Forests and Deforestation
A forest is any ecological community that is structurally dominated by tree-sized woody plants. Forests occur anywhere that the climate is suitable in terms of length of the growing season, air and soil temperature, and sufficiency of soil moisture. Forests can be classified into broad types on the basis of their geographic range and dominant types of trees. The most extensive of these types are boreal coniferous, temperate angiosperm, and tropical angiosperm forests. However, there are regional and local variants of all of these kinds of forests. Old-growth tropical rainforests support an enormous diversity of species under relatively benign climatic conditions, and this ecosystem is considered to represent the acme of Earth's ecological development. Within the constraints of their regional climate, temperate and boreal forests also represent peaks of ecological development.
Many countries have developed national schemes for an ecological classification of their forests. Typically, these schemes are based on biophysical information and reflect the natural, large-scale patterns of species composition, soil type, topography, and climate. However, these classifications may vary greatly among countries, even for similar forest types.
An international system of ecosystem classification has been proposed by a scientific working group under the auspices of the United Nations Educational, Scientific and Cultural Organization (UNESCO). This scheme lists 24 forest types, divided into two broad classes: closed-canopy forests with a canopy at least 16.5 ft (5 m) high and with interlocking tree crowns, and open woodlands with a relatively sparse, shorter canopy.
Forests are among the most productive of Earth's natural ecosystems. Mature forests store more carbon (in biomass) than any other kind of ecosystem. This is especially true of old-growth forests, which typically contain large trees and, in temperate regions, a great deal of dead organic matter. Because all of the organic carbon stored in forests was absorbed from the atmosphere as carbon dioxide (CO2), these ecosystems are clearly important in removing this greenhouse gas from the atmosphere. Conversely, the conversion of forests to any other type of ecosystem, such as agricultural or urbanized lands, results in a large difference in the amount of carbon stored on the site. That difference is made up by a large flux of CO2 to the atmosphere. In fact, deforestation has been responsible for about one-half of the CO2 emitted to the atmosphere as a result of human activities since the beginning of the industrial revolution.
Because they sustain a large biomass of foliage, forests evaporate large quantities of water to the atmosphere, in a hydrologic process called evapotranspiration. Averaged over the year, temperate forests typically evapotranspire 10–40% of their input of water by precipitation. However, this process is most vigorous during the growing season, when air temperature is highest and the amount of plant foliage is at a maximum. In fact, in many temperate forests evapotranspiration rates during the summer are larger than precipitation inputs, so that the ground is mined of its water content, and in some cases streams dry up.
Intact forests are important in retaining soil on the land, and they have much smaller rates of erosion than recently harvested forests or deforested landscapes. Soil eroded from disturbed forests is typically deposited into surface waters such as streams and lakes, in a process called sedimentation. The resulting shallower water depths makes flowing waters more prone to spilling over the banks of rivers and streams, causing flooding.
Forests are also important in moderating the peaks of water flow from landscapes, both seasonally and during extreme precipitation events. When this function is degraded by deforestation, the risk of flooding is further increased.
Although trees are the largest, most productive organisms in forests, the forest ecosystem is much more than a population of trees growing on the land. Forests also provide habitat for a host of other species of plants, along with numerous animals and microorganisms. Most of these associated species cannot live anywhere else; they have an absolute requirement of forested habitat. Often that need is very specific, as when a bird species needs a particular type of forest, in terms of tree species, age, and other conditions.
Generally, forests provide the essential habitat for most of Earth's species of plants, animals, and microorganisms. This is especially true of tropical rain forests. Recent reductions of forest area, which since the 1950s have mostly been associated with the conversion of tropical forest into agricultural land-use, are a critical environmental problem in terms of losses of biodiversity. Deforestation also has important implications for climate change and access to natural resources.
Forests are an extremely important natural resource that can potentially be repeatedly harvested and managed to yield a diversity of commodities of economic importance. Wood is by far the most important product harvested from forests. The wood is commonly manufactured into paper, lumber, plywood, and other products. In addition, in most of the forested regions of the less-developed world firewood is the most important source of energy used for cooking and other purposes. Potentially, all of these forest products can be sustained indefinitely. Unfortunately, in most cases forests have been irresponsibly over-harvested, resulting in the "mining" of the forest resource and widespread ecological degradation. It is critical that in the future all forest harvesting is conducted in a manner that is more responsible in terms of sustaining the resource.
