Chapter 3 Why are forests important?
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Forest for wood and energy
Non-wood forest products
Forests as dwelling places
Environmental benefits of forests
Genetic resources and biodiversity
Forests and global climate
Forests provide a wide range of benefits at local, national and global levels. Some of these benefits depend on the forest being left untouched or subject to minimal interference. Others can only be realized by harvesting the forest for wood and other products. Yet other benefits from forests, despite being frequently claimed, are illusory.
Forest for wood and energy
Wood is one of the most versatile and ubiquitous products in human use. Wood and the products derived from it are found in every area of modern existence, from the timber used in construction, furniture and a myriad of industrial and domestic uses to fibre board, chipboard, paper, newsprint and cardboard.
As a construction material, wood is strong, light, durable, flexible and easily worked. It has excellent insulating properties. In contrast to the substitutes for wood in structural and architectural uses such as brick, concrete, metals and plastics, wood can be produced and transported with little energy consumed and the products are renewable and usually biodegradable (Koch, 1991).
Wood is also of major economic importance. The world demand for timber and wood products has been growing at 1 to 2 percent per year over the past decade. The total world production of industrial timber in 1990 was about 1 600 million cubic metres, of which about three-quarters came from the industrialized countries. Paper production was about 235 million tonnes and wood pulp production was 160 million tonnes; in both cases, over 80 percent of the production was in the industrialized world.
The present international trade in roundwood, sawnwood and wood products is worth about US$ 36000 million per year, of which about US$ 10000 million comes from developing country exports. The trade in paper and pulp accounts for a further US$ 60000 million, of which the share of the developing world is just US$ 2500 million. The overwhelming majority of the world's exports of wood and wood products comes from the industrialized rather than the developing countries, contrary to popular belief.
The 1990 estimate by FAO of world fuelwood and charcoal consumption was about 1 800 million cubic metres (FAO, 1992a). Of this, nearly 90 percent was in the developing world, where for many people wood meets virtually all energy needs. Since most of the fuelwood consumption and charcoal-making occur on a small-scale, informal basis in rural areas, where no statistics are kept, any such estimates must be treated with considerable caution. The figures nevertheless suggest that the world consumption of wood for energy is roughly similar to that of industrial wood.
Non-wood forest products
In traditional forest management, forests have primarily been seen as wood-producing units and other products have conventionally been referred to as "minor forest products"; their supply was not given high priority. It is only in recent years that proper attention has been paid to the importance of these products, especially in their local context where they may be considerably more valuable than the wood obtained from the forest. It is now becoming accepted that in many cases recognition of the economic and social importance of these other forest products may be the key to the active involvement of people in forest management.
Non-wood forest products include plants for food and medicinal purposes, fibres, dyes, animal fodder and other necessities. For instance, the Kayapó people of Gorotire village in southern Pará State, Brazil, utilize over 98 percent of the 120 species occurring within the local scrub savannah (campo cerrado). The Kayapó even prepare a planting medium from litter and termite and ant nests which they take to the woodland and in which they plant useful wild species (Anderson and Posey, 1989).
The wildlife in forests can also make a major contribution to food supplies. Surveys in Cameroon, Côte d'lvoire, Ghana and Liberia found that forest wildlife accounted for 70 to 90 percent of the total animal protein consumed. People surveyed in one area in Ghana considered the loss of such meat supplies to be the worst impact of local forest destruction (FAO, 1993g).
Forest products may also be commercially important at a local level, where they are traded in markets and shops or are sent to the larger towns and cities. In India, for example, the tendu leaf (Diospyros melanoxylon) is used for the locally made bidi cigarette. It is estimated that the collection and processing of the leaves provides part-time employment for up to half a million women.
Some of these forest products are valuable export commodities. They include gums and resins, bamboos, various oils, turpentine, tanning materials, honey, spices, bark and leaves and medicinal plants. Rattan, the long thin stem of a climbing palm (mainly Calamus sp.), has become an important export for Indonesia, Malaysia and the Philippines. Portugal, Morocco and other Mediterranean countries export large quantities of cork derived from the cork oak, Quercus suber. The Republic of Korea has built up an export trade in edible forest fungi, while gum arabic from Acacia senegal has long been an important export from Sudan.
