Tolmačenje

DEFORESTATION

Deforestation is the conversion of forested areas to non-forest land use such as arable land, urban use, logged area or wasteland. Historically, this meant conversion to grassland or to its artificial counterpart, grain fields; however, the Industrial Revolution added urbanization. Generally this removal or destruction of significant areas of forest cover has resulted in a degraded environment with reduced biodiversity. In developing countries, massive deforestation is ongoing and is shaping climate and geography.

Deforestation results from removal of trees without sufficient reforestation; however, even with reforestation, significant biodiversity loss may occur. There are many causes, ranging from slow forest degradation to sudden and catastrophic wildfires. Deforestation can be the result of the deliberate removal of forest cover for agriculture or urban development, or it can be an unintentional consequence of uncontrolled grazing (which can prevent the natural regeneration of young trees). The combined effect of grazing and fires can be a major cause of deforestation in dry areas. In addition to the direct effects brought about by forest removal, indirect effects caused by edge effects and habitat fragmentation can greatly magnify the effects of deforestation.

While tropical rainforest deforestation has attracted most attention, tropical dry forests are being lost at a substantially higher rate, primarily as an outcome of slash-and-burn techniques used by shifting cultivators.

Deforestation alters the hydrologic cycle, altering the amount of water in the soil and groundwater and the moisture in the atmosphere. Forests support considerable biodiversity, providing valuable habitat for wildlife; moreover, forests foster medicinal conservation and the recharge of aquifers. With forest biotopes being a major, irreplaceable source of new drugs (like taxol), deforestation can destroy genetic variations (such as crop resistance) irretrievably.

Shrinking forest cover lessens the landscape's capacity to intercept, retain and transport precipitation. Instead of trapping precipitation, which then percolates to groundwater systems, deforested areas become sources of surface water runoff, which moves much faster than subsurface flows. That quicker transport of surface water can translate into flash flooding and more localized floods than would occur with the forest cover. Deforestation also contributes to decreased evapotranspiration, which lessens atmospheric moisture which in some cases affects precipitation levels downwind from the deforested area, as water is not recycled to downwind forests, but is lost in runoff and returns directly to the oceans; in deforested north and northwest China, the average annual precipitation decreased by one third between the 1950s and the 1980s [1].

Long-term gains can be obtained by managing forest lands sustainably to maintain both forest cover and provide a biodegrable renewable resource. Forests are also important stores of organic carbon, and forests can extract carbon dioxide and pollutants from the air, thus contributing to biosphere stability and probably relevant to the greenhouse effect. Forests are also valued for their aesthetic beauty and as a cultural resource and tourist attraction.

Present causes

While short-sighted, market-driven forestry practices are often one of the leading cause of forest degradation, the principal human-related causes of deforestation are agriculture and livestock grazing, urban sprawl, and mining and petroleum extraction. Causes include demand for farm land and fuel wood. Underlining causes include poverty, lack of reform. The causes of deforestation are complex and often differ in each forest and country. Government policies, such as ones in Brazil, make it a priority to resettle some of the country's numerous landless people. The largest cause as of 2006 is slash-and-burn activity in tropical forests. Slash-and-burn is a method sometimes used by shifting cultivators to create short term yields from marginal soils. When practiced repeatedly, or without intervening fallow periods, the nutrient poor soils may be exhausted or eroded to an unproductive state. Slash-and-burn techniques are used by native populations of over 200 million people worldwide.

Theories of deforestation

Three schools of thought exist with regards to the causes of deforestation - the Impoverishment school, which believes that the major cause of deforestation is "the growing number of poor", the Neoclassical school which believes that the major cause is "open-access property rights" and the Political-ecology school which believes that the major cause of deforestation is that the "capitalist investors crowd out peasants". The Impoverishment school sees smallholders as the principal agents of deforestation, the Neoclassical school sees various agents, and the Political-ecology school sees capitalist entrepreneurs as the major agents of deforestation. Actual data support the first two theories as widespread numerical impacts.

Atmospheric pollution

Deforestation is often cited as one of the major causes of the enhanced greenhouse effect. Trees and other plants remove carbon (in the form of carbon dioxide) from the atmosphere during the process of photosynthesis. Both the decay and burning of wood releases much of this stored carbon back to the atmosphere. A.J.Yeomans asserts in Priority One that overnight a stable forest releases exactly the same quantity of carbon dioxide back into the atmosphere. Others state that mature forests are net sinks of CO₂ (see Carbon dioxide sink and Carbon cycle).

