The significance of forestry impacts on the environment is therefore determined largely by whether the forest is managed or developed within its limits as a renewable resource, or whether it is simply "mined" for the value of its standing timber without regard for the future. Forestry development projects generally involve the cutting of trees for their wood. If trees are only selectively cut and removed, then the forest may be able to recover, but if the trees are clear cut or the land is converted to other uses, then the values of the forest are lost.
Impacts of forest loss
Damage or destruction of the forest brings many kinds of environmental changes:
--- There is generally an important loss of habitat for native plants and animals. Many plants depend on the forest trees to provide the right environment for their growth; they will disappear when the trees are cut. Sometimes even the trees themselves are local species of considerable rarity and conservation interest. Many native birds, animals and insects are forced to retreat to ever smaller areas of undisturbed forest where they can still find food and shelter, but their numbers will diminish as their habitat shrinks. Some rare plants and animals may eventually be driven to extinction.
--- The trees shelter the land, release moisture into the atmosphere, and otherwise moderate and improve the climate. Cutting the trees can bring local changes in climate, such as reduced rainfall and greater extremes of high and low temperatures.
--- The loss of the forest reduces the capacity of the land to catch and hold water. This increases both the danger of flooding downstream and the effects of drought, when streams and springs may dry up completely. In many parts of the world, the destruction of the forest has increased the number and the size of natural disasters affecting the nearby human populations.
--- Since much of the fertility in tropical forests is in the vegetation rather than in the soil, cutting the forest will lead to a serious loss of soil fertility after only a brief improvement. The exposed soil is also subject to erosion which can result in the rapid loss of the top-soil, leaving the land permanently degraded.
--- The commercial harvesting of trees, usually for export, means the loss of many important forest resources for local use, such as fruits, nuts and other food from forest plants, medicinal plants, wood for construction and carving, fuel wood, vines, fibres and other useful materials. These secondary resources may be very important to local people, but their loss is seldom calculated among project costs.
The extent of these and other losses obviously depends on the kind of forest exploitation and whether it is reversible. Light selective logging may leave many forest values nearly intact, but it can lead to a change in forest composition unless efforts are made to ensure the regrowth of the species that are cut. Heavy selective logging which leaves only "trash" trees will produce a less rich and productive forest, but one that still provides some soil protection and water management once the immediate damage from logging is overgrown.
Clear cutting may be followed by reforestation, but the trees planted are often fast-growing exotic species intended to be cut again in a few decades. Such a plantation forest is much less valuable as wildlife habitat, as a source of local materials, or for maintaining soil fertility, and it will not hold water as efficiently as the native forest. Clear cutting with no attempt at restoration generally leads to a weed-covered wasteland which may slowly regenerate into a poor secondary forest of little value.
Frequently, once the forest is cut, farmers move in and plant subsistence gardens until the soil is exhausted. This usually prevents even the regrowth of a secondary forest, and the land may remain as grassland or scrub which is occasionally cleared again for relatively unproductive gardens. The forest values of such land are permanently lost and its contribution to island productivity becomes marginal.
Impacts of logging and clearing
Projects for cutting and removing forest trees or the clearing of forest land can have direct impacts on the environment and other resources. Since this work usually requires heavy machinery like bulldozers, there is a risk of direct damage to the soil, particularly where it is wet as it often is in the tropics. Heavy tracked machines can cause severe soil compaction, with the soil becoming so hard that plants cannot easily take root. One logged island area had 15% of the logged area so seriously compacted that it was worthless for future forest production. Vehicle tracks are also very susceptible to erosion and gullying.
The disturbance of the land caused by logging and clearing, combined with the movement of vehicles and people in and out of the area, results in the invasion of the disturbed areas by weeds and introduced species. These aggressive fast-growing plants out-compete and smother native island plants, prevent or slow forest regeneration, make reforestation difficult and expensive, and may spread into the surrounding forest upsetting the natural balance of species.
Forestry projects require the construction of access roads into the areas being exploited. These roads make it possible for people to enter areas previously protected by their inaccessibility, and can lead to uncontrolled development, the establishment of squatters, and the planting of crops in areas where such use is inappropriate and can only lead to degradation of the land. Such unplanned settlement may be socially and politically difficult or impossible to control. It may even block the implementation of development projects, and create difficult problems of legal rights and land titles.
Forestry projects are attractive because the trees can be converted into a quick source of cash income. However, the destruction of the forest frequently means the loss of other development opportunities which might provide a better return in the long term. Logged areas lose any tourism potential they may have had. The damage to water supplies may hurt agricultural or urban development. Coastal fisheries development and aquaculture may be blocked by the increased sedimentation resulting from soil erosion and flooding. The cost to the country of the loss of these development possibilities needs to be balanced against the benefits of the immediate income.
Reforestation, or the replanting of trees to make forest areas, is a much more constructive type of development project. Tree planting projects can have several possible aims.
Forest plantations may be established to produce wood as a crop. The economists' desire for a rapid return on investment often leads to the planting of fast-growing exotic species like Caribbean pine or eucalyptus. Such trees may provide low grade lumber like fence posts, or be converted to wood chips to be exported for the manufacture of paper, but their suitability has not been demonstrated for all parts of the region. More valuable timber trees may also be planted. Since usually only one kind of tree is planted in an area, such plantation forests lack diversity and provide few of the values of native forests. These forests are also intended to be cut again when they reach maturity. It is too early to know whether such regular tree harvests can be continued in island environments without using up soil nutrients.
Where a natural forest is cleared to make way for a plantation forest, the result is generally a degradation in forest values. However, such plantations can also be created on grasslands or other areas where the soil has already been degraded, and can thus reconvert wastelands to productive use. The reforestation of damaged areas can produce many environmental improvements as tree cover is re-established on the land.
There have been proposals for energy tree plantations of fast-growing species that can be harvested to provide firewood, or to supply fuel for wood-burning electric power plants. These would have some of the same advantages and disadvantages as other plantation forests.
Reforestation can also play a part in the establishment or maintenance of village forest reserves or wood lots. These areas of forest near a village are designed to provide a handy supply of fuel wood, building materials and other forest products to the village. The tree planting can be done by the villagers themselves using the species most useful to them.
Planting trees may also be done more for the benefits of environmental protection or restoration than for the production of wood. Reforestation may aim to restore watershed capacity, control erosion and flooding, improve wildlife habitat, regenerate depleted soils, cover mine wastes or protect against wind damage. Even in village and town areas trees can contribute environmental benefits, such as shading streets and gardens, reducing heat accumulation, and making built-up areas more attractive for residents and tourists. On small islands where land is scarce, there is no reason why the land along roads and in urban areas should not make a contribution to the production of wood and other tree products as well as improving the urban environment.