A. MÉTRO
SILVICULTURE IS A MEANS to an end, not an end in itself. It is a tool to fulfill the aims of policy, which itself is affected by the economic and social background, by the technological needs of wood-using industries and the general pattern of land usage. Silvicultural techniques based on significant results derived from closely controlled small-scale research must be equally well suited to large-scale application, with its limitations imposed by finance, staff, equipment or infrastructure, before forest management can be expected to adopt them. In a sense, therefore, silviculture is the handmaid of policy, management, utilization and integration of planning and financing. But the coin has two sides. Biological limitations are often the most intractable of all; it is difficult to grow trees in a swamp or a desert. Thus the biological feasibility of an afforestation project, though not the only or even necessarily the most important one, is the most fundamental. Good silviculture is the basis for the success of man-made forests.
The silvicultural problems and techniques of manmade forests differ considerably from those of natural forests. With the continual extension of afforestation into increasingly varied environmental conditions, they have become enormously diverse. The most fruitful analysis can be made only by concentrating on certain of the more essential problems specific to man-made forests.
This is why work in forest genetics and tree breeding, and also fertilization, was spotlighted so brilliantly during the symposium and the study tours. Another instance, as the Hon. David Fairbairn, Minister of National Development, emphasized in his closing speech at Canberra, is the highly specialized type of forest plantation comprising shelterbelts and windbreaks.
The survey, classification and mapping of sites for afforestation constitute one of the starting points from which the formulation of afforestation policy or the planning of a planting project can be begun.
The solutions applicable to the problems of site survey and classification depend on the size and variability of the projects concerned, on the degree of accuracy required, on the level of development of the area, and on the general scientific and technical documentation that is available. The problems vary according to whether they are considered on a world, a national, a regional, or a purely local scale. In many cases, however, the problem of site evaluation, or the estimation of the productivity of a site for forestry, is a complex one. Even if the production objective is well defined (e.g., pulpwood), the actual productivity of a site is always the result of the interaction of a number of factors, which include not only the bioclimatic and pedological characteristics of the environment and the species that is to be planted there, but also the technical (tending, fertilizing, etc.), financial and social means that are available for establishment and cultivation.
Nevertheless, a certain number of rules are generally applicable, as proposed by C. W. Ralston (1967). The best basis for estimating the productivity of a given tree species on a given site is always a stand of exploitable size of the same species already growing on that site. The best measure of the productivity of a stand of this kind is provided by a conventional site index based on the relationship of the height of the dominants to age. Since volume is more affected by stand density than is height, the use of direct volume measurement is less generally applicable than that of height measurement; direct volume measurement can, however, be used successfully in managed and fully stocked man-made forests. It is, moreover, perfectly possible for site indices based on height measurements to be expressed in terms of volume production, as has been done in the recent management tables for Great Britain (Brodley, Christie and Johnson, 1966). The applicability to new plantations of site indices derived from pre-existing stands may be modified by differences of a genetic nature and by changes in growing conditions (soil fertility, weed competition, disease and insect attacks, etc.) and it is essential to bear this possibility in mind.
If no stands of sufficient age of the species concerned are available on the spot and thus direct measurement of productivity is impossible, the best indirect estimation of site productivity is provided by a site index obtained from another species, provided that it has been possible to establish correlations in other similar environments between the indices that are valid for this other species and those that are valid for the species to be planted. This is the method known as comparative indexing.
If there are no trees at all on the site that is to be planted, an approximate estimate of the suitability of the site for the species concerned may sometimes be obtained indirectly by combining data on the climate and physical geography; it is particularly important that this analysis of the environment should include the characteristics of' the soil. Of these the soil water balance is one of the most important, and methods of comparing this balance through the year between different geographic regions have been described by Golfari (1967) for Pinus radiata.
Soil characteristics include the nature of the parent rock, depth, moisture relations, aeration and nutrients and may involve a large number of independent and interdependent factors. Thanks to progress in the basic sciences and to modern electronic methods of computing, formulas for indexing sites by indirect methods may take into account a large number of' soil factors related to one another by more or less complex functions (Czarnowski, Gentle and Humphreys, 1967). Where such facilities are not available, it is usually advisable to simplify the description and evaluation of soils to be afforested and to concentrate on those factors which seem likely to be limiting for tree growth. Thus in the dry woodland conditions of' Zambia, most attention is paid to soil depth and soil texture (Sanders, 1967).
The types and series of soils that have been mapped for agricultural purposes can sometimes give an approximation of the forest productivity of a site, but this is not always the case. In particular, these agricultural soil types frequently take too little account of the depth of the soil, which influences forest productivity considerably.
Wherever it is possible to establish good correlation between the existence of well-defined natural plant communities or of individual indicator species and forest productivity, this method of evaluation can be extremely useful.
In general, the most rational approach consists in combining the direct and indirect methods, taking into account simultaneously, for each site, any indices that may be known and the most important factors of the physical environment; of these, particular emphasis must be placed on those related to the water, air, and nutrient supply of the soil, as well as temperature, rainfall and humidity variation throughout the year.
It must be accepted that cases frequently occur where direct methods of site evaluation are impossible due to absence of existing stands of the species to be planted and where indirect methods are not precise enough to justify the large investment of an afforestation project. In these circumstances the establishment of species trials becomes essential. The efficiency of species trials, however, is dependent; on the forester's ability to differentiate and to recognize the main soil types, each of which needs to be tested separately. For example, it may be impossible to say that the red clay of soil type A will have a higher productivity for a particular introduced species than the coarse sand of soil type B. but it is nonetheless important to be able to say that they are obviously different arid should therefore be sampled separately by any species trials program.
The principles of site evaluation through comparison with other sites of known productivity have been. described in the preceding section. The method has a relatively precise applicability only within the natural distribution range of the species concerned, or one of its major introduction areas, such as, for example, that of spruce in Europe, Monterey pine in Australia, loblolly pine in the southern. United States. To extrapolate comparisons to very great distances or to totally foreign floristic regions, where the only possible factors for comparison are those of rainfall and temperature measurements collected over a few years, and of soil analysis, would be too hazardous. It therefore becomes necessary in a large number of countries to start afforestation programs by carrying out species and provenance acclimatization trials. They should be carried out in accordance with -the principles summarized by R. Morandini (1967). A clear distinction should be made between the three following phases.
The first phase, called the "elimination phase," consists in selecting locations that are typical of the major site types, and planting in each of them a number of tree species or varieties, of which each is represented by a relatively small number of individuals. As their name indicates, the primary purpose of these elimination trials is to obtain results, both negative and positive, that are detectable within relatively short periods of time, of the order of a few years, in order to simplify the longer term experimental work which follows. It is, however, desirable that trials should be repeated in time, in order that the negative results should be as valid as possible, and not caused by circumstances that are fortuitous, secondary, or easily remedied (for example, absence of mycorrhizae, exceptional seasonal climatic conditions, etc.).
FIGURES 18, 19 and 20. - Site preparation at Nyamusika, Toro, Uganda. Left; mechanical slashing of elephant grass Pennisetum purpureum about 12 feet high prior to line plowing. Right, line plowing in slashed elephant grass; planting will be done into the furrows. Below, Pinus patula 6 months after planting in line-plowed furrow in elephant grass.
In order that the positive results should also be valid, observations should be prolonged for several years so that account can be taken of the requirements of the various species beyond the juvenile stage, and of their growth rhythm, and so on.
The second phase consists in establishing a network of comparative plots of a reduced number of species. Each plot should be large enough to provide information on the volume production potential of each of the species that has shown most promise in the elimination phase.
The third phase consists in determining for the most promising species the provenances which are best suited to the ecological conditions of the region that is to be afforested and to the objectives, both technical and economic, that are to be pursued there. These comparative provenance trials should cover the largest possible number of well-identified provenances, including seed sources located in countries where the species has been introduced with long-standing success. A general account of the principles, aims and methods of provenance trials has been given by Lines (1967), while early results from a specific example dealing with Populus x euramericana in Italy have been described by Avanzo (1967).
All these phases of investigation are advisable for regions where major programs of afforestation are proposed. However, they can be implemented only by experienced research staff and organizations. They necessitate the setting up of careful, detailed procedures, both for the collection of seed of the most promising species and all their provenances, its storage, distribution, and use, and for the planning and execution of the experimental program. Results must be suitable for interpretation, not only in scientific but also in practical and economic terms.