Many other plant products can also be collected from forests, such as fruits, nuts, mushrooms, and latex for manufacturing rubber. In addition, many species of animals are hunted in forests, for recreation or for subsistence. Forests provide additional goods and services that are important to both human welfare and to ecological integrity, including the control of erosion and water flows, and the cleansing of air and water of pollutants. These are all important forest values, although their importance is not necessarily assessed in terms of dollars. Moreover, many of these values are provided especially well by old-growth forests, which in general are not very compatible with industrial forestry practices. This is one of the reasons why the conservation of old-growth forest is such a controversial topic in many regions of North America and elsewhere. In any event, it is clear that when forests are lost or degraded, so are these important goods and services that they can provide.
The global area of forest of all kinds was about 8.4 billion acres (3.4 billion ha) in 1990, of which 4.3 billion acres (1.76 billion ha) were tropical forest and the rest temperate and boreal forest. That global forest area is at least one-third smaller than it was prior to extensive deforestation caused by human activities. Most of the deforested land has been converted to permanent agricultural use, but some has been ecologically degraded into semi-desert or desert. This global deforestation, which is continuing apace, is one of the most serious aspects of the environmental crisis.
Deforestation refers to a longer-term conversion of forest to some other kind of ecosystem, such as agricultural or urbanized land. Sometimes, however, the term is used in reference to any situation in which forests are disturbed, for example by clear-cut harvesting, even if another forest subsequently regenerates on the site. Various human activities result in net losses of forest area and therefore contribute to deforestation. The most important causes of deforestation are the creation of new agricultural land and unsustainable harvesting of trees. In recent decades, deforestation has been proceeding most rapidly in underdeveloped countries of the tropics and subtropics.
The most important ecological consequences of deforestation are: the depletion of the economically important forest resource; losses of biodiversity through the clearing of tropical forests; and emissions of carbon dioxide with potential effects on global climate through an enhancement of Earth's greenhouse effect. In some cases, indigenous cultures living in the original forest may be displaced by the destruction of their habitat.
There are numerous references in historical, religious, and anthropological literature to forests that became degraded and were then lost through over harvesting and conversion. For example, the cedars of Lebanon were renowned for their abundance, size, and quality for the construction of buildings and ships, but today they only survive in a few endangered groves of small trees. Much of the deforestation of the Middle East occurred thousands of years ago. However, even during the Crusades of the eleventh century through the thirteenth century, extensive pine forests stretched between Jerusalem and Bethlehem, and some parts of Lebanon had cedar-dominated forests into the nineteenth century. These are all now gone.
Similar patterns of deforestation have occurred in many regions of the world, including most of the Mediterranean area, much of Europe, south Asia, much of temperate North and South America, and, increasingly, many parts of the subtropical and tropical world.
In recent decades, the dynamics of deforestation have changed greatly. The forest cover in wealthier countries of higher latitudes has been relatively stable. In fact, regions of Western Europe, the United States, and Canada have experienced an increase in their forest cover as large areas of poorer-quality agricultural land have been abandoned and then regenerated to forest. Although these temperate regions support large forest industries, post-harvest regeneration generally results in new forests, so that ecological conversions to agriculture and other non-forested ecosystems do not generally occur.
In contrast, the rate of deforestation in tropical regions of Latin America, Africa, and Asia has increased alarmingly in recent decades. This deforestation is driven by the rapid growth in size of the human population of these regions, with the attendant needs to create more agricultural land to provide additional food, and to harvest forest biomass as fuel. In addition, increasing globalization of the trading economy has caused large areas of tropical forest to be converted to agriculture to grow crops for an export market in wealthier countries, often to the detriment of local people.