Forests as dwelling places
Forests and woodlands provide a dwelling place for more than 200 million people in the tropics (Brown et al., 1991). They include those who have lived there for generations, often referred to as indigenous or tribal peoples; people who have recently moved into the area, often described as settlers, squatters or encroachers; and people who live part time in the forest working as small-scale loggers or harvesters of forest products. The numbers vary with time and among different areas, but all need to be taken into account when forest management is being considered.
Indigenous people, contrary to widespread assumption, are not only hunter-gatherers with minimal impact on the forests, but may be shifting agriculturalists as well. Their traditional slash-and-burn fallow systems have generally provided them with a stable existence, though at a low standard of living, while at the same time retaining the basic forest structure. But the decreasing area of forest available for farming and rising populations are resulting in shorter fallow periods, and the system is becoming less effective in maintaining the people.
Settlers are frequently "shifted" agriculturalists who have been unable to find the land required to grow enough food in the area in which they originate. When they move into a forest, because they lack the local knowledge and traditional skills of indigenous peoples, they often tend to be more destructive, quickly exhausting a patch of land through cultivation techniques that are inappropriate to the local conditions and then moving on to repeat the process elsewhere. Some may be interested mainly in clearing an area and selling the timber in order to raise enough capital to start a small business in the city. Others may be sponsored by wealthier people wishing to extend their landholdings by clearing and laying claim to areas of forest.
Because they lack heavy equipment, most of these settlers are unable to penetrate deeply into dense forests; they therefore tend to operate around the edges. But when roads are built into the forest for logging, mining or general transport they provide settlers with easy access. Much of the forest encroachment taking place in the humid tropics is along such routes.
Environmental benefits of forests
Forests and woodlands have an important role in protecting the environment at a local and even regional level. This is particularly true of steeply sloping watersheds where the tree roots are important in binding the soil and protecting it against erosion and landslide.
Uncontrolled clearing of forests from such upland areas, in addition to its local effects, can also have major repercussions further downstream. The eroded soil carried by streams and rivers is mainly deposited in reservoirs for irrigation and hydroelectricity and reduces the capacity and shortens the life of these costly investments.
Forests can also play a major part in areas that are covered with snow in winter. During the spring, forests help regulate the rate at which the snow melts and also reduce the danger of avalanches. In Colorado, USA, for example, the regulation of the snow melting in the spring is seen as the most important benefit from the forest cover on slopes.
At a local level trees can also bring many environmental benefits. They provide protection against wind erosion. They can help increase the rate at which rainwater infiltrates and recharges the groundwater. Used judiciously in farming systems they help maintain the fertility of the soil as the nutrients drawn up by their roots are recycled into the top layers of the soil by leaf fall. They provide shade for animals and humans; the microclimate under trees may be several degrees cooler and more humid than out of their shade. In coastal areas, mangroves protect the land against erosion by the sea as well as providing breeding grounds for fish and shrimps.
Forests also have an increasingly important role as havens for wildlife and for the protection of endangered species of plants and animals. Often allied to this is their role in recreation, tourism and what has come to be known as "ecotourism". Hiking, camping, nature study and simply getting out of the city are increasingly important to urban people in their often stressed and polluted world. Aesthetic appreciation of trees and forests as well as the cultural and spiritual values that they epitomize are very important to rural and urban dwellers alike.
The development of ecotourism may pay significant dividends. A wide public has been sensitized to nature and the environment by books and television documentaries. Many want to see for themselves and are willing to pay substantially to do so. Gameviewing safaris are already well established and cater to a wide market. More specialized holidays, offering hiking, camping, bird watching and opportunities to study nature in detail, are becoming steadily more popular. To tap this market by providing an increasingly knowledgeable public with what it wants, while at the same time avoiding creating social or environmental problems, requires a continued and expanding development effort. Measures have to be taken to ensure that the money generated by ecotourism benefits the country and the communities concerned and is invested in conservation of the resource itself.