In is, therefore, essential that all efforts in this field should be well co-ordinated at international level. The specialized sections and working parties of the International Union of Forest Research Organizations (IUFRO) are the appropriate bodies to assume responsibility for this and to propose to their member institutes a code of experimental procedure which has received general agreement. This is already being done in the case of provenance trials.
Mention should be made of what has been and still is being achieved, under the aegis of FAO, by several governments in collecting seed of various provenances of Australian eucalypts and tropical, Mexican, Carribbean or southeast Asian pines, and by IUFRO for provenances of species that are of importance for plantation forestry in the temperate zones. All these operations constitute the logical development of resolutions passed at recent international forestry meetings. It is essential to extend and supplement them. It has been suggested, for example, that certain institutes which have adequate funds and technical means at their disposal should act as wholesalers and distribute, for a suitable fee, supplies of seed of different provenances to those who request them.
Among fast-growing tropical plantation species, most attention has been concentrated until now on eucalypts and tropical pines; for the biological plasticity of eucalypts has enabled several species of this genus to adapt themselves readily to a very wide variety of sites. These species have very quickly passed the "elimination phase" in tropical and subtropical environments and have displayed a remarkable ability to respond to favorable site conditions. The symposium recognized the need to concentrate provenance research on a limited number of species: Eucalyptus globulus, E. camaldulensis, E. gomphocephala, E. viminalis, E. tereticornis, E. grandis, E. robusta, E. citriodora, E. microtheca, E. deglupta, E. dalrympleana, E. bicostata and E. occidentalis and, in addition, to intensify research on the acclimatization and cultivation of the species of the group Renantherae, the technological and economic interest of which is well known, but which have rarely given satisfactory results in the elimination trials in which they have been used.
In spite of the adaptability of a number of species of eucalyptus, their usefulness appears to be limited by difficulties in economic utilization. In many countries where these species have made it possible to launch large afforestation campaigns, a move is now being made toward a kind of reconversion, with increasing importance being attached to conifer plantations (southern and eastern Africa, Brazil, northern Africa, etc.). Hence the value of general analyses of the future potential of eucalypts (Pryor, 1967) and tropical conifers (Lamb, 1967), as well as more specific accounts of introductions into individual countries, such as those in Congo, Brazzaville (Groulez, 1967b, 1967c).
In the case of tropical conifers, it is desirable that the excellent work undertaken by tile Commonwealth Forestry Institute on low altitude tropical pines be extended to species of medium and high altitude tropical conifers.
Similar studies, particularly in the field of provenance research, should be extended to other fast-growing tropical species, such as Gmelina, Cedrela, teak, limba (Terminalia superba), certain American mahoganies, bamboos, etc. In some cases studies are already in progress. Such projects should be made generally known, to enable all interested institutes to take part as was done, for example, for a number of provenances of Eucalyptus camaldulensis and E. dalrympleana collected by the seed center at Canberra. In some tropical countries it may be advisable to pay particular attention to species that are able to coppice or to regenerate naturally within the rotation.
Whatever the species in question, it is important to safeguard the sources of seed of all the species and provenances which, in one region of the globe or another, in their country of origin or of introduction, are regarded as species that are useful for plantation forestry.
Successful results in forest nurseries are often due primarily to devoted care and attention to detail, rather than to modern technology. Everything appears to be dominated by local conditions, and the subject of "forest nursery techniques" does not lend itself well to generalizations of worldwide application. In particular, conditions for nursery growth in the cool temperate zones (De Philippis and Giordano, 1967; Aldhous, 1967) differ considerably from those in the tropics (Iyamabo, 1967; Groulez, 1967e, 1967f; Foot, 1967; Procter, 1967).
Nevertheless, a certain number of common principles, objectives and practices do exist. A suitable medium for germination, adequate water, air, nutrient supply and space for growth, and protection against weather, weeds, pests and diseases are necessities common to nursery plants everywhere. So is the need for cutting and the importance of correct methods of preparation and transport of plants, to ensure that good nursery stock is equally good after being planted in the field. The same practice may be adopted for more than one reason. In Scandinavia, for example, where, thanks to the "kindness" of the climate, the use of bare-rooted planting stock has been the general rule until now, the present shortage of labor has made it necessary to plant throughout the year, including the dry season.
This has led local officers to develop methods of raising ball-rooted plants, which previously seemed essential only in zones with a long, severe drought season.
BALANCE OF COSTS AND PROFITS
However, there is a more important common denominator; it is now fully appreciated everywhere that the value of any nursery technique, like all forest plantation techniques, must be judged by its effect on the long-term cost/benefit ratio of the plantation.
The quality of the young plants is an important factor affecting this ratio, not because their cost constitutes a high percentage of the initial investment (only about 5 percent), but because their quality and uniformity directly govern the "percent take," and thereafter the rate and uniformity of growth during the early years in the life of the plantation. The nursery is the best point for quality control of the planting stock; it is here that the first step toward the success of the plantation can be taken. To achieve this objective some increase in nursery costs is frequently acceptable.
TENDENCY TOWARD CONCENTRATION
The attempt to reduce nursery production costs explains the tendency toward the concentration of nurseries into large units capable of producing several million plants per year (for example, the Kaingaroa nursery, visited during the study tour in New Zealand, which produces 19 million plants per year). This concentration facilitates the mechanization of cultivation techniques, and ensures that production of healthy, uniform planting stock can be supervised by only a few specialists.
This tendency toward concentration has as its immediate corollary long distances from the planting site. The location of nurseries must in any case be such that the possible difference between the ecology of the nursery and that of the planting site will not have an adverse effect on the "take" of the plants. In order to reinforce subjective ideas on this subject by precise factual data, further searching studies, principally of a physiological and pedological nature, are required (Stone and Goor, 1967). In any case there must be an efficient organization for the transport of plants from nursery to planting site. This is particularly important in more arid tropical and subtropical regions, where the success of planting is most often governed by the successful cushioning of the shock suffered by young plants when moved from the privileged environment of the nursery to the rough conditions of the planting site. The example is sometimes quoted of countries where transport is carried out in isothermal vehicles.
On the other hand, a fine network of smaller nurseries, provided they are efficient - and it is often difficult to ensure this - has the opportunity of supplying plants to the public, of presenting the "forestry image" and of winning the goodwill of local communities in a more intimate way than can be done by a few very large nurseries.
TRAINING STAFF
The need to give specialized training to the staff responsible for their management is another feature that is common to all nurseries whatever their location, and the professional training of this category of staff must not be neglected in drawing up a training policy for man-made forests.
SEED QUALITY CONTROL
The supply of nurseries with good quality seed is still an important problem which has not been solved satisfactorily everywhere, and which must continue to be the subject of sustained efforts. A remarkable example of seed production and supply policy has been illustrated in Queensland (Slee and Reilly, 1967).
FUTURE DEVELOPMENTS AND SPECIAL TECHNIQUES
There is room for further development in the size and type of individual container. The elimination of root deformation, which has been a serious problem in some countries, notably in north Africa, is of high priority, while reduction in container size and thus in cost, of both materials and transport, is another obvious line of investigation. In contrast is the use of very large-sized plants, both of poplars and conifers, for example in Italy and Yugoslavia; in this case the increased nursery costs will, it is thought, be more than offset by a reduction in tending costs and in rotation. Further research can be expected on the use of hydroponics and artificial substrates instead of soil, synthetic mulches and improvements in fertilizers, sterilizers and herbicides. Some special techniques, such as the temporary greenhouses now in general use in Scandinavian countries (Siren, 1967) or techniques for the cold storage of young plants, are likely to be of far-reaching value in temperate climates. Extension of the photo-period through artificial lighting is also a future possibility in those regions.
In addition, it is impossible to overemphasize the fact that vegetative propagation techniques - such as those developed with poplars, for example - have made it possible to obtain results that are ideally suited to the needs of the timber economy of several countries in the temperate zones of the globe. This should stimulate intensification of the studies that have already been undertaken by several research institutes, and have yielded more than encouraging results on the vegetative propagation of certain species of eucalypts, conifers or other groups of species (Giordano, 1967).
Finally, a critical aspect of the acclimatization of many exotic species, not only conifers, relates to the establishment of mycorrhizac (Bakshi, 1967; Rambelli, 1967). It should be noted, however, that the FAO/IUFRO Symposium on Internationally Dangerous Forest Diseases and Insects (Oxford, July 1964) drew the attention of afforestation workers to the dangers inherent; in transporting from one country to another samples of soil containing mycorrhizae. It is in this way that nematodes, bacteria, fungi and other pathogenic organisms can be spread. It is necessary to act with great caution, and the aim should eventually be to transport only pure cultures.
