The clearing of the Earthâ€™s tropical forests has been occurring on a large scale for many centuries. This process, known as deforestation, involves the cutting down, burning, and damaging of forests. The loss of tropical rain forest is more profound than merely destruction of beautiful areas. If the current rate of deforestation continues, the world's rain forests will vanish within 100 years - causing unknown effects on global climate and eliminating the majority of plant and animal species on the planet.
Why Deforestation Happens
Deforestation occurs in many ways. Most of the clearing is done for agricultural purposes - grazing cattle, planting crops. Poor farmers chop down a small area (typically a few hectares) and burn the tree trunks - a process called â€œSlash and Burnâ€ agriculture. Intensive, or modern, agriculture occurs on a much larger scale, sometimes deforesting several square kilometres at a time. Large cattle pastures often replace rain forest to grow beef for the world market.
Commercial logging is another common form of deforestation, the trees being sold as timber or pulp. Logging can occur selectively - where only the economically valuable species are cut - or by clear-felling, where all the trees are cut. Commercial logging uses heavy machinery, such as bulldozers, road graders, and log skidders, to remove cut trees and build roads, which is just as damaging to a forest overall as the chainsaws are to the individual trees.
The causes of deforestation are very complex. A competitive global economy drives the need for money in poorer tropical countries. At the national level, governments sell logging concessions to raise money for projects, to pay international debt, or to develop industry. For example, Brazil had an international debt of $159 billion in 1995, on which it must make payments each year. The logging companies seek to harvest the forest and make profit from the sales of pulp and valuable hardwoods such as mahogany.
Figure 1 (right). Deforestation in the Brazilian Amazon in 1986. The darker the area, the more forest that remains
Deforestation by peasant farmers is often to raise crops for subsistence agriculture and is driven by the need for food. Most tropical countries are very poor, and farming is a basic way of life for a large part of the population. In Brazil, for example, the average per capita earnings are $5400 p.a., compared to $26,980 in the U.S.A. (World Bank, 1998). In Bolivia, which holds part of the Amazon rain forest, this figure falls to just $800 p.a. Farmers in these countries do not have the money to buy necessities and must raise crops for food and to sell. There are other reasons for deforestation, such as to construct towns or dams which flood large areas, yet these latter cases constitute only a very small part of the total deforestation.
The Rate of Deforestation
The actual rate of deforestation is difficult to determine. Scientists study the deforestation of tropical forests by analyzing satellite imagery of forested areas that have been cleared. Figure 2 (left) illustrates how scientists classify the landscape. Within the image are patches of deforestation in a distinctive "fishbone" along roads. This leaves fragments of isolated forest cut off from the larger forest area. Regrowth - also called secondary forest - is abandoned farmland or timber cuts that are regenerating. The majority of the picture is undisturbed, or "primary," forest, with a network of rivers draining it.
The Food and Agriculture Organization (FAO) estimated that 53,000 square miles of tropical forests were destroyed each year during the 1980s. Of this, they estimate that each year 21,000 square miles were in South America, mainly in the Amazon Basin. Based on these estimates, an area of tropical forest equivalent to Wales is deforested each year!
The rate of deforestation varies from region to region. Recent research suggests that in the Brazilian Amazon, the rate of deforestation was around 6200 square miles/yr from 1978-1986, but fell to 4800 square miles/yr from 1986 -1993. By 1988, 6% of the Brazilian Amazon had been cut down (90,000 square miles, an area the size of the UK). However, due to the isolation of fragments and the increase in forest/clearing boundaries, a total of 16.5% of the forest (230,000 square miles, an area nearly 50% bigger than France) was affected by deforestation. Scientists are analyzing the current rate of deforestation, as well as studying how deforestation changes from year to year.
The much smaller region of Southeast Asia (Cambodia, Indonesia, Laos, Malaysia, Thailand, and Vietnam) lost nearly as much forest per year as the Brazilian Amazon from the mid-1970s to the mid-1980s, with 4800 square miles per year converted to agriculture or cut for timber.
Deforestation and the Global Carbon Cycle
Deforestation increases the amount of carbon dioxide (CO2) and other trace gases in the atmosphere. The plants and soil of tropical forests hold 460-575 billion tonnes of carbon with each hectare of tropical forest storing about 550 tonnes of carbon. When a forest is cut and burned, the carbon that was stored in the wood is released into the atmosphere as CO2.
The loss of forests has a profound effect on the global carbon cycle. From 1850 to 1990, deforestation worldwide released 122 billion tonnes of carbon into the air, with the current rate being approximately 1.6 billion tonnes yr-1. In comparison, fossil fuel burning (coal, oil, and gas) releases about 6 billion tonnes yr-1, so it is clear that deforestation makes a significant contribution to the increasing CO2 in the atmosphere, thus enhancing the greenhouse effect, and contributing to an increase in global temperatures.
