Deforestation is the destruction or clearing of forested lands, usually for the purposes of expanding agricultural land or for timber harvesting. When the process is conducted by clearcutting (removal of most or all of the canopy tree growth, leaving few or no live or dead trees standing) or when mass forest burning occurs, significant losses of habitat and biodiversity may result, including the erosion of biological community structure and the extinction of species. Deforestation is proceeding at a rapid pace in may areas of the world, especially in the tropical and boreal forest regions of the earth, with annual net loss of forests during the 1990s estimated in the range of nine to sixteen million hectares per annum. Large scale deforestation may have adverse impacts on biosequestration of atmospheric carbon dioxide, exacerbating greenhouse gas buildup, through the release of stored carbon in tree biomass and reduced CO2 fixation rates due to loss of trees. Deforested regions are often subject to accelerated rates of soil erosion, increased surface runoff and sedimentation of streams and rivers, reduced infiltration and ground water recharge, with adverse water quality impacts on surface water and ground water resources.
Root causes of deforestation include a broad range of economic and social factors, such as (a) poorly formulated property rights systems, (b) widespread poverty and overpopulation, placing pressure on marginally productive lands for subsistence, (c) expansion of agriculture to feed a dramatically increasing human population, (d) short term view of forest management economics at the expense of long term forest productivity and (e) lax forest management. The impacts of deforestation can include the displacement of indigenous peoples from their historic living areas, or the loss of traditional livelihoods and food production and procurement systems. At the current time there is a strong correlation between widespread deforestation and countries which have a low per capita income, deforestation fom commercial timber harvesting is still a problem in many industrialized countries as well.
Types of deforestation
Chief methods of deforestation are: (a) land clearing to prepare for livestock grazing or expansion of crop planting, (b) commercial logging and timber harvests, (c) slash-and-burn forest cutting for subsistence farming, and (d) natural events such as volcanic eruption, stand windthrow from hurricanes, catastrophic forest fires, or changes in local climate and rainfall regimes. It is important to note that those natural factors which may cause deforestation represent only a small fraction of observered deforestation worldwide during historical time.
Causes of deforestation
See Main Article: Causes of deforestation
The predominant driver for deforestation world wide is the clearing of trees to expand agriculture, according to the United Nations Framework Convention on Climate Change. Subsistence agriculture in poor countries is responsible for 48% of deforestation; with commercial agriculture is responsible for 32% of deforestation; and commerical logging is responsible for only 14% of deforestation; charcoal and other fuel wood removals comprise less than 6% of deforestation, but those uses can generally be assigned to subsistence practises.
The degradation of forest ecosystems has also been traced to economic incentives that make forest conversion appear more profitable than forest conservation. Many important forest functions lack readily visible markets, and hence, are without economic value that is apparent to the forest owners or the communities that depend on forests for their well-being. Considerable deforestation arises from a lack of security of property rights and from the absence of effective enforcement of conservation policies, both factors particularly prominent in developing countries; in some cases, terrorism and governmental corruption are concomitant factors in forest losses.
Small scale deforestation was practiced by some societies tens of thousands of years before the present, with some of the earliest evidence of deforestation appearing in the Mesolithic period. These initial clearings were likely devised to convert closed forests into more open ecosystems favourable to game animals. With the advent of agriculture in the mid-Holocene, greater areas were deforested, and fire becamet an increasing methodl to clear land for crops. In Europe by 7000 BC. Mesolithic hunter-gathers employed fire to create openings for red deer and wild boar. From pollen core records, in Great Britain, shade-tolerant species such as oak and ash were being replaced by hazels, brambles, grasses and nettles. Removal of the forests led to decreased transpiration, resulting in increased formation of raised peat bogs. Widespread decrease in elm pollen across Europe between 8400-8300 BC and 7200-7000 BC, starting in southern Europe and gradually moving north to Great Britain, likely represents land clearing by fire at the onset of Neolithic agriculture.
The Neolithic period ushered in extensive deforestation for agriculture. Stone axes were being made from about 3000 BC not only from flint, but from a wide array of hard rocks from across Africa, Britain, Scandinavia and North America. They include the noted Langdale axe industry in the English Lake District, quarries at Penmaenmawr in North Wales and numerous other locations. Rough-outs were made locally near the quarries, wih some polished locally to yield a fine finish. This step not only increased mechanical strength of an axe, but also facilitated penetration of timber. Flint continued to be utilised from sources such as Grimes Graves as well as numerous mines across Europe. Evidence of deforestation has been found in Minoan Crete; for example the environs of the Palace of Knossos were severely deforested in the Bronze Age.
Ancient and medieval times
In ancient Greece, regional analyses of historic erosion and alluviation demonstrate that massive erosion follows eforestation, by about 500-1,000 years the introduction of farming in the various regions of Greece, ranging from the later Neolithic to the Early Bronze Age. The thousand years following the middle of the first millennium BC saw substantial instances soil erosion in numerous locales. The historic siltation of ports along certain coasts of Europe (Bruge) and the coasts of the Black Sea and southern coasts Asia Minor (e.g Tulcea, Clarus, and the ports of Ephesus, Priene and Miletus, where harbours were reduced in use or abandoned because of the silt deposited by the Danube and Meander Rivers) and in coastal Syria during the last centuries BC.
By the end of the Middle Ages in Europe, there were severe shortages of food, fuel and building materials since most of the primordial forests had been cleared. Transition to a coal burning economy and cultivation of potatoes and maize allowed continuity of the already large European population to survive. Easter Island has suffered from an ecodisaster, aggravated by agriculture and deforestation. The disappearance of the island's palm trees slightly predates and suggests correlation with the significant decline of its civilization starting at least as early as the 1600s AD; the societal collapse of that period can be linked to deforestation and over-exploitation of other resources.
Post industrial era
Since the mid nineteenth century worldwide deforestation has sharply accelerated, driven by the expanding human population and industrialisation. Approximately one half of the Earth's mature tropical forests (between 7.5 million and 8.0 million sq. km of the original 15 million to 16 million sq. km that until 1947 covered the Earth) have now been cleared. Some scientists have asserted that unless significant forest protection mitigation measures are adopted, by the year 2030 that 90 percent of the planet's forest will have been removed, as well as hundreds of thousands of flora and fauna species rendered extinct.
The adverse environmental impacts associated with largescale deforestation can include significant changes in ecological, hydrological, and climatic processes at local and regional levels. The ecological consequences include habitat loss and habitat fragmentation and adverse changes in local species richness and biodiversity. In some cases, increased local species diversity associated with the destruction or fragmentation of old-growth forests may actually erode biological diversity at regional scales, through the replacement of rare species with restricted distributions (e.g., spotted owls, spectacled bears, colobus monkeys) by common species that are habitat generalists, human commensals, or invasive species. Hydrological impacts stem from the loss of infiltration capacity associated with canopy interception and leaf litter detritus absorbtion, with resulting acceleration of surface runoff flows at the expense of groundwater recharge; these impacts aggravate problems from water pollution and sedimentation, and may alter the balance and volumes of ground water and surface water flows regimes available to sustain riparian ecosystems. Soil loss may occur as the result of active surface erosion, and through the loss of organic matter accumulation. Climate impacts relate to the carbon sink reductions engendered by deforestation, which long term effects have contributed to the buildup of atmospheric carbon dioxide.
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