It is obvious that establishment methods affect the whole history of a plantation and the entire sequence of subsequent silvicultural treatments: thinning, pruning and, to a certain extent, final felling.
In addition, site preparation, planting and tending often constitute the most important part of the investment in manmade forests. In the final financial balance sheet of the afforestation operation, their cost is weighted by the fact that they have to carry interest charges over the entire rotation.
This part of the activities of the afforestation worker must therefore be subjected to a close analysis, as regards both its immediate cost and the consequences of every kind which will result from it, right up to the final sale of the trees that are produced and even beyond that, to the conversion and timber marketing stages.
It must, however, be recognized that correlation between the cost of a given technique and the benefits which result from it is often difficult to establish precisely. For frequently this can be done only long after the technique is applied, when the relationship may have become obscured by numerous independent variables of all kinds (biological, financial, economic, and social).
As reviewed exhaustively by Stuart-Smith (1967), establishment techniques are considered to include techniques for site preparation, planting proper or direct sowing and tending after planting until canopy closure or until the first pruning or thinning, whichever may be earliest.
All these techniques are closely dependent upon local circumstances, of which the most important are: climate; site, including the initial flora and fauna; the tree species that is to be cultivated; the category of seed or planting stock that is available; and the kind of wood that it is hoped to produce, which influences initial spacing as well as subsequent treatment. Other factors which may affect establishment practice indirectly must also be taken into consideration, such as the economic and social situation which may point to the full use of manual labor rather than to mechanization; or the need to avoid cultivation methods which damage soil conservation in certain catchment areas; or recreational requirements which call for species mixtures and varied spacing.
SITE PREPARATION
There is an almost universal tendency, even in countries that are densely populated and with a rugged relief, such as Japan, toward more intensive and more highly mechanized site preparation methods, because of the benefits they confer in increased survival rate and early growth. This tendency may be noted both in temperate (Siren, 1967) and in tropical zones (Allen, 1967; Endean, 1967; Groulez, 1967a, 1967d). Intensive site preparation also reduces the amount of subsequent tending required, as well as fire hazards.
Plantations are being established on an increasing scale on areas that were previously l vested., either wholly or partially, where site clearing operations may have to be carried out by mechanical methods, because manual labor is either too expensive or incapable of developing the power required for this type of work Cleaning the site, after clearing, is often achieved by burning, and completed by plowing or cultivation methods which may be applied either to a proportion or the whole of the area to be planted. Complete site preparation appears to be indispensable in regions that have a long dry season and it is common practice in plantations of fast-growing species that are to be based on very short rotations. Although it is expensive, this method is beneficial in that it allows the crowns of the young trees to close canopy rapidly.
When the soil moisture is a less restricting biological factor, or when the fast growth of the young trees during the early years is less imperative, partial cultivation of the area, often in the form of strips of varying width, is frequently sufficient.
Current practice rightly tries to avoid undue interference with the soil structure. Thus, clearing is carried out with a drag chain rather than with a bulldozer, subsoiling is preferred to deep plowing, and scraping and surface treatment is kept to a minimum in regions where there is a threat of erosion (in a tropical environment this may sometimes include slopes as gentle as 5 percent).
The use of herbicides for preliminary site cleaning still presents some disadvantages, but can be useful in certain circumstances, for example to suppress the new growth of adventitious vegetation immediately before planting.
TAUNGYA -SHAMBA METHODS
The methods that are now well known under the name of "taungya" or "shamba," principally in Asia and Africa (combining for several years the growing of agricultural crops with the planting of young trees) display undeniable social and economic advantages in numerous cases. Conditions for success are the presence of an industrious group of cultivators, a sufficient demand for land to make the prospect of clearing a proportion of fresh land each year an attractive proposition, and that the land controlled by tile forest service should be both suitable for agriculture and close enough to markets for the surplus produce to be sold (:Kenya Forest Department, 1967). Under these conditions the forest workmen obtain land to cultivate and guaranteed forestry work for a certain period every year, while agreeing for their part to do the initial land clearing, to keep the trees weeded for one or more years after they are planted, and to work for the forest service each year for the period agreed. The value of the shamba system in Kenya may be judged by the fact that in 1965 four fifths of tile total area afforested had been cleared by this means. The system, however, is a relatively inflexible one and does not allow for rapid changes in rate of planting. It is also unsuitable for species which, in a moist tropical environment, need to be surrounded by natural regrowth if they are to develop natural pruning and satisfactory form.
Planting or direct sowing
There are valid reasons for believing that methods of planting proper will continue to prevail over methods of direct sowing as far as exotic species are concerned. This situation will last as long as the production of large quantities of genetically selected seed remains difficult.
This does not, however, mean that direct sowing methods should be neglected. In the United States and Canada, for example, they have been improved very substantially during the last ten years. In the southern United States the area afforested by these methods increased between 1956 and 1963 from 4,000 to 60,000 hectares per year. The cost is very low, approximately one half or one third of that of planting but, despite the notable advances achieved, it still entails a veritable squandering of seed, and the method cannot be expected to come into general use until the problem of seed supplies has been resolved. In addition, in cases where there are mycorrhiza problems, these are more easily solved by planting techniques than by direct sowing.
PLANTING PROPER
As already stated, the success of the plantation depends partly upon the quality of the plants at the moment of planting. The size of planting stock is a frequently controversial factor which merits a methodical approach (Schmidt-Vogt, 1967). Tendencies toward both smaller and larger plants, depending on local circumstances, have already been noted in the section on nursery techniques. Bare-rooted plants are suitable for use in favorable planting conditions, but some form of ball-rooted plant is necessary wherever dry spells are to be expected during or soon after planting. Where polythene containers are used there are still opposing schools of thought on whether the polythene should be removed or not. In some countries, such as Malawi, it has proved possible to use polythene containers more than once. For certain tropical hardwood species, notably teak, the use of stumps is still standard practice - for example, in Tanzania (Wood, 1967).
The date of planting is vital, particularly in dry regions. In most cases, the planting date in such regions is fixed according to the amount of rain that has already fallen (on the assumption that it will continue to fall). The improvement of long-term weather forecasting should encourage the strengthening of collaboration between afforestation workers and the meteorological services. The use of planting machines makes it possible to take rapid advantage of the optimum time for planting, where labor is short. They are used almost exclusively in developed countries; in others, the majority of planting is done by hand.
Deep planting, or the placing of the root collar below the level of the soil, often makes it possible to obtain a better take in dry conditions. It is known that, as an extreme measure, large saplings of poplar can be planted to a depth of more than 3 meters. In tropical areas where termites are numerous the use of insecticides is essential for the protection of eucalyptus, and in these circumstances the advantages of deep planting against drought may be outweighed by the danger of bringing the young trees into contact with untreated soil. Recent work with antitranspirants has indicated the value of pursuing further investigations into this method for arid regions.
The use of fertilizers is discussed later. It can promote the survival and growth of young plants and thus reduce tending operations. Some species of eucalypts are giving very encouraging responses.
TENDING
The need for tending operations after planting is governed by the same factors as site preparation. It is the best sites which normally require the most intensive tending operations, and justify them by increased profitability.
The use of herbicides is still not in general use, but it is probable that it will increase rapidly. Research on this subject is concerned not only with the type of herbicide, which must not affect the growth of the young trees, but also with the time of application (Bachelard and Boughton, 1967). Competition from adventitious plants must be eliminated not only during the driest season but also during the period of most active root growth. Woody regrowth and creepers, for example Mikania in Malaysia, are usually the most difficult and expensive competitors to eliminate on closed forest sites and may require intensive research programs, while on savanna sites grass, together with the fire danger which accompanies it, is the biggest danger.
RESEARCH
It cannot he overemphasized that successful manmade forests can only be achieved by the integration of all phases of management. Nevertheless, planting techniques are of particular importance. Afforestation workers should never feel satisfied with results obtained, however brilliant, but must be prepared for continuing research to compare existing with possible improved practices. They should not neglect to apply new advances in agricultural technology, particularly in mechanical and chemical methods. In addition, in this field of planting techniques a better knowledge of tree physiology would be of inestimable value.
PRESENT SITUATION
Wardle (1967) has summarized current practice in spacing, thinning and pruning in three of the principal regions of the globe where large planting programs have been undertaken: the southern hemisphere, North America and northern Europe.