Deforestation and the Hydrologic Cycle
Tropical deforestation also affects the local climate by reducing the evaporative cooling that takes place, since, as trees and plants are cleared away, the forest canopy quickly diminishes. Recent research suggests that up to 90% of the rainfall in a tropical rain forest is a result of its moist, green canopy. Water lost from the leaves returns water to the local atmosphere, promoting the formation of clouds and precipitation. Less evaporation means that more of the Sun's energy is able to warm the surface and the air above, leading to a rise in temperatures.
Deforestation and Biodiversity
There are estimated to be between 5 and 80 million species comprising the biodiversity of planet Earth. Tropical rain forests - covering only 7% of the total dry surface of the Earth - hold over half of all these species - only about 1.5 million of which have even been named and far fewer have been studied in depth.
Many of the rain forest organisms can only be found in small areas because they require a special habitat in which to live, which in turn, makes them very vulnerable to deforestation. If their habitat is destroyed, they may become extinct, so every day species are disappearing from the tropical rain forests as they are cleared. We do not know the rate of extinction, but estimates indicate that up to 137 species disappear worldwide each day, the loss of which will have a great impact on the planet. We are losing species that might show us how to prevent cancer or help us find a cure for AIDS. Other organisms are losing species they depend upon, and thus face extinction themselves.
What happens after a forest is felled is very important to any subsequent regeneration. Different felling techniques and uses of the land affect the soil and surviving organisms that make up a rain forest. In a tropical rain forest, nearly all of the life-sustaining nutrients are found in the plants and trees, not in the ground. When the plants and trees are cut down to sow the land, farmers usually burn the tree trunks to release these nutrients creating a fertile soil. When the rains come, they wash away most of these nutrients, leaving the soil much less fertile. In as little as 3 years, the ground is no longer capable of supporting crops.
When the fertility of the ground decreases, farmers seek other areas to clear and plant, abandoning the nutrient-deficient soil and this is left to grow back to a rain forest. However, just as the crops did not grow well because of low nutrients, the forest will grow back equally slowly because of poor nutrients. After the land is abandoned, the forest may take up to 50 years to grow back.
Another type of farming practiced in rain forests is called shade agriculture. In this type of farming, many of the original rain forest trees are left to provide shade for shade-loving crops like coffee or chocolate. When the farm is abandoned, the forest grows back very quickly, because much of it was left unharmed in the first place. After this type of farming, forests can regenerate in as little as 20 years.
Other types of farming can be more devastating for forest re-growth. Intensive agricultural systems use large quantities of pesticides and fertilizers. These chemicals kill many of the organisms in the area, and leach into the soil and surrounding areas. On banana plantations, pesticides are used on the plants and in the soil to kill pest animals. However, these pesticides also kill other animals as well, and weaken ecosystem health. Banana plantations also use irrigation ditches and underground pipes for water transport, changing the water balance of the land. After the abandonment of a banana plantation, or other intensive agricultural system, it can take many centuries for a forest to re-grow.
Figures 3a and 3b.
Deforestation in continental Southeast Asia (excluding Malaysia and Indonesia) from 1973 to 1985. The dark grey represents forest, the lighter areas deforestation. The white box-like areas on the 1985 map are places for which no satellite information was available. During this period, about 50,000 square miles were deforested.
A study in Indonesia found that when only 3% of the trees were cut, a logging operation damaged 49% of the trees in the forest. Yet, even with that much damage, the rain forest will grow back relatively quickly if left alone after selective logging, because there are still many trees to provide seeds and protect young trees from too much sun.
Clear felling is much more damaging to a tropical rain forest. When the land is commercially felled and all the trees removed, the bare ground is left behind with very little re-growth. There are almost no nutrients left behind because all the tree trunks were removed (cf â€˜slash-and-burnâ€™). A clear-felled forest can require many years to regenerate - in fact, scientists do not yet know how long it takes for these forest to grow back.
The deforestation of tropical rain forests is a threat to life worldwide. Deforestation may have profound effects on global climate and cause the extinction of thousands of species annually. Stopping deforestation in the tropics has become an international movement.
Because the loss of rain forests is driven by many complex factors, the solutions are equally complex. Simple solutions that do not address the nature of world economics and rain forest ecology have little chance of succeeding. The future requires solutions based on solving the economic crises of countries holding rain forests, and improving the living conditions of the poor people often responsible for deforestation.
The deforestation of the temperate regions of Europe took place in the Middle Ages and took 500 years; tropical deforestation only began about 50 years ago, but is proceeding at a much greater rate. The causes in both cases are essentially the same, but whilst we can do little about the past, we can do something about the present (and future). It is through our purchases that much of the demand for tropical hardwoods comes and it is our politicians that set the framework for international conventions. There is a popular scientific belief that the Worldâ€™s climate is changing and that Man is, at least in part, responsible for global warming. Tropical deforestation will certainly affect our climate in ways that we can only guess at, but they are unlikely to be benign. Habitat destruction removes species that might be essential for our own survival in the future and reducing biodiversity can never be good. The causes of tropical deforestation may be known but the consequences are not. Therefore we should stop. Now.
Hope that helps