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9. Maintenance of the plantation
Once a plantation has been established, the work should not be considered finished. It will be necessary, for example, to protect the plantation against weather, fire, insects and fungi, and animals. A variety of cultural treatments also may be required to meet the purpose of the plantation.
9.1 Protection
Weather Phenomena: the occurrence of damaging weather phenomena is usually unpredictable. Little can be done to protect forest plantations against the damage caused by weather, except to grow tree and shrub species known to be resistant to the detrimental effects of local weather patterns, or locating the stands of trees or shrubs in sheltered areas. Some tree and shrub species are more windfirm than others, or are less prone to crowns and branches breaking off in high winds. Other species are more tolerant to salt spray and, therefore, can be used for planting in belts along exposed seaward flanks to give protection to other less tolerant species forming the main plantation. Thin-barked species are more susceptible to damage and to subsequent attacks by insects or fungi than are other species.
Fire: damage by fire imposes a serious threat to plantations. The fire risk is generally high in the dryer climatic regions; but, even in relatively moist or high rainfall areas, there may be warm and dry spells when the fire risk is high. Fire risk should be a major consideration from the early stages of plantation development.
Fires can originate from natural causes, such as lightning, but many occur as a result of the activities of man. Plantation fires can start from fires spreading from farmland on the perimeter, from the activities of hunters, or from burning by herdsman to improve livestock grazing. There have been instances of deliberate burning to create employment (in the fire suppression and subsequent replanting) or to show disapproval of forest policies. It is not possible to prevent a climatic build-up of fire hazard conditions, but much can be done to minimize the risk of fire through public education and involving local people in forestry.
A main principle in protecting forest plantations against fire is that, where there is insufficient combustible material to allow a ground fire to develop, there is little or no fire risk. Dangerous and damaging plantation fires can only develop when fire is able to occur at ground level.
In many parts of the world, annual or periodic burning of vegetation is commonly practiced to improve grazing conditions, to reduce the build-up of fuels, or to improve soil fertility through accumulation of ash.
Insects and fungi: most insects and fungi are selective of the host species. In their natural environment, trees and shrubs normally attain a state of equilibrium with indigenous pests. However, when exotic trees and shrubs are planted, exotic pests can also be introduced. Quite often, these exotic pests readily adapt themselves to the conditions of their new habitat. In general, the risk of damage from pests is higher when the plants are physiologically weakened from planting on unsuitable sites, improper site preparation, inefficient planting, adverse climatic conditions, or neglect of weeding and other maintenance operations. But even healthy trees and shrubs are attacked at times. For many insects and fungi, no control measures are available; when this is the case, the best precaution is to plant tree and shrub species or varieties known to be resistant to the pests.
The main precautions to be taken in guarding against possible future damage from insects and fungi are to plant tree or shrub species that are suitable to the climatic and soil conditions of the site, and to make surveys of indigenous pests to ensure that none are among the known forms to which the selected species is susceptible; but this is seldom easy, especially in view of the gaps in available knowledge on site requirements and susceptibility of exotic species to insects and fungi. To obtain this needed information, carefully controlled experiments should be initiated before developing large-scale planting programmes.
Care taken in establishment and maintenance operations during the early years of a plantation (resulting in healthy vigorous young trees or shrubs) can help to make a plantation more resistant to insects and fungi. However, when evidence of pest attack appears, it should be investigated promptly and the cause identified. Various control measures are available; these may be silvicultural, chemical, biological, or mechanical.
Silvicultural measures include well timed, careful thinnings after establishment of the forest plantation. Through thinning, poor and suppressed stems are eliminated, maintaining the plantation in a thrifty and vigorous growing condition. In young plantations, prompt removal and destruction of infested trees and shrubs can be effective in preventing the spread of the pest attacks to the rest of the plantation. Where a threat of infection is known to exist, planting of tree or shrub mixed species also can be considered a silvicultural control measure.
One disadvantage of mixed plantings is that subsequent forest management can be complicated; however, this may be avoided, at least partially, by planting alternate blocks or wide belts with different tree or shrub species, forming barriers to the spread of a pest or disease from the initial point of infection.
Insects and fungi can often be checked by applications of appropriate chemical insecticides or fungicides. Usually, these chemicals are available as liquids (or wettable powder), dusts, or smokes. Spraying with hand-operated spray guns or portable mist-blowers is frequently used to control attacks in young plantations; with canopy closure, aerial spraying and dusting or smoking can be more effective and cheaper. Only previously tested and environmentally sound insecticides and fungicides should be prescribed for use.