Genetic resources and biodiversity
The term "genetic resources" refers to the economic, scientific or social value of the genetic variation found among and between species. If properly managed, these resources are renewable; they can be used, without ever being used up (Ledig, 1986).
Genetic variation has a number of fundamentally important functions; it constitutes a buffer against changes in the environment (including those brought about by pests, diseases and climate change) and it provides humans with the building blocks for selection and breeding to adapt plants and animals to a range of environments and end uses (Palmberg, 1987). Intensive selection and breeding for increased yields and uniformity has long been practiced for agricultural crop species. While such selection can improve certain traits in the short term, it may also reduce genetic variation when done over many plant generations, and the more genetically uniform populations have a decreased ability to respond to changing environmental pressures (including attack by pests and diseases) or shifting human needs. Therefore the use of narrowly based genetic materials grown for short term productive purposes must always be paralleled by conservation of the maximum amount of available variation through the establishment of reserves and managed resource areas and through the inclusion of genetic conservation concerns in improvement and breeding strategies.
An example of the adverse effects of a narrow genetic base has recently been seen in the decline of the widely planted neem tree (Azadirachta indica) in Sahelian countries, believed to be due to environmental stress (Ciesla, 1993). Narrowing of the genetic base occurred because the neem seed quickly loses viability, which restricted the original importations to the region from its native Asia. The effect of this may have been to reduce the tolerance of Sahelian populations of the species to moisture stress. Another example has recently been provided by the widespread planting of the giant varieties of ipil ipil (Leucaena leucocephala) in the tropics. The most widely used varieties were propagated from a small number of individuals. They have recently come under heavy attack by the Leucaena psyllid (Heteropsylla cubana), an insect which has spread rapidly from its native habitat in Mexico and Central America to the Pacific Islands, Asia and, most recently, eastern Africa.
Narrowing of the genetic base in tree breeding is carried to its extreme in clonal forestry, where there is no genetic variation among individuals in each monoclonal block. Poplars and willows have long been selected for commercial properties and desirable individuals have been propagated through cuttings, but the number of widely used clones has been relatively limited. Insects and diseases have quickly adapted, with the result that most of the early clones have been heavily attacked. Poplar breeders now give first priority in their breeding strategy to resistance to such attacks, and only secondarily consider attributes such as growth rate, form or wood properties. This strategy relies entirely on the ability to return repeatedly to the gene pool of the wild species in the search for genes conferring resistance.
The world's forests, especially those in the tropics, are both laboratories for the natural selection of genetic resources of plants and animals, on a scale which cannot be matched by today's or any conceivable future research stations, and dynamic storage banks for those genes. In an era of increasing pressure on resources and significantly changing environmental conditions, they provide one of humanity's most effective ways of buffering itself against a highly uncertain future.
Biodiversity (or biological diversity) refers to the variety of life forms, the ecological roles they perform and the genetic diversity they contain
(Wilcox, 1984). It is estimated that the tropical forests contain at least 50 percent and probably a considerably higher proportion of all the living species on the planet, including a great proportion of higher plants and mammals. There are, for example, 50 indigenous tree species in Europe north of the Alps. In Malaysia, in contrast, an area of forest covering just 50 ha was found to contain 830 tree species, and in Peru nearly 300 species of trees have been recorded on a single hectare (Whitmore, 1990).
No financial value can be placed directly on the almost infinite variety of living species in the forests. Only a tiny proportion are ever likely to be studied in detail, let alone be found useful to humanity. Yet the loss of each individual species makes the world a biologically poorer place. It must also be understood that such a loss affects the interlinkages and symbiotic relationships with other species.
It is possible to point to some very tangible gains that have been obtained from the resources other than wood available in the world's forests. Many common products, rubber for example, are of forest origin; so also are various fruits. Various widely used medicines were originally discovered by analysis of forest plants, often those traditionally used by forest dwellers.