The tendency in the southern hemisphere to adopt spacings that are generally wider than those used in Europe originated from the intention of obtaining saw-timber rapidly, and has been maintained by the need to reduce labor and to conserve planting stock while continuing to plant a large area each year. Spacing is commonly 2 to 2.7 meters (7 to 9 feet).
In North America, where the areas planted are still relatively small compared with the total forested area, the spacings adopted are of the same order as in the southern hemisphere. In northern Europe, the spacings adopted continue to be closer - 1.2 to 2 meters (4 to 7 feet) - than those used in the other two regions, but a fairly strong tendency toward wider spacing can be observed, for example, in the United Kingdom, partly to reduce initial investment, partly to facilitate access for machines, and partly to reduce or avoid the need for early thinnings, which have little chance of being merchantable.
Very wide spacings are frequently used when planting (other than taungya planting) is done after clear-felling of tropical high forest. This is due to the fact that thinnings are often unsalable, and that certain species, such as the Terminalias, are naturally wide crowned, while others - for example Ancoumea - benefit from the natural regrowth which promotes natural pruning and improved stem form in the planted trees.
Quantitative information on current thinning practice is scarce. In the southern hemisphere it is aimed essentially at releasing the dominant trees to achieve exploitable size as rapidly as possible, with a view to the production of saw-timber (Borota and Procter, 1967; Grut, 1967). In North America, thinning plays a less important role, although with some notable exceptions in the southeastern United States. Where thinning is practiced the aim is apparently to obtain maximum merchantable timber at the end of the rotation. It is interesting to note that in the United Kingdom the closest attention is being paid to the economic aspect of thinnings. It is thought there that spacings closer than 2 meters (7 feet) are not generally justified, when account is taken of the cost of the planting investment and the effect on the size of early thinnings. The advantages of easy access may lead to still wider spacing between the rows or to line thinning in the first thinning operations. Throughout the thinning period the highest intensity of thinning consistent with maximum volume production is advocated. The alternative of no thinning may be preferable in areas where windthrow is expected to terminate the crop's life before optimum rotation age, and in areas of low yield or poor access, especially those in which roads have not yet been constructed.
Pruning is practically nonexistent in North America. In the southern hemisphere, on the contrary, it is regarded as an indispensable complementary operation in widely spaced plantations intended to produce saw-timber on a short rotation. Pruning is to 6 meters (20 feet) and sometimes up to 10.5 meters (35 feet), but final high pruning is restricted to a limited number of 250 to 375 per hectare (100 to 150 per acre) of the main crop trees. In Europe pruning is usually confined to the removal of branches to head height to allow access for thinning but even this application would be reduced if line or strip thinning is generally adopted. High pruning will probably be applied to only one species, Pinus sylvestris, in order to eliminate black knots.
With the possible exceptions of teak and poplars, there is even less information about spacing, thinning and pruning in broadleaved than in coniferous crops. Principles are likely to be similar, but broadleaved trees usually have wider crowns than conifers and the sharp differentiation between sapwood and heartwood which is frequent may also influence silvicultural treatment.
INFLUENCE OF SOCIAL AND ECONOMIC CONDITIONS ON SILVICULTURAL PRACTICE
Social and economic conditions have plainly exercised a strong influence on the development of silvicultural practice. In the southern hemisphere the lack of adequate quantity or quality of wood in the indigenous forests and the high cost of importing from overseas led to the policy of producing saw-timber as rapidly as possible and thus to the practices of wide initial spacing and frequent pruning. Shortage of labor in New Zealand led to delayed thinning. North American practice was dominated by the existence of a vast resource of natural timber available to the market at low cost; this made profitable thinning in plantations virtually impossible. In Europe the high level of demand, including a demand for small-size material by rural markets, favored intensive management and frequent thinning.
Economic conditions change. The competition from natural forests of the northern coniferous belt will become less overwhelming as exploitation moves into less accessible stands of poorer quality. Rural and pitprop markets for small-size material in Europe are likely to continue to decline, but the increasing demand for pulpwood may assist the disposal of thinnings in a number of countries. Labor costs rise everywhere and this is likely to favor the development of mechanical methods such as line thinning.
It is essential that those who lay down thinning and pruning practices should take full account of economic considerations and the cost/benefit ratio which is expected to result, not only under the current economic conditions but also, as far as possible, under any future changes of conditions which may be anticipated. The maximum degree of flexibility must be incorporated, to allow managerial staff discretion to allow for local or changing conditions.
RESEARCH
The problem of thinning in relation to the sale of small-size timber has already been considered by IUFRO. The program and conclusions of the working party set up for this purpose were discussed at its 14th congress (Munich, September 1967). Attention has also been drawn to the use that can be made of measurement of basal area development as a means of regulating thinning intensities (Bevege, 1967; Borota and Procter, 1967).
Wardle (1967) has defined the different elements of silvicultural treatments on which research should obtain information as:
1. physical consequences of treatments;
2. call on resources needed to carry out the treatments;
3. effect of the physical consequences of the treatments on the value of the product.
The physical consequences should include not only effects on quantity of yield, both total and at various stages, but also effects on the number of stems and the possibility of selection, effects on quality of production (size of log, taper, ring width, wood density, fiber characteristics and distribution of knots) and indirect effects on resistance to pests and diseases and on wind stability. Resources needed to carry out the treatments include finance, equipment, infrastructure, labor and technical expertise. Economic analysis is needed to convert physical effects into terms of value and to integrate the appraisal of the whole sequence of resource use/treatment/physical effect value.
It is certain that between now and the end of the century, the problem of the regeneration of forests planted by man will present itself over large areas, whereas, until now, preoccupation has been much more with the creation of new plantations than with the sound management or regeneration of old ones.
Lewis (1967) has dealt with this question, but it should be noted that there is very little in the literature on the subject.
The following preliminary observations can be made, (cf. Van Miegroet, 1967a).
1. Natural regeneration is not associated exclusively with natural forest.
2. Artificial regeneration is not associated exclusively with man-made forests.
3. Both can be used in combination in both cases.
4. Artificial regeneration should be employed as soon as it becomes apparent that natural regeneration will not suffice to achieve rapidly and economically the aims in view.
PRESENT SITUATION
Up to the present, the only important examples of regeneration of man-made forests are for:
1. species cultivated in high forest in Europe;
2. Pinus patula and P. radiata in the southern hemisphere;
3. certain cultivars of the genus Populus;
4. eucalypts in tropical and subtropical zones;
5. Acacia mearnsii in tropical and southern Africa.
TRENDS
In all cases the tendency is to make increasing use of artificial regeneration, owing to the uncertainties caused by the irregularity of seed years and good seeding conditions, and the excessively long delay that usually has to elapse before natural regeneration is complete. The future will inevitably see an increase in the use of fast-growing species in monocultures. This type of crop needs intensive management, the high cost of which can only be justified by keeping the rotation short. The result will be an increasing need to have immediate uniform and complete regeneration, with regular spacing and ready access for all operations, which will be increasingly mechanized.
These criteria will be satisfied more easily by artificial regeneration techniques than by natural regeneration.
The only exceptions will be where it is necessary to maintain the canopy for soil and water conservation; or when the species in question coppice easily up to a considerable age (eucalypts, for example), or produce very abundant seed, as sometimes occurs in the southern hemisphere with Pinus patula and P. radiata; or when there is a shortage of labor. Even in some of these instances the need for genetic improvement through the use of seed produced from seed orchards or seed stands will favor artificial regeneration.
For some countries the choice between natural and artificial regeneration may have to be based primarily on financial conditions. These can impose the creation of stands which do not conform to optimum density standards, for example in tropical zones.
In any event to enable him to choose, objectively and in advance, the type of regeneration that is the most efficient for each site, the practicing forester will need much more precise scientific data than have been available in the past.
Research
The major points with which research should be concerned are the following:
1. cultivation between rotations;
2. slash disposal, or burning;
3. protection against animals, "repellents;"
4. blank filling in both natural and artificial regeneration;
5. long-term maintenance of productivity.
The problem of maintaining site productivity is discussed in Chapter III. It is clearly of great importance in determining the choice of regeneration method. With certain combinations of species and soil, including those based on an initial incorrect site choice such as occurred in some spruce stands in Europe, it may be necessary to practice rotation of crops just as is done in agriculture. In other cases it may be possible to grow a number of consecutive rotations of the same species without serious deterioration.
Conclusion
To conclude, the progress that can be expected from research on provenances and tree breeding, the changing pattern of industrial demand and of the techniques used to meet it will ensure that plantation forestry and artificial regeneration, well suited as they are to the world of today, will be still better suited to the world of tomorrow.