Biological control of insects has been employed with success in some situations; in most instances, the introduction of a parasite to control the insects is required. The greatest success in biological control is usually achieved after the problem has grown to epidemic proportions.
Mechanical control, either by physically removing and destroying the pests or by eliminating the alternative hosts, can be effective.
Wild animals: damage to forest plantations by wild animals mainly takes the form of tree browsing or de-barking. In general, there are three orders of wild animals responsible for damage: rodents (rats, mice, and moles and squirrels); lagomophs (hares and rabbits); and artiodactyls (deer, antelopes, pigs and buffaloes). The principal methods of controlling damage by wild animals involves the use of fences, hedges or ditches, trapping and removal, and poison baits.
Domestic animals: in some countries, grazing or browsing by sheep, goats and cattle can be a menace to young plantations. At times, hedges and fences are used to prevent intrusion by domestic animals. Where fencing costs are high, trespass by livestock can be controlled by guards.
In many dry areas, grazing by goats is a traditional land use. Extensive enclosures of forest plantations can impose drastic changes in the habits and economies of the rural communities affected. In such situations, it would be unwise to initiate planting programmes unless alternative means of livelihood can be provided beforehand; generally, this requires the integration of community development schemes (for example, improved agriculture or animal husbandry, better communications, schools, or medical welfare) and increased opportunities for employment by the development of rural industries (such as afforestation programmes and rural forest industries).
9.2 Cultural treatments
Cultural operations are required to promote the conditions that are favorable to the survival and subsequent growth and yield of the trees or shrubs in the plantation. In most forest plantations, cultural operations are concerned with preventing the trees and shrubs from being suppressed by competing vegetation; quite often, this treatment is called weeding. Other cultural treatments are thinning to achieve a desired spacing among the trees or shrubs, and the periodic watering of the plants.
Weeding: weeding is a cultural operation that eliminates or suppresses undesirable vegetation which, if no action were taken, would impair the growth of the plantation crop. This undesirable vegetation competes with trees and shrubs for light, water, and nutrients; weeding increases the availability of all or the most critical of these elements to the trees and shrubs. A primary objective of weeding is to promote growth and development of the plantation crop, while keeping the costs of the operation within acceptable limits.
A main factor affecting the intensity and duration of weeding treatments is the relationship between the tree or shrub crop and the weeds. On some sites, the plantation crop eventually grows through the weeds, dominates the site, and becomes established; on such sites, the function of weeding is to increase crop uniformity and speed up the process of establishment and growth. On other sites, the type or density of the weed growth is such that, in the early stage of a forest plantation, it may suppress and kill some or all of the planted trees or shrubs; in such areas, the main purpose of weeding is to reduce mortality and maintain an adequate stocking of trees or shrubs.
The methods of weeding involve either suppression or elimination of the competing vegetation. Suppression of weeds consists of physically beating down or crushing them, or cutting the weeds back at or above ground level. Weed elimination can be achieved by killing the weeds, destroying the whole plant either by cultivation or by the use of chemicals. Weeding may be total or partial.
Thinning: thinning of forest plantations, particularly those established for wood production, may be required to obtain the desired spacing between the trees. In general, this spacing is a compromise between a "wide" spacing to reduce planting costs and inter-tree competition in times of drought, and a "close" spacing to attain early canopy closure, the suppression of weeds, the reduction of weeding costs, and natural pruning of branches through shading.
In "first-rotation" forest plantations, the thinning objective is frequently to adjust the initial spacing among plants, so that the size and type of tree or shrub required is attained on a short rotation, without secondary thinning treatments. Where a tree or shrub of larger size and higher quality is required, closer than final spacing is often prescribed in an initial thinning; usually, some form of secondary thinning is necessary as a subsequent treatment. The element of selection in thinning should ensure that the increment growth of the final crop is concentrated on the best stems.
Regardless of the purpose of the thinning operation, it should follow closely the timing and spacing requirements that are outlined in a prescribed thinning schedule for the area.
