Deforestation, tropical

Human activity, particularly in the developing world, continues to make it more difficult to sustain the world s biomass growth areas. It has been estimated that tropical forests are disappearing at a rate of tens of thousands of hm per year. Satellite imaging and field surveys show that Brazil alone has a deforestation rate of approximately 8 x 10 hm /yr (5). At a mean net carbon yield for tropical rain forests of 9.90 t/hm yr (4) (4.42 short ton /acreyr), this rate of deforestation corresponds to a loss of 79.2 x 10 t/yr of net biomass carbon productivity. 

Land use changes in the tropics have resulted in a landscape characterized as a mosaic of logged forests, cleared fields, and successional forests. This results in the transformation from extremely fire resistant rainforest ecosystems to anthropogenic landscapes in which fire is a common event (16, 17), Fires occur in disturbed tropical forests because deforestation has a dramatic effect on microclimate. Deforestation results in lower relative humidities, increased wind speeds, and increased air temperatures. In addition, deforestation results in increased quantities of biomass that are susceptible to fire. This biomass may be in the form of forest slash, leaf litter, grasses, lianas or herbaceous species (16, 18). 

Abusive forest practices are neither a recent phenomenon nor limited to developing tropical countries. Deforestation has brought about economic and social declines of civilizations for over 5,000 years 19). Postel and Ryan (1) estimated only 1.5 million ha of primary forests remain out of the 6.2 biUion ha... 

The burning of slash following deforestation, whether intentional or unintentional, results in far greater direct and indirect losses of nutrients than deforestation alone. This is particularly true in many tropical forests where only a small fraction (if any) of the aboveground biomass is removed prior to burning. Carbon losses from slash fires in the tropical dry forest were 4-5 fold greater than C losses from wood export (Table IV) (55). Slash fires in tropical dry forests resulted in N losses of 428-500 kg ha whereas fuel wood export of the relatively N-poor coarse woody debris amounted to approximately 41 kg N ha" Losses of P increase with increasing fire severity. P losses of 10-77 kg ha" as a result of severe fires is not uncommon (Table TV) (53, 58, 60). 

Much of the surface soil erosion and hence nutrient loss occurs when deforestation and biomass burning removes and/or consumes the organic materials that protect the soil surface. Significant losses may occur by dry ravel or overland water erosion associated with precipitation events. Under a shifting cultivation system in a tropical deciduous forest ecosystem in Mexico, Maass et al. 61) reported first year losses of N, P, K, and Ca were 187, 27, 31, and 378 kg ha" respectively. In contrast, losses in adjacent undisturbed forests were less than 0.1 kg ha for all nutrients except Ca (losses were 0.1-0.5 kg ha for Ca). 

The clearing of forests in the United States early in the century, combined with a large increase in postwar tropical deforestation, where much of the wood was burned, released carbon dioxide to the air and changed the atmospheric components. 

Globally, it is estimated that there is 3870 million ha of forests (30 % of the Earth s land area), of which 95 % is natural or semi-natural (Table 1.1). At the present time, in most developed countries, the forest area is increasing and this is likely to continue given the present rates of harvesting. However, in tropical parts of the world, the forest area continues to decline, giving rise to serious concerns. It is estimated that between 1990 and 2000, there was a loss of 9.4 million ha per annum of forest worldwide (an annual deforestation rate of 14.6 million ha and an increase in forest area of 5.2 million ha per year Table 1.2). The only way to reverse this trend is to place sufficient economic value on forest resources, with incentives to encourage sustainable forest management. 

Although timber production and utilization can result in substantial environmental benefits compared with materials extracted from nonrenewable sources, timber utilization and forestry have become associated with negative environmental impacts. Tropical deforestation... 

The carbon cycle has continued in this manner for millions of years. However, scientists have detected an imbalance in the carbon cycle due to the increase in the amount of carbon dioxide produced through burning fossil fuels and the deforestation of large areas of tropical rain forest. The Earth s climate is affected by the levels of carbon dioxide (and water vapour) in the atmosphere. If the amount of carbon dioxide, in particular, builds up in the air, it is thought that the average temperature of the Earth will rise. This effect is known as the greenhouse effect (Figure 13.14). 

Veldkamp, E. (1994). Organic-carbon turnover in 3 tropical soils under pasture after deforestation. Soil Sci. Soc. Am. J. 58,175-180. 

According to present-day estimates, up to 17 million ha of tropical forests are being destroyed every year, 6 million ha of which are in the Amazon basin. By the end of 1988, an area of 21 million ha was deforested, and 10 years later this area reached 27.5 million ha, which exceeds the size of the U.K. 

Over virgin tropical forests, about 75% of incoming solar radiation is spent on evapotranspiration. Therefore, WTF deforestation will result in radical changes in energy exchange and global atmospheric circulation. Still more substantial will be changes in local climate, especially from the viewpoint of the rain rate which could decrease by as much as 65%. Another problem is that we still do not know the threshold level for WTF destruction. This level determines an ecosystem s ability to self-support. If, for instance, this level is 20%, then the threshold has already been exceeded. 

In much the same way as Scenario (2.3), it is necessary to write similar relationships for all known types of plant formations and, using a global model, to evaluate the consequences of substitution of one type of vegetation cover for another (see Section 3.6.2 for details). But, of course, most important for studies is the process of substitution of forests for agricultural lands. At the present time, of the 148,000,000 km2 (57 million mi2) of land approximately 31,000,000 km2 (12 million mi2) are arable, and they expand at a rate of 100,000km2 (38,610mi2) per year. A major element of arable land loss is deforestation which continues to the present day, primarily in tropical countries through commercial over-exploitation of tropical forest. 

Zhang H. Henderson-Sellers A. and McGuffie K. (2001). The compounding effects of tropical deforestation and greenhouse warming on climate. Climatic Change, 49(3), 309-338. 

In comparison, the average tree consumes only 1 ton of COz in a lifetime, and an acre of rainforest consumes about 500 tons yearly. When agribusiness, the ethylene industry, or pulp and paper corporations turn forests or rainforests into farmland, they also destroy an effective consumer of COz. The world s fastest-disappearing forests are in Indonesia, where they are cut down either to make paper pulp or to be replaced by palm oil plantations. Palm oil is mostly used to make biodiesel fuels. Tropical deforestation not only results in COz emissions (20% of the global total), but it also poisons the rivers. 

The increasing world population exerts massive pressure on the often fragile ecosystems and land resources of the Tropics and Subtropics. This situation leads to resource overuse, such as excessive deforestation, overgrazing, overextraction of nutrients, and other types of agricultural mismanagement. These frequently manifest themselves in various forms of soil degradation, which are accelerated by the low ecosystem resiliency common in the Tropics and Subtropics. Responsible for this fragility are mainly the climatic conditions of past and present and the processes that dominated soil development. 

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