PROBLEMS ASSOCIATED WITH DROUGHT
Drought in all its forms constitutes one of the most frequent hazards encountered by afforestation workers.
Afforestation practices in arid and semiarid areas have been developed mainly by practical foresters working through trial and error under local conditions. There has been little in the way of co-ordinated research. It is therefore important to analyze and classify the problems involved. Stone and Goor (1967) have done this with particular reference to site preparation, choice of species, production of planting stock (there would be no question of direct sowing), planting techniques, and subsequent tending.
Site preparation techniques are extremely important in arid zones. They must first of all stabilize the site and, if necessary, reduce its sensitivity to wind or water erosion. Sand dune fixation presents a special problem which can be tackled by a variety of methods, as is done in Tunisia (Ben Aissa, 1967). At the other extreme are the hard crusts and pans which occur, for example, in parts of Morocco and which must be broken up by heavy subsoiling machinery before successful planting can be carried out (Bennouna, 1967). Techniques must be appropriate to ensure that the soil moisture is used at the optimum time for the growth of the young plants, without any unnecessary losses caused by overdeep infiltration, excessive runoff or direct evaporation, or consumption by other adventitious plants. This moisture may need to be concentrated in the places most favorable for the young plants; hence the importance of terraces, subsoiling, and surface cultivation.
Special opportunities and problems exist in areas where there is low rainfall but abundant river water, for example in the Nile and Indus valleys. Irrigated tree plantations must usually compete for land and water with the needs of agriculture, and must therefore achieve the maximum possible productivity from a Limited area. This implies research into the possibilities of using new and higher yielding species, for example the possible replacement of Dalbergia sissoo by eucalypts in West Pakistan (Siddiqui, 1967).
Techniques based on the concentration of the more fertile topsoil in ridges and planting on the ridges are frequently beneficial in promoting early growth.
Techniques for the conditioning of young plants in nurseries, so that they can be transported and planted without suffering from drought, are also very important. Only very rarely can bare-rooted plants be used with success, unless in the form of "stumps." On the other hand, in practice a very wide variety of containers are used (including polythene veneer, metal, banana leaf, compressed peat, etc.) which may or may not be regarded as expendable. Soil blocks (small polyhedral blocks of compacted soil), which may contain powdered fertilizers or specific pesticidal chemicals and which serve simultaneously as soil and container, are also much in favor.
Before adopting a method on a field scale, the forester must check, some time after planting, that the root systems of the young trees have not suffered from the type of container used. The malformations which may result sometimes do not appear until several years have passed. It is also essential that the root/shoot ratio of the young plants be properly balanced.
For successful survival the choice of planting time, in relation to the build-up of soil moisture and to the growth rhythm of the roots, is crucial. Any measure which reduces evaporation during the first days or weeks after planting is likely to be beneficial. A great many trials have been carried out on the application of synthetic products (mostly derived from petroleum) such as transpiration retardants and mulches, but none have yet reached the stage of large-scale field application.
Tending operations after planting are essential until the young trees are large enough to close canopy, form their own microclimate, and shade out weed species. They usually have to be continued over two or three years and are expensive.
Planting techniques in arid zones present a number of new problems, and have neither been developed through well-established local traditions nor from a sufficient body of methodical research. A well-co-ordinated program of intensive and fundamental research is needed, to be carried out by integrated research teams, covering several disciplines. Priority should be given to bioclimatic, pedological and physiological problems (evapotranspiration phenomena; growth rhythms of roots and shoots, etc.). Economic research should follow. These multidisciplinary research teams, where they can be formed, should coordinate their efforts at the international level under the aegis of FAO, IUFRO, or through direct liaison.
Any effort of international collaboration should investigate the possibility of developing a common system of comparative site evaluation, following the general principles discussed above, but suited particularly to the somewhat narrow range of highly specialized conditions in the arid zones. Such a system should be easily applicable by the practical forester and should include site criteria based on the natural vegetation, which can be of great value in areas where detailed climatic observations are lacking.
PROBLEM ASSOCIATED WITH EXCESS MOISTURE
Mikola (1967) has estimated the total area of swamps and peatlands in the world at over 200 million hectares. In addition there are millions of hectares of waterlogged mineral soils. The peatlands of the boreal coniferous belt in Eurasia and North America account for the biggest area; in Finland, for example, they constitute 32 percent of the land area. There are also substantial areas of tropical swamp forests, such as those in Sarawak, the Amazon basin, parts of Costa Rica and Panama, and in Colombia and Ecuador, in addition to coastal and deltaic mangrove forests.
Much of the swamp and peatland area carries some form of tree growth and wood production can be increased through the improvement of growing conditions and the use of natural regeneration. But it is probable that there are at least 50 million hectares of treeless bogs existing in parts of the world where the climate would permit forest growth (Mikola, 1967). Afforestation with suitable species, combined with drainage and where necessary with the application of fertilizers, offers the means of rendering some at least of these areas productive. Yet, it is probable that less than 10 million hectares have been reclaimed so far.
The increase in production that results from drainage measures is a substantial one; it can range, in countries where their development has increased considerably in recent years - the Scandinavian countries, the U.S.S.R., Canada and the United States - from 1 to 8 cubic meters per hectare per year, depending on the quality of the soil.
Phytosociological studies are of great value in deciding on the most appropriate treatments. In the peatlands of northern Europe, evaluation of the suitability for drainage and afforestation and the need for fertilization, as well as the selection of tree species and of the method of afforestation, is based on the composition of vegetation and the quality of peat.
Two methods of forestry drainage in peatland are in use on a large scale: shallow ditches closely spaced (for example 25 to 40 centimeters deep and 3 meters apart) and deep ditches widely spaced (for example 40 to 60 centimeters deep and 40 to 60 meters apart). Since the second world war mechanized ditching has increasingly replaced digging by hand and has made it possible to cover much larger areas annually; the biggest area drained manually in Finland in a year was 40,000 hectares or 10,000 kilometers of ditches, compared with recent achievements of over 200,000 hectares or 50,000 kilometers of ditches by mechanized methods. Ditches must be regularly maintained. Trees increase evapo-transpiration and it may be possible to dispense with a proportion of closely spaced ditches after a tree-crop is established, but some at least of the ditches must be kept in good condition.
On some sites drainage alone is not enough. Fertilizers must be added also. Phosphorus is the element most commonly deficient in peat soils, while potassium may also be in short supply; nitrogen is usually adequate, though liming may be necessary to mobilize it and make it available to plants.
The best methods of establishing trees for different local conditions must be found by experience. In Finland, both direct broadcast seeding and notch-planting are used successfully, while in the United Kingdom and parts of Norway planting into upturned turves, after plowing, has given the best results. Suitable species for local conditions may be extremely limited. In the peat swamps of Sarawak, Shorea albida is the species planted; it grows fast but is frequently of poor form and competition from weed trees presents serious problems.
Though it must be admitted that cost/benefit analysis indicates a fairly low rate of interest on the investment in reclamation of poorly drained peat bogs and other wet soils, increasing demand for wood products and pressure from competing interests on the higher quality sites are certain to accelerate this activity. Finland, for example, plans to drain and to afforest or reforest an area of 5 million hectares of poorly drained land over the next 20 years; it is estimated that the realization of this program will increase the annual growth of the Finnish forests by 10 million cubic meters, or 20 percent.
Closer co-operation between organizations carrying out research in this field in different countries is very desirable and was the subject of discussion during the fourteenth IUFRO congress at Munich.
PROBLEMS ASSOCIATED WITH TOPOGRAPHY
Otsuka (1967) has surveyed the work being done in afforestation at high altitudes and in steep topography in Japan, a country which has had long experience in this field.
At the planning stage, the survey, classification and selection of sites to be afforested in mountainous regions should take special account not only of the bioclimatic and edaphic factors mentioned earlier, but also of the geomorphology of the sites and of the types of erosion which may possibly develop there during the phase of soil preparation for planting. Position on the slope, aspect and microtopography can all introduce great variations in growing conditions which must be taken into account when selecting species. In Japan, for example, Cryptomeria is usually planted on the deeper soils on the lower slopes, while Chamaecyparis is planted on the upper slopes and ridge tops.
It is useless to expect an economically profitable timber production on certain profiles with convex or complex slopes. The fixation of such slopes may well be achieved more easily, quickly and economically by mere protection of the natural vegetation, whether woody or herbaceous, than by afforestation.