Wildlife

Some forests are rich in biological diversity. Deforestation can cause the destruction of the habitats that support this biological diversity - thus causing population shifts and extinctions. Numerous countries have developed Biodiversity Action Plans to limit clearcutting and slash and burn agricultural practises as deleterious to wildlife, particularly when endangered species are present.

Hydrologic cycle and water resources

Trees, and plants in general, affect the hydrological cycle in a number of significant ways:

• their canopies intercept precipitation, some of which evaporates back to the atmosphere (canopy interception);
• their litter, stems and trunks slow down surface runoff;
• their roots create macropores - large conduits - in the soil that increase infiltration of water;
• they reduce soil moisture via transpiration;
• their litter and other organic residue change soil properties that affect the capacity of soil to store water.

As a result, the presence or absence of trees can change the quantity of water on the surface, in the soil or groundwater, or in the atmosphere. This in turn changes erosion rates and the availability of water for either ecosystem functions or human services.

The forest may have little impact on flooding in the case of large rainfall events, which overwhelm the storage capacity of forest soil if the soils are at or close to saturation.

Soil erosion

Deforestation generally increases rates of soil erosion, by increasing the amount of runoff and reducing the protection of the soil from tree litter. This can be an advantage in excessively leached tropical rain forest soils. Forestry operations themselves also increase erosion through the development of roads and the use of mechanized equipment.

China's Loess Plateau was cleared of forest millennia ago. Since then it has been eroding, creating dramatic incised valleys, and providing the sediment that gives the Yellow River its yellow color and that causes the flooding of the river in the lower reaches (hence the river's nick-name 'China's sorrow').

Removal of trees does not always increase erosion rates. In certain regions of southwest US, shrubs and trees have been encroaching on grassland. The trees themselves enhance the loss of grass between tree canopies. The bare intercanopy areas become highly erodible. The US Forest Service, in Bandelier National Monument for example, is studying how to restore the former ecosystem, and reduce erosion, by removing the trees.

Landslides

Tree roots bind soil together, and if the soil is sufficiently shallow they act to keep the soil in place by also binding with underlying bedrock. Tree removal on steep slopes with shallow soil thus increases the risk of landslides, which can threaten people living nearby.

Farming

New methods are being developed to farm more food crops on less farm land, such as high-yield hybrid crops, greenhouse, autonomous building gardens, and hydroponics. The reduced farm land is then dependent on massive chemical inputs to maintain necessary yields. In cyclic agriculture, cattle are grazed on farm land that is resting and rejuvenating. Cyclic agriculture actually increases the fertility of the soil. Selective over farming can also increase the nutrients by releasing such nutrients from the previously inert subsoil. The constant release of nutrients from the constant exposure of subsoil by slow and gentle erosion is a process that has been ongoing for billions of years.

Forest management

Efforts to stop or slow deforestation have been attempted for many centuries because it has long been known that deforestation can cause environmental damage sufficient in some cases to cause societies to collapse. In Tonga, paramount rulers developed policies designed to prevent conflicts between short-term gains from converting forest to farmland and long-term problems forest loss would cause, whilst during the seventeenth and eighteenth centuries in Tokugawa Japan the shoguns developed a highly sophisticated system of long-term planning to stop and even reverse deforestation of the preceding centuries through substituting timber by other products and more efficient use of land that had been farmed for many centuries. In sixteenth century Germany landowners also developed silviculture to deal with the problem of deforestation. However, these policies tend to be limited to environments with good rainfall, no dry season and very young soils (through volcanism or glaciation). This is because on older and less fertile soils trees grow too slowly for silviculture to be economic, whilst in areas with a strong dry season there is always a risk of forest fires destroying a tree crop before it matures.

Afforestation

Today, in China, where large scale destruction of forests has occurred, the government has required that every able-bodied citizen between the ages of 11 and 60 plant three to five trees per year or do the equivalent amount of work in other forest services. The government claims that at least 1 billion trees have been planted in China every year since 1982. In western countries, increasing consumer demand for wood products that have been produced and harvested in a sustainable manner are causing forest landowners and forest industries to become increasingly accountable for their forest management and timber harvesting practices.

The Arbor Day Foundation's Rain Forest Rescue program is a charity that helps to prevent deforestation. The charity uses donated money to buy up and preserve rainforest land before the lumber companies can buy it. The Arbor Day Foundation then protects the land from deforestation. This also locks in the way of life of the primitive tribes living on the forest land.