these favorable qualities have permitted the use of a variety of forest practices, often dependent in moist and wet types on the ability of desirable species to establish themselves adequately under the shade of natural forest (as in much rain forest), or to regenerate satisfactorily at the time of cutting (as with teak), if assisted properly with necessary measures to assure adequate seed production, soil preparation and tending. Knowledge is still incomplete for all types, even for species as advanced as teak, while the problems of securing the establishment of adequate quantities of desirable species in most moist and wet forests, and their proper maintenance by tending, leave unsolved a great multitude of complex problems. The question of establishment of natural regeneration of any kind is also made increasingly difficult as rainfall quantity becomes lower and recurrent drought more frequent and severe. Sal, though in general an aggressive species, illustrates the difficulty of securing establishment in an environment in which frost, over-wet periods, recurrent drought, frequent fires and similar undesirable environmental features, are coupled with periodic seeding and a limited period of seed viability. This causes difficulties in securing natural regeneration by canopy manipulation or other appropriate silvicultural measures at any given time or place, the only condition under which a silviculturist may be properly said to have adequate control of practice. In most dry forest types, though the floristic pattern is less complex, the example of teak and sal illustrates the tendency towards purity of composition. In these types, natural regeneration is only feasible in vegetative form, as through coppice sprouts. Similar though diverse problems for the special types discussed, mangrove, bamboo and conifers and the special research needs growing out of them, are covered in the pertinent literature, including papers presented to the Fourth World Forestry Congress, and are not further elaborated in the study, Tropical Silviculture, Volume I.
In summary, though much progress has been made in spite of incomplete knowledge and limited markets, material expansion of natural regeneration practices will require not only the improvement and perfection of present knowledge and methods but the extension of knowledge and practice to forests and species not now merchantable. Admittedly, this is a formidable task, in view of the complexity of tropical forests and their wide geographic distribution, and despite the general similarity in environment and silvical character within each major formation in all world regions, and the probability that silvical problems and patterns may be alike in broad context, the focal point of this particular study. Under these circumstances, an expansion of research activities and large-scale trials to furnish information on costs and practicability of methods, as well as silvicultural results, would seem essential. Though more extensive knowledge on as wide a scale as possible seems highly desirable, it appears to be a practical necessity to concentrate on the most promising species and species groups and on the most productive fields of botanical and silvical knowledge. Tropical forestry literature (including the report of, and the papers presented to, the Tropical Forestry Section of the Fourth World Forestry Congress) contains many useful and detailed suggestions as to fields and projects for research activity by major forest formations and types.
It would indeed seem worthwhile, therefore, to indicate only the major fields of definite value and promise.
In this connection, past progress has obviously advanced most rapidly by:
1. Broadening markets(permitting expansion of silvicultural practice to many areas now economically inaccessible due to current lack of such species or now operable by commercial selection only).2. Silvical research
(on methods of enriching future stands by increasing the proportion of the more promising species Such silvical aspects as seeding habits, factors affecting establishment, crop protection, behavior of various species under the shade of mature forest and ability to compete in mixed forest and form a satisfactory commercial crop both quantitatively and qualitatively, have proved especially important).3. Investigation of reduction of costs
(tending complex stands, such as by improved poison-girdling practices or reduction in expensive weeding operations).
Courtesy, Jorgenson
Courtesy, Pyinmana Division, Burma
Future programs might well include:
(a) Inventories to show what species are available in accessible stands in economic sizes and quality and in sufficient quantity to assure a satisfactory commercial supply. Accurate botanical identification of such trees is required, including identification in seedling and pole stages.(b) Technological studies to determine the more promising of these species, regardless of present marketability, i.e., those with superior wood qualities from a commercial standpoint. Such studies should include not only normal tests of anatomical, physical and mechanical properties, but also tests of seasoning and machining qualities, durability, ease of preservative treatment, etc.
(c) Studies of silvical qualities especially of the more promising species, and of methods of increasing the proportion of these species in future stands. In this respect the following aspects are of the greatest importance in the design of initial silvicultural practice:
(i) Seeding habits. Frequency and abundance of seeding; character of dissemination; conditions necessary for germination.(ii) Tolerance. Especially ability to survive under full shade, form advance reproduction, respond and grow satisfactorily after release and under degrees of light and competitive conditions.
(iii) Hardiness. Resistance to insects, disease and other destructive agents.
(iv) Productivity. Rates of growth: ability to form a satisfactory commercial crop, in both quality and quantity.
(v) Reduction in silvicultural costs. Studies and large-scale trials, aimed at eliminating or reducing the costs of expensive cultural operations, e.g., eliminating unnecessary clearings or release cuttings. The proper use of controlled fire and grazing, appropriate measures to train and supervise skilled labor crews, and improvement of poison-girdling techniques.
(d) Large-scale comparisons of promising regeneration methods to determine what is economically feasible and practical, in a forest where commercially desirable species are ordinarily few and sparsely scattered through the forest, extraction costs high, and funds, labor and technical skills often very limited.
Substantial and expanding work along these lines is now under way in all three world areas. Sound research organizations are essential, staffed by intelligent and imaginative personnel well trained in modern research techniques. This will be particularly import tent as practice intensifies and new and possibly less aggressive species must be dealt with. Close association with administrative and operating staff is also needed in developing areas to help conduct the pilot plant or commercial-sized tests under sufficient controls, to test fairly new methods and procedures in actual application, and to furnish adequate information on costs. Each country and region might well examine its research and technical organizations with these thoughts in mind.
Finally, greatly expanded interchange of information and personnel between the various tropical areas is justified by the impressive similarities between various tropical formations in silvical features and the resultant possibilities of parallel silvicultural patterns in widely separated world areas. Such exchange should involve (i) published material (as in technical articles and bulletins); (ii) large-scale world conferences (as the Fifth World Forestry Congress, the Inter-African Forestry Conferences and the British Commonwealth Forestry Congresses); and (iii) direct contact of personnel (technical seminars and study tours).
FIGURE 5. - Applying sodium arsenite to a frill girdle - rain forest in Malaya.
Courtesy, Malayan Forest Service