Steep topography makes road-making expensive, which in turn puts up the cost of transporting labor, plants and materials. Physical difficulties in working reduce work output, while possibilities for mechanization are restricted. In Japan tractors can work on slopes up to 20 degrees, while manual operations can be performed up to 45 degrees. Light hand-operated machines, such as brush cleaners and hole diggers, are being developed for this sort of condition. The danger of soil erosion may rule out the more intensive site preparation and tending operations, such as burning and clean weeding, which would be preferred on flat land. On some steep slopes some form of terracing is essential for success. This reduces surface runoff, soil erosion and, in the colder areas, snow creep, and assists in concentrating rainfall in the rooting zone of the trees.
It is to be hoped that the volume of work being undertaken in this field will expand rapidly throughout the world. Erosion causes the loss of thousands of hectares of potentially productive land every year, and afforestation is one of the methods of remedying the situation. For example it has been estimated (Falla Ramirez, 1967) that some 1.2 million hectares in the mountain areas of tropical America are in need of reforestation.
PROBLEMS ASSOCIATED WITH FERTILITY
The growth and yield of trees, like those of any other plants, are closely influenced by their nutrient supply. It is surprising therefore that the importance of the provision of adequate mineral elements has been recognized in plantation forestry only comparatively recently.
Swan (1967) has given an exhaustive review of the problem, and in particular of the diagnosis of mineral deficiencies by the physical or chemical analysis of the soil, by soil bioassays, by the study of visual symptoms, and by foliar analysis. Only a correct and complete diagnosis can form the basis for a sound treatment.
Application may be by hand, by inter-row spreaders, by dust blowers, by soil injector or by aircraft. Aerial application is rapidly becoming standard practice in countries with large areas to be treated and the costs range from $25 to $100 per hectare ($10 to $40 per acre) according to conditions.
It is evident that in many plantations the trees have an inadequate supply of one or more of the 13 essential mineral elements, the macronutrients (N, P. K, Mg, Ca, S) and the micronutrients (B. (Cu, Zn, Fe, Mn, Mb, Cl), and it is only by means of fertilizers that it is possible to remedy this. The significant positive results that have been published are many in number, and are increasing continually. The advantages of the increased growth rates resulting from fertilization lie not only in the increase in the total volume of wood produced but also in reduced harvesting costs, since a given volume of wood can be harvested from a smaller area and fewer trees. Experience in Scandinavia indicates that increases of 30 to 50 percent in periodic annual increment are readily obtainable. The duration of the response of the stands varies from 5 to 40 years, depending principally upon the type and quantity of the fertilizers, the soil type, drainage, and climate, and hence the site index (Gentle and Humphreys, 1967). As regards the quality of the wood that is produced, there is no indication that it is inferior.
As an example, Richards and Bevege (1967) have shown that the benefit/cost ratio of a suitably fertilized plantation of Pinus taeda in Queensland planted at 2,250 stems per hectare (900 stems per acre) was 1.33, compared with 1.03 for an unfertilized plantation planted at 1,000 stems per hectare (400 stems per acre) after both had yielded their first pulpwood thinning at age 7.
The application of fertilizer toward the end of the rotation, provided its biological effectiveness can be demonstrated, may have greater economic advantages than application at the time of planting, since the interval between the making of the investment and the realization of the asset is so much shorter. Nevertheless, application at planting is frequently employed, particularly in tropical and subtropical countries, for it is very important in plantations of fast-growing species that all the trees planted should get off to a good start, and as uniformly as possible. The application of suitable fertilizers has on many occasions been found to encourage the rapidity and uniformity of initial growth, and may reduce the number of expensive weeding operations.
In many countries, however, there is still a lack of the basic information required to derive optimum benefit from the general remarks that have just been made. Information on the nutrient demands of individual species (Lubrano, 1967) is clearly of fundamental importance. On the one hand, without the silviculturist being aware of it, the trees may be in a state of " hidden hunger," a state intermediate between that of optimum nutrient status and that of the acute deficiency which is detectable from visual symptoms. On the other hand, fertilizers are not a panacea; they must be applied with discretion, and the possibility of other limiting factors for tree growth, for example clay or ironstone pans, inadequate drainage, etc., should not be neglected.
A specific instance of a problem associated with fertility is presented by the afforestation of spoil mounds, and wasteland resulting from opencast mining (Knabe, 1967). Here the neutralizing of excessive and toxic quantities of chemicals is often as big a problem as the supply of deficient nutrients.
Tree breeding is a highly specialized field. It was discussed exhaustively during the meeting organized by the Swedish Government on behalf of FAO and IUFRO (Stockholm 1963). The highly condensed report of this meeting and the proceedings which were published in extenso give a very complete account of it. Recent developments have been described by Kedharnath (1967)
There now exists ample evidence of the effectiveness of the application of genetics to tree improvement, in increasing growth rates and in improving wood properties and resistance to diseases and insects. Results from tree improvement research can be exploited most fully through the creation of man-made forests. In several developing countries, under the stimulus of the Stockholm meeting, large financial investments, both national and international, have been allocated to this field. It must be acknowledged that the investments required for research into and initial applications of forest genetics are long-term ones of which the precise level of profitability is still uncertain. This may have prompted commercial or private interests to give priority to other sectors of silviculture, particularly cultivation techniques, the results of which can be seen within a few years.
But it would be a mistake to present the problem in the form of an alternative. Numerous admirable examples can be quoted in which tree-breeding techniques have been employed in conjunction with other modern forestry techniques with excellent results. This is particularly true in the Scandinavian countries and in the southeastern United States, but considerable progress has also been achieved recently in this respect in Australia (Slee and Reilly, 1967) and in Africa (Václav, 1967; Cooling, 1967; Burley, 1967). With the rapid growth rates obtainable in the tropics and subtropics, significant results can, in fact, be obtained in those areas from tree improvement within 5 to 10 years.
It is a fact that up to now the work of forest geneticists has been concentrated on a limited number of species, and that there are grounds for extending it to species from other regions. Consideration should be given to the compilation of a list of the tree species which are most likely to make a rapid and substantial contribution to plantation forestry, particularly in the developing tropical and subtropical countries, and which are recommended for accelerated genetics research. In addition, breeding must be carried out with a view not only to the adaptability of the trees to the environment for which they are intended, but also to their adaptability to the silvicultural techniques which may be applied to them (for example, selection of clones which propagate readily, strains which have a robust root system suitable for mechanical planting).
In his review of the current position, Kedharnath (1967) has stated his opinion that conventional breeding methods are likely to be more fruitful for a given input than all other methods, for some time to come. Productive seed orchards established with clones of superior parents and mass production of seeds of inter-specific crosses are already playing an important role. Clonal are still generally preferred to seedling seed orchards, though suggestions have been made that separate seedling seed orchards be established simultaneously with progeny trials. Though breeding for visible characters such as vigor, stem form and branch habit is still of great importance, particularly in the early stages of a new breeding program, the improvement of the less readily observed characteristics of wood quality and disease resistance is attracting world-wide and increasing attention. Insurance is needed against possible loss of resistance due to mutational changes in the pathogenicity of the organisms. Induced polyploidy and mutations, through both radiation and chemical treatment, are beginning to have a significant impact but their full potential has still to be realized. Finally, the complex nature of the inheritance patterns of many important characters, and the increasing use of population genetics, imply a rapid expansion of the use of biometrics as a tool of the forest geneticist.
SHELTERBELTS AND WINDBREAKS
Though shelterbelts and windbreaks may confer incidental benefits on production and soil conservation, their primary function relates to a third aspect of forestry, the control of the environment. The review by Ostrom and Read (1967), which describes the subject on a world scale, reveals a great disparity between the situation in different countries, with a glaring lack of information from most developing countries, while there are divergent views on the value of shelterbelts among both foresters and agriculturists. It must be concluded from this that frank interprofessional exchange of experience and a great deal of research are still essential. In some areas there may be scope for a co-operative effort between governments, companies and private individuals, such as that being practiced in the semiarid region of Argentina (Yussem Favre, 1967). At the international level close liaison should be maintained with both the World Meteorological Organization (WMO) and the International Biological Program, both of which are interested in this subject.