Watering: often, forest plantations in arid regions need at least periodic watering during the first growing season to obtain a satisfactory survival rate. Watering should begin after the cessation of rains, when the moisture content of the soil has fallen to near the wilting coefficient; then watering should be repeated at intervals until the onset of the next rainy season. Before each watering, the area around the tree should be cleared of weeds, and a shallow basin should be made around the stem of each tree or shrub to collect as much water as possible.
Watering can be an expensive operation, especially on terrain too steep or too rough for the passage of tank vehicles. Pack animals may be required to carry drums of water to the plantation site. Watering can be uneconomic for large forest plantations, particularly when the source of water is a long distance from the plantation, but it may be justified in the case of small plantations or for establishing roadside avenues.
In some instances, regular cultivation and weeding, especially during the first growing season, are sufficient measures to conserve soil moisture for satisfactory survival of the plants, eliminating the need for watering.
For forest plantations that are established for purposes of wood production, trees and shrubs are harvested once they attain the "optimum size" for the wood product wanted. From a biological standpoint, trees and shrubs should not be cut until they have at least grown to the minimum size required for production utilization. Beyond attaining the minimum size, the question of when to harvest must still be answered, however.
Quite often, the average annual growth rates of a forest plantation can be used as a guide in determining when to harvest wood. In general, the average annual growth of trees and shrubs increases slowly during the initial years of plantation establishment, reaches a maximum, and then falls more gradually, as illustrated in Figure 4.11. Trees and shrubs usually should not be allowed to grow beyond the point of maximum average annual growth, which is the age of maximum productivity; foresters call this the "rotation" age of the forest plantation.
To determine the average annual growth rate of a forest plantation at a point-in-time, the volume and age of the trees or shrubs must be estimated; then the average annual growth (at the specified point-in-time) is determined by dividing the standing volume by the corresponding age. Again, careful measurements of volumes and known ages are necessary for this determination.
Figure 4.11 Relationship between tree age and tree growth indicating the rotation age.
Economic considerations also help to determine when to harvest trees and shrubs for wood products. When based solely on market factors, the time to harvest is when the profit is maximized. Profit is maximized when the returns generated from harvesting and selling the wood minus the costs of harvesting and (when required) processing the wood into the desired products is the greatest.
The methods of felling trees and shrubs, cutting the stems and branchwood into the desired lengths, and removing the wood from the plantation site should be chosen to minimize degradation of the site. Axes, saws, wedges and sledges may be all that are necessary to fell the trees and shrubs and cut them into the desired lengths. Power-chainsaws are used in many instances; while their use makes harvesting easier, their high cost of operation can make then uneconomical.
Once the trees and shrubs are felled and cut into desired lengths, they must be carried or pulled to loading points for transport to processing sites or directly to a market place. When stem lengths are too heavy to carry, a simple drag or sled can be employed to move them, using an available power source, such as a domestic animal or a tractor. When residual trees or shrubs are left in the forest plantation, the harvesting operation should be carried out to prevent damage to this standing resource.
It is important that the methods of harvesting should be selected to "match" the skills of the people who will harvest the trees or shrubs. Once again, advanced planning will be necessary to ensure that the labour and required equipment will be available for use at the needed time.
In selecting appropriate tree and shrub species for plantations in arid zones, site condition information (on climate, soils, topography, biotic factors, vegetation, and water table levels), as well as knowledge of socio-economic factors, must be known. Once the site and species have been selected for planting, fencing which marks boundaries and protects the trees and shrubs should be erected, if required. For a successful tree or shrub crop, site preparation is often necessary; site preparation may include removal of competing vegetation, enhancing water catchment, reducing water runoff, providing good soil conditions, eliminating fire hazards, and preparing the soil.
In general, planting coincides with the rainy season. After planting, use of opaque plastic films can impede evaporation and inhibit weed growth around potted stock. The spacings of the plantings should be wide enough to prevent competition for soil moisture, or when part of management, to accommodate machinery used for irrigation.
Maintenance of forest plantations includes protecting the plants from detrimental climatic conditions, fire, insects and fungi, and animals. Maintenance may include measures that are silvicultural (such as well-timed and careful removal of damaged trees and shrubs), chemical (with insecticides or fungicides), biological (with parasites), or mechanical (removing or destroying pests, erecting fences, etc.). Because trespass by man can threaten the success of a planting programme, planning should also include methods of dealing with this potential problem.
Cultural treatments to promote favorable growth conditions include weeding, thinning, and watering. Harvesting operations, when required, should match the skills of the people who will perform the tasks.