In 1990, the global area of forest was 4.23 billion acres (1.71 billion ha), equivalent to 91% of the forest area existing in 1980. This represents an annual rate of change of about -0.9% per year, which if projected into the future would result in the loss of another one-half of Earth's remaining forest in only 78 years. During this period of time, deforestation (indicated as % loss per year) has been most rapid in tropical regions, especially West Africa (2.1%), Central America and Mexico (1.8%), and Southeast Asia (1.6%). Among nations, the most rapid rates of deforestation are: Côte d'Ivoire (5.2%/year), Nepal (4.0%), Haiti (3.7%), Costa Rica (3.6%), Sri Lanka (3.5%), Malawi (3.5%), El Salvador (3.2%), Jamaica (3.0%), Nicaragua (2.7%), Nigeria (2.7%), and Ecuador (2.3%).
These are extremely rapid rates of national deforestation. A rate of forest loss of 2% per year translates into a loss of one-half of the woodland area in only 35 years, while at 3% per year, the half-life is 23 years, and at 4%, it is 18 years.
Potentially, forests are a renewable natural resource that can be continually harvested to gain a number of economically important products, including lumber, pulp for the manufacture of paper, and fuel wood to produce energy. Forests also provide a habitat for game species and also for the much greater diversity of animals that are not hunted for sport or food. In addition, forests sustain important ecological services related to clean air and water and the control of erosion.
Any loss of forest area detracts from these important benefits and represents the depletion of an important natural resource. Forest harvesting and management can be conducted in ways that encourage the regeneration of another forest after a period of recovery. However, this does not happen in the cases of agricultural conversion and some types of unsustainable forest harvesting. In such cases, the forest is "mined" rather than treated as a renewable natural resource, and its area is diminished.
At the present time, most of Earth's deforestation involves the loss of tropical forests, which are extremely rich in species. Many of the species known to occur in tropical forests have local (or endemic) distributions, so they are vulnerable to extinction if their habitat is lost. In addition, tropical forests are thought to contain millions of additional species of plants, animals, and microorganisms as yet undiscovered by scientists.
Tropical deforestation is mostly caused by various sorts of conversions, especially to subsistence agriculture, and to market agriculture for the production of export commodities. Tropical deforestation is also caused by unsustainable logging and fuel wood harvesting (about two thirds of tropical people use wood fuels as their major source of energy. Less important causes of tropical deforestation include hydroelectric developments that flood large reservoirs and the production of charcoal as an industrial fuel. Because these extensive conversions cause the extinction of innumerable species, tropical deforestation is the major cause of global biodiversity concern.
Mature forests contain large quantities of organic carbon, present in the living and dead biomass of plants, and in organic matter of the forest floor and soil. The quantity of carbon in mature forests is much larger than in younger, successional forests, or in any other type of ecosystem, including human agroecosystems. Therefore, whenever a mature forest is disturbed or cleared for any purpose, it is replaced by an ecosystem containing a much smaller quantity of carbon. The difference in carbon content of the ecosystem is balanced by an emission of carbon dioxide (CO2) to the atmosphere. This CO2 emission always occurs, but its rate can vary. The CO2emission is relatively rapid, for example, if the biomass is burned, or much slower if resulting timber is used for many years and then disposed into an anaerobic landfill, where biological decomposition is very slow.
Prior to any substantial deforestation caused by human activities, Earth's vegetation stored an estimated 990 billion tons (900 billion metric tons) of carbon, of which 90% occurred in forests. Mostly because of deforestation, only about 616 billion tons (560 billion metric tons) of carbon are presently stored in Earth's vegetation, and that quantity is diminishing further with time. It has been estimated that between 1850 and 1980, CO2 emissions associated with deforestation were approximately equal to emissions associated with the combustion of fossil fuels. Although CO2 emissions from the use of fossil fuels has been predominant in recent decades, continuing deforestation is an important source of releases of CO2 to the atmosphere.
The CO2 concentration in Earth's atmosphere has increased from about 270 ppm prior to about 1850, to about 360 ppm in 1999, and it continues to increase. Many atmospheric scientists hypothesize that these larger concentrations of atmospheric CO2 will cause an increasing intensity of an important process, known as the greenhouse effect, that interferes with the rate at which Earth cools itself of absorbed solar radiation. If this theory proves to be correct, then a climatic warming could result, which would have enormous implications for agriculture, natural ecosystems, and human civilization.
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