There are numerous published data on:
1. Tree species used, and their improvement: notably poplars, willows, eucalypts, elms, Robinia, oaks, ash, pines, junipers, spruce and cypresses. Trees should normally have moderate to dense, but not very widespreading, crowns, stout straight boles, good retention of lower limbs and fairly uniform height growth. Fast-growing broadleaved species are most generally favored. Windbreaks often require more than one kind of tree or shrub having different growth characteristics, to provide foliage density at various height levels over a period of years (Ostrom and Read, 1967). Very little selection of ecotypes for the specialized conditions of shelterbelts and windbreaks has been done and there is need for the collection and testing of a much wider range of germ plasm.2. Design and methods for establishment and protection of windbreaks. Considerable differences exist in the number of rows used. Ten rows were common in the United States 30 years ago, but more recently there has been a tendency to favor much narrower breaks of 1 or 2 rows, both in Canada and in the United States; 5 to 8 rows are common in the U.S.S.R. An intermediate crown density (permeability of about 50 percent) is favored and intervals between windbreaks of 250 to 500 × 1,000 meters are common practice.
3. Effects on microclimate. The effect on wind velocity and thus on the velocity of driven rain, snow, sand, etc., is the most important. When wind direction is perpendicular to the windbreak, wind velocity is reduced by 20 percent or more over a zone extending from 3 to 5 times tree height to windward to 20 times tree height to leeward. The denser the windbreak, the greater the reduction in velocity close in, but the narrower the zone of protection to leeward. In the protected zone evaporation may be reduced by from 8 to 35 percent and the diurnal temperature range is normally reduced also.
The effects on crop and livestock yield are still the major gap in present knowledge (Ostrom and Read, 1967). Though there is cumulative evidence that yields are increased, there is a lack of quantitative comparisons between the increased agricultural yield, any yield of wood from the windbreak, the area lost to agriculture, which includes a strip on either side of the windbreak affected by shade and competition from tree roots, and the possible risk of loss from birds and insects. The effects on crop yield in irrigated land require special study. In many countries foresters feel that demonstration of the effect of windbreaks on crop yields under local conditions is essential, since few people will accept practices based on research in other countries.
Forest shelterbelts are not, however, concerned solely with the improvement of agricultural production; they contribute widely to the improvement of the entire way of life of populations. Man is becoming increasingly aware of this aspect as modern life assails him with the fumes, dust, noise and eyesores which are the most immediately obvious attributes of the manmade environment.
The symposium stressed the need for a worldwide effort of co-operation to be concentrated on this subject, which is embodied in one of the symposium recommendations.
ROW PLANTATIONS
There are few kinds of plantation which are so closely integrated with agriculture as this. This is true right from the stage of planting up to that of conversion, since in many cases the timber from row plantations is intended for numerous domestic and industrial purposes, including the packaging of the products from the neighboring field.
The role of these plantations is a multiple one, including production, protection, and recreation. Although in many cases the three functions are fulfilled simultaneously, it is important to have a clear idea of which is to predominate, since this will govern the choice of species or clones, the spacing, the cultivation techniques, the lifetime of the plantation, and so on.
Since the protective function of row plantations has been considered under the preceding section, the present section is concerned primarily with their productive role. A large number of species have been used in different countries, but poplars are unquestionably the most important. Populus nigra (italica and thevestina) is widely used in the Near East for windbreaks and also to produce small-sized wood for use in rural buildings, cottage industries and match manufacture, while P. deltoides and P. x euramericana are planted in areas of heavier rainfall in Europe, where logs of larger size are needed by industry (Castellani, 1967).
For comparable sites, poplars growing in properly managed row plantations have a similar rate of growth to those in a normal plantation. They :must be properly looked after (for example by pruning and protection against parasites). Studies in the Po valley have shown that, even in areas not greatly exposed to wind, where the windbreak effect of row plantations is negligible, the financial yield from the poplar wood more than balances not only the cost of planting and upkeep but also the reduction in yield from agricultural field crops caused by the competition of the poplars for light, water and nutrients (Castellani, 1967).
ENRICHMENT PLANTING IN TROPICAL FORESTS
Planting at very wide espacement or what has come to be called "enrichment planting" has been the subject of much research and application in tropical Africa and elsewhere for several decades.
At first many foresters considered it too dangerous to destroy the complex biological equilibrium of tropical high forest and to replace it by plantations at close espacement. It was also considered unlikely that thinnings from such plantations could be sold. The plot, strip planting, Okoumé, Limba, and other methods used have all been stages in a slow development. Highly cautious to begin with, owing to the lack of knowledge at the time and to certain glaring examples of site degradation resulting from ill-judged human intervention, practice is now moving closer to conventional plantation forestry and involves increasingly radical treatment. These methods have been described by Catinot (1967). On the other land, retention of a relatively high proportion of the overwood and planting at very low densities are often necessary for the Meliaceae in Latin America, as a protection against shootborer.
The most recent version of the Okoumé method in tropical western Africa consists in creating pure stands - of Okoumé, for example - that are intended to be exploited on a short rotation, of the order of 40 to 50 years, after the trees have reached diameters much smaller than those which trade and industry have been accustomed to handling from natural forest. In order to give the young trees full light as early as possible, the underwood is felled by bulldozer before planting, while the larger canopy trees are girdled. Woody regrowth is allowed to grow up as a nurse, in order to promote natural pruning and straight stems among the okoumé, The young trees are planted at an espacement of 4 × 5 meters, weeded regularly for 4 years and thinned twice to leave them at a final spacing of 12 × 12 meters at 12 to 15 years. The Limba method is similar, except that the trees are planted at final spacing of 12 × 12 meters, since Limba thinnings, unlike Okoumé, are unsalable; forest destruction is even more thorough and weeding needs to be continued for 6 years.
The various methods in use can all be modified to suit local conditions. Further studies are needed on the cost aspect and on methods of protection against game and parasites.
To summarize, whereas during its early days tropical silviculture restricted itself to sporadic enrichment of the natural forest, present-day methods are aimed at obtaining fully stocked plantations. In view of rapid growth rates and the early development of wide crowns, there is now a tendency to increase the spacing of the young trees to reduce or eliminate thinning. This parallels a similar trend already mentioned as occurring in Europe. Thus planting in the tropical forest promises to become no longer a "special form of afforestation" but simply one ecological variant of the man-made forests spread round the world.
There is a widening gap between the sensitivity of research being carried out in the fundamental sciences relevant to forestry and the lack of information on the applicability of more general silvicultural techniques to manmade forests on a field scale. It is a paradox that, while forest geneticists are studying the behavior of units smaller than the chromosome, there is still inadequate evidence for a rational decision to be made between planting and direct sowing in the pine forests of the southern United States (Stevenson, 1966).
Still more striking is the gap between the volume of research being done in the developed as compared with the developing countries. In the tropics and subtropics, despite some admirable exceptions at both national and international level, the resources of forest research organizations are still meager in comparison with the magnitude and variety of the problems they are tackling. The fact that the establishment of manmade forests is a comparatively recent innovation and is expanding so rapidly makes it imperative that forest research organizations should be alert, well staffed and properly financed. When preliminary research results are lacking, the afforestation worker is forced to take an unnecessary gamble. Moreover, the frequent use of monospecific and even-aged crops in man-made forests makes the silvicultural problems less complex than those of natural forests and results may be looked for correspondingly more quickly. The fast growth rates which are common in many plantation species in the tropics and subtropics is a further factor which ensures that research in such areas pays early dividends.
Plantation forestry needs considerable investments right from the start. This has tempted numerous afforestation workers to economize to an excessive extent on initial expenditure (site preparation, planting, tending) and the result has usually been disappointment and failure. Experience, both in Europe and in enrichment planting in tropical high forest in Africa (Catinot, 1967), illustrates a common tendency in the development of afforestation methods, irrespective of latitude: based initially on ideas of cheapness and simplicity and the use of narrow strips, small plots and minimum site preparation, they have evolved toward a progressive intensification of treatment. This has justified itself on both technical and financial grounds.
Wherever production is the primary aim, plantation silviculture thus has closer affinities with agriculture than with traditional natural forest silviculture, in which preservation of the environment plays a dominant role (cf. Van Miegroet, 1967b). On the contrary, the freedom of the afforestation worker, as of the agriculturist, to manipulate the environment to suit the crop is facilitated by every possible modern technique. The choice of varieties suitable for the site, the improvement of the site by means of mechanization, fertilizers and irrigation, tending methods, and the use of herbicides and insecticides are all essential technical aspects of this chapter, as they would be in any international meeting concerned with plant production.