There is no reason to suppose that this flow of beneficial products from the forests will be reduced in the future, provided that due attention is given to their conservation and sustainable management. The natural ecological systems of the forests will continue to evolve as they have always done. In the continuing human battle against diseases and pests the need for new products from the forests is likely to be as great in the future as it has been until now.
Forests and global climate
There is no doubt about the reality of the greenhouse effect. Certain gases in the atmosphere trap heat which would otherwise be radiated into space. Without this naturally occurring greenhouse effect, the average temperature of the earth would be about 30 C lower than it is.
There is also no doubt that the proportion of greenhouse gases in the earth's atmosphere is increasing as a result of human activities. The main increase to date has been in carbon dioxide. If the increase continues at the same rate as in recent decades, the global warming that is likely to result could ultimately have severe, if not catastrophic, consequences for hundreds of millions of people.
The main contribution to the increasing concentration of greenhouse gases in the atmosphere has been from the burning of fossil fuels. This is likely to remain the case over the next few decades. The Intergovernmental Panel on Climate Change (IPCC) projected that fossil fuel consumption would contribute 65 percent of the greenhouse effect from 1990 to 2025. The contribution from deforestation and biomass burning over the same period is projected to be 15 percent (IPCC, 1990).
Because trees, like all green plants, absorb carbon dioxide during photosynthesis, reforestation is often suggested as a means of countering the increase in greenhouse gases in the atmosphere. But trees only absorb carbon dioxide when they are growing. Carbon uptake is greatest in the early years when the rate of growth of the tree is at its maximum and tapers off as the tree reaches maturity. Eventually the trees in the forest simply act as a carbon store when no further net growth of the forest is taking place. When the trees die or are harvested, a portion of the carbon stored as woody tissue is once again released into the atmosphere; the portion of carbon released vanes with the product made from the tree. If the tree is burned as fuelwood then a high proportion of fixed carbon is released, but if it is made into a durable product with a long-term use (such as furniture) then the carbon will remain fixed for a long time.
The areas of forest plantation required to have a significant impact on the amount of carbon dioxide in the atmosphere are colossal. For example, one study (Sedjo and Solomon, 1989) indicates that the current net annual increase in atmospheric carbon (approximately 3 000 million tonnes) could be sequestered in approximately 465 million hectares of plantation forests for about 30 years, or as long as they remained alive. This corresponds to an increase of more than 10 percent in the current area of all forests on the earth's surface or an increase of more than four times the present plantation area in the world to sequester only the current net annual increase in atmospheric carbon. Even this enormous estimate is based on the assumption of an average annual growth of 15 m³ per hectare per year, which is unlikely to be achieved in temperate regions.
The effect of the planting of trees outside closed forests on the global climate and on carbon sequestration will depend upon its scale. Largescale tree planting projects in agricultural areas, such as the "four around" schemes in China reported to have been carried out on 6.5 million hectares of agricultural land in the decade 1981 to 1990, must sequester considerable quantities of carbon, as well as providing other environmental benefits.
Urban trees, in contrast, sequester minor amounts of carbon but can make other contributions to reducing the effects of climate change which are much more significant. A well-placed urban tree is about 15 times more valuable than a forest tree from the carbon cycle perspective. Urban trees break up "heat islands" by providing shade. This can lessen air-conditioning use which requires inputs of fossil fuels. Heating during winter months can also be reduced because trees can provide shielding from winds. It has been estimated that three strategically placed trees per house can reduce home air conditioning needs by 10 to 15 percent, especially in developed areas of subtropical and middle latitudes (FAO, 1990a).
There is great uncertainty about virtually all aspects of the global warming question. It will probably be at least ten years, and some scientists now say 30 to 40 years, before data are adequate for firm scientific conclusions on what exactly is happening and what its effects are likely to be at the regional level and below. In the meantime, the high stakes involved indicate a need for prudence on the part of humanity. Both slowing the rate of deforestation and planting trees can be important in reducing the risk of global warming, and both actions would simultaneously produce a range of other benefits for humanity.
If, at the same time, the necessary actions to curb the emission of greenhouse gases from all sources are not taken, and global warming occurs on a significant scale, there could be far-reaching consequences for the world's forests. On the one hand, an increase in atmospheric carbon and higher temperatures could result in increased growth rates. The natural ranges of many tree species could advance to higher latitudes or higher altitudes as temperatures increased. On the other hand, those forests that were stressed by climate change would become more susceptible to damage by fire, insects, pollution and disease. Genetic variation could be greatly reduced, leaving only the most resistant genotypes, and large areas of forest and even tree species could be lost.
There is no doubt about the immense importance of forests and the many benefits they can bring. There is, therefore, no need for untrue or exaggerated claims. Bad arguments can undermine good cases and can make advocates for forests look less credible in the eyes of knowledgeable people.
The belief that forests increase local rainfall, for example, is almost invariably false. Most rain is carried by air currents to the area on which it falls and is usually derived from ocean evaporation; droughts are caused by changes in the patterns of the rain-bearing winds. Denuding an area of trees will have little or no effect on whether rain-bearing winds blow across it or rain falls upon it.
The belief that forests provide additional oxygen supplies to the areas where they are growing sometimes suggested as one of the advantages of pert-urban plantations is totally false. During the process of photosynthesis, oxygen is indeed emitted. But relative to the volume of the atmosphere, the amounts are tiny and the increase in the oxygen content of the air in a forest is negligible. Moreover, during the hours of darkness, trees are net absorbers of oxygen and emitters of carbon dioxide.
The notion that forests regulate the flow of streams and rivers by acting as sponges, absorbing rainwater and releasing it gradually, sometimes believed to be a seasonal phenomenon, also false. In the case of light rainfall, a forest intercepts a high proportion of the water, preventing it from reaching the ground. With heavier rainfall, the forest does initially slow down the rate of water runoff and this will tend to increase infiltration, though this will be at least partly balanced by the water taken up by the roots and emitted through the leaves. In very heavy rain, the ground beneath the tree is quickly soaked with water. Once this has happened, any additional water simply flows away into the streams and watercourses. The seventy of floods that occur in the lower areas of river systems is primarily determined by the amount of rainfall and how it is distributed over the river basin. A distribution that results in all the tributaries or rivers flowing into the flood plain at the same time will produce the worst flooding.
The belief that trees are always benign elements in the farming landscape is also a myth. Trees can compete with crops for light and nutrients, causing significant reductions in crop yields. They can spread as weeds, ruining the land for grazing or crops. They can lower the water table, causing wells and water-holes to dry up. Poorly designed windbreaks can cause funnelling of winds, increasing plant damage and soil erosion. Equally, ill-chosen or badly positioned trees on hillsides can increase water erosion. They can harbour tsetse fly and highly destructive pests such as the Quelea bird.
A dramatic and tragic example of the damage that can be caused by poorly planned tree planting is the November 1988 flooding in Thailand. Large numbers of people were killed as whole villages were buried in mud and tree trunks swept down from the hills above them. The blame was placed upon careless logging, and so great was the public outrage that further logging was banned in the country's primary forests. It is now emerging that the real cause of the disaster was the expansion of rubber plantations on steep slopes. Cloned rubber saplings were planted on terraces constructed not for erosion control but for ease of rubber tapping. When the floods came, the terraces and the trees growing on them were swept down on to the visages below.
Care is always needed in the selection and planting of trees,
not only to match the species to the site and to the uses to
which people expect to put them, but also to anticipate their
effects, especially on agricultural land. The objections or lack
of enthusiasm shown by local people for many of the initiatives
so eagerly pressed upon them by tree planting enthusiasts are not
always as ill-founded as has tended to be assumed in the past.
|Mean carbon storage (t/ha)
Data for temperate zone species indicates that plantations of Acer saccharinum can absorb 5 tonnes C per year under optimal conditions and that Platanus occidentalis absorbs as much 7.5 tonnes C per hectare per year (FAO, 1990a). Carbon is also stored to varying degrees in forest soils.
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