Yet paradoxically, in limiting our forestry subject to manmade forests, we are enlarging our public to some extent. Whereas in the still recent past the tendency shown by foresters in their teaching, training schools and international meetings was to place emphasis on what it was that distinguished their techniques from those of agriculturists, those responsible for silviculture in the field of man-made forests have the major advantage of broadening their contacts, of deriving inspiration from methods that have often led agriculture to brilliant technical results, while in their turn making their own discoveries, their common ideal, and indeed their prestige radiata toward broader horizons.
In silviculture, even more than in agriculture, the evaluation of the biological and economic consequences of treatments is an arduous task. Whereas it is relatively easy to establish the cost of each of the silvicultural operations involved in a plantation program, it is, on the other hand, extremely difficult to correlate cause and effect. Effects may be delayed over several years and thus be confounded by interaction with other effects caused by the same kind of treatment repeated later (for example successive thinnings) or by different kinds of treatments (for example spacing, tending, thinning, pruning). Only experiments with fairly complex factorial designs will enable the researcher to determine quantitatively what proportion of subsequent growth effects is due to a particular treatment.
A clear appreciation is needed of the multiple functions possible for man-made forests. The majority of the studies that have been published on plantation silviculture is concerned almost exclusively with plantations of which the major function is production. Too little is known about plantations which are intended primarily for soil and water conservation, climate amelioration or recreation, and which necessarily involve different aims and principles and consequently impose different methods and techniques. For example in these cases unit costs, which relate to the volume that is produced in production forestry, must be applied to the area that is protected or tile water that is used or the numbers of visitors.
*ALDHOUS, J.R. 1967 A review of research and development in nursery techniques in Great Britain. Canberra.
*ALLAN, T.G. 1967 Industrial plantation establishment methods in Zambia. Canberra.
*AVANZO, E. 1967 Preliminary observations on a young comparative plantation of Populus x euramericana (Dode) Guinier on the outskirts of Rome. Canberra.
*BACHELARD, E.P. & BOUGHTON, V.H. 1967 The effect of weed-icicles on the growth of Pinus radiata seedlings. Canberra.
*BAKSHI, B.K. 1967 Mycorrhiza: its role in man-made forests. Canberra.
*BEN AISSA, J. 1967 Fixation et reboisement des dunes littorales en Tunisie. Canberra.
*BENNOUNA, A. 1967 La mécanisation dans l'implantation du rideau forestier de l'Oriental marocain. Canberra.
*BEVEGE, D.I. 1967 Thinning of slash pine in Queensland with special reference to basal area control. Canberra.
*BOROTA, J. & PROCTER, J. 1967 A review of softwood thinning practice and research in Tanzania. Canberra.
BRADLEY, R.T., CHRISTIE, J.M. & JOHNSTON, D.R. 1966 Forest management tables. London, Forestry Commission. Booklet No. 16
*BURLEY, J. 1967 Pinewood quality studies in Central Africa. I. Introduction, objectives and materials. Canberra.
*CASTELLANI, E. 1967 Plantations en alignement. Canberra.
*CATINOT, R. 1967 Formes spéciales de boisement: plantations en ligne, plantations d'enrichissement, rideaux coupe-vent et brise-vent. Canberra.
*COOLING, E.M. 1967 Improvement of seed of exotic forest trees for use in Zambia. Canberra.
*CZARNOWSKI, M.S., GENTLE, S.W. & HUMPHREYS, F.R. 1967 Site index as a function of soil and climatic characteristics. Canberra.
*DE PHILIPPIS, A. & GIORDANO, E. 1967 Practice and research in nursery techniques in the temperate zone. Canberra.
*ENDEAN, F. 1967 Research into plantation silviculture in Zambia. Canberra.
*FALLA RAMIREZ, A. 1967 Reforestación en zonas altas de América tropical. Canberra.
*FOOT, D.L. 1967 Nursery and establishment technique on the Vipya Plateau, Malawi, with special reference to the formation of a man-made pulpwood forest. Canberra.
*GENTLE, S.W. & HUMPHREYS, F.R. 1967 Experience with phosphatic fertilizers in man-made forests of Pinus radiata in New South Wales. Canberra.
*GIORDANO, E. 1967 Preliminary observations on the rooting capacity of cuttings of 20 provenances of Eucalyptus camaldulensis. Canberra.
*GOLFARI, L. 1967 El balance hídrico de Thornthwaite como guía para establecer analogías climáticas. Canberra.
*GROULEZ, J. 1967a Création des bambusaies à Bambusa vulgaris sur sols de savane au Congo-Brazzaville. Canberra.
*GROULEZ, J. 1967b Introduction d'eucalyptus au Congo-Brazzaville. Canberra.
*GROULEZ, J. 1967c Premiers résultats des essais d'acclimatation de résineux tropicaux au Congo-Brazzaville. Canberra.
*GROULEZ, J. 1967d Technique d'afforestation en exotiques à croissance rapide au Congo-Brazzaville Canberra.
*GROULEZ, J. 1967e Techniques de pépinière de pins tropicaux au Congo-Brazzaville. Canberra.
*GROULEZ, J. 1967f Les techniques de pépinière utilisée au Congo-Brazzaville pour l'éducation des plants d'eucalyptus. Canberra.
*GRUT, M. 1967 Determining the most profitable thinning grades and rotations for Pinus radiata D. Don. Canberra.
*IYAMABO, D. 1967 Practice and research in nursery technique in the tropical zone. Canberra.
*KEDHARNATH, S. 1967 Tree improvement: its impact on man-made forests. Canberra.
*KENYA. FOREST DEPARTMENT. 1967 Taungya in Kenya: the "Shamba system." Canberra.
*KNABE, W. 1967 Man-made forests on man-made ground. Canberra.
*LAMB, A.F. 1967 Choice of pines for lowland tropical sites. Canberra.
*LEWIS, N. 1967 Regeneration of man-made forests. Canberra.
*LINES, R. 1967 The planning and conduct of provenance experiments. Canberra.
*LUBRANO, L. 1967 Researches on the nutrient-demands of some species of Eucalyptus. Canberra.
*MIKOLA, P. 1967 Special techniques for poorly drained sites, including peat bogs, swamps, etc. Canberra.
*MORANDINI, R. 1967 Planning of species and provenance trials. Canberra.
*OSTROM, C.E. & READ, R.A. 1967 Shelterbelts and wind-breaks. Canberra.
*OTSUKA, T. 1967 Planting program on slopes. Canberra.
*PROCTER, J. 1967 A review of nursery practice and research in Tanzania. Canberra.
*PRYOR, L.D. 1967 Eucalyptus in plantations: present and future. Canberra.
*RALSTON, C.W. 1967 Recognition and mapping of site types for afforestation. Canberra.
*RAMBELLI, A. 1967 Pinus radiata and its relationship with root symbionts. Canberra.
*RICHARDS, B.N. & BEVEGE, D.I. 1967 Effect of cultivation and fertilizing on potential yield of pulpwood from loblolly pine. Canberra.
*SANDERS, M. 1967 Soil survey and site selection in Zambia. Canberra.
*SCHMIDT-VOGT, H. 1967 Influence of plant size on survival and growth of young forest plantations. Canberra.
*SIDDIQUI, K.M. 1967 Irrigated forest plantations in West Pakistan. Canberra.
*SIRÉN, G. 1967 Mechanizing forest regeneration in Sweden. Canberra.
*SLEE, M.U. &; REILLY, J.J. 1967 The production of improved tree seeds in Queensland. Canberra.
STEVENSON, D.D. 1966 Management of industrial pine plantations in the southern United States: problems and solutions. Sixth World Forestry Congress, Madrid.
*STONE, E.C. & GOOR, A.Y. 1967 Afforestation techniques for arid conditions. Canberra.
*STUART-SMITH, A.M. 1967 Practice and research in establishment techniques. Canberra.
*SWAN, H.S.D. 1967 The fertilization of man-made forests. Canberra.
*VÁCLAV, E. 1967 Tree breeding in Tanzania. Canberra.
*VAN MIEGROET, M. 1967a Natural and artificial regeneration. Canberra.
*VAN MIEGROET, M. 1967b La définition du but de la sylviculture. Canberra.
*WARDLE, P.A. 1967 Practice and research in spacing. thinning and pruning. Canberra.
*WOOD, P.J. 1967 Teak planting in Tanzania. Canberra.
*YUSSEM FAVRE, R. 1967 Cortinas forestales rompevientos en la región semi-árida de la República Argentina. Canberra.
NOTE: *Paper submitted to World Symposium on Man-made Forests and their Industrial Importance, Canberra, April 1967.
