Homepage | DESERTLINKS Homepage
| © DESERTLINKS 2004
English-EN | Español-ES | Italiano-I | Ελληνικά-GR | Portuguese-PT
|Desertification Indicator System for Mediterranean Europe|
g Description of reasons for deforestation and why it is an issue in the context of desertification
Authors: Agostino Ferrara <firstname.lastname@example.org>, Raffaella Lovreglio
Introduction. A large area of the Earth's land surface is covered by forests; a very valuable resource to human populations, biodiversity, edaphic conditions, water storage, and the condition of the atmosphere. Human populations originally used forests only for timber, for shelter, as fuel for burning, and for other food- products. At that time the forests were used in a sustainable way, without excessive threat of extinction of species through over-exploitation. From these early times of forest usage to modern times, there has been a huge increase in the amount of timber being harvested daily, for a number of reasons such as:
Now forests are threatened by factors such as climatic hazards, diseases prompted by insects or pathogens, threats of a purely anthropogenic nature, fires, atmospheric pollution, deforestation and the increase in social pressure. Man is partly responsible for most of these threats. Indeed the mark of human actions is always present: it is relatively moderate concerning climatic disorder despite the emissions of greenhouse gases, responsible for significant damage; it is average in the sensitive growth of certain artificial forestry stands prone to parasitic attacks; it is implicated most in the phenomena of atmospheric pollution or of deforestation.
The estimated rates of global deforestation are astonishing. According to the World Resources Institute (http://pubs.wri.org/pubs_content.cfm?PubID=3018), more than 80 % of the Earth's natural forests have already been destroyed (at a rate of about 40 million hectares per year). Up to 90 % of West Africa's coastal rain forests have disappeared since 1900. Brazil and Indonesia, which contain the world's two largest surviving regions of rain forest, are being stripped at an alarming rate by logging, fires, and land clearing for agriculture and cattle-grazing. Loss of habitat is among the obvious consequences of deforestation. 70% of the Earth's biodiversity is present in forests. Rain forests help generate rainfall in drought-prone countries elsewhere. Studies have shown that destruction of rain forests in such West African countries as Nigeria, Ghana, and Côte d'Ivoire may have caused two decades of droughts in the interior of Africa. (http://pubs.wri.org/pubs_content.cfm?PubID=3018).
Deforestation may have catastrophic global effects as well. Trees are natural consumers of carbon dioxide, one of the greenhouse gases whose build up in the atmosphere contributes to global warming. Destruction of trees not only removes these "carbon sinks," but in addition tree burning and decomposition pump even more carbon dioxide into the atmosphere, along with methane, another major greenhouse gas.
Climatic conditions. Slightly more than one-half of the world's climates are classified as arid and semi-arid. Further sub-division of the arid and semi-arid areas is loosely defined, but based on mean annual rainfall. Generally speaking, deserts receive 100 mm rainfall or less, arid lands between 100 and 250 mm and semi-arid lands between 250 and 500 mm, with the following additional distinctions:
Although all climatic areas are subject to desertification, the arid and semi-arid areas in general and the arid areas in particular, are highly susceptible, because they are only a few millimetres of rain away from being true deserts.
Weather Fluctuations. Arid and semi-arid areas have harsh climates made more difficult by weather fluctuations. There are normally many more years of below average rainfall than average or above. Prolonged droughts occur frequently. Droughts occur so frequently in northeast Brazil, for example, that the area is called the drought polygon. Weather variation, per se, does not cause desertification, but it is an accelerator to desertification, especially when man continues land uses supportable only by average or above average rainfall. Failure to plan for the "bad" years and modify land use accordingly accelerates the desertification process. Although droughts increase the rate of land degradation, they are not the cause, nor does desertification have a direct relation to a nearby desert. Soil degradation may begin in any cultivated field.
Desertification and deforestation involve a drastic change in microclimates. For instance, if shrubs and trees are felled, the noonday sun will fall directly on hitherto shaded soil; the soil will become warmer and drier, and organisms living on or in the soil will move away to avoid the new harsher conditions. The organic litter on the surface - dead leaves and branches, for example - will be quickly oxidized, the carbon dioxide being carried away. So too will be the small amount of humus stored in the soil. The local-scale, mesoscale, and large-scale climatic impacts of deforestation are demonstrated. The difference in radiation and energy balance between forest and clearing produces higher air temperatures in the clearings, particularly in the dry season. In areas of substantial deforestation, higher sensible heat fluxes from the cleared forest produce deeper convective boundary layers, with differences in cloud cover being observed, and mesoscale circulations being predicted.
All these microclimate changes also bring about ecological changes. The ecosystem is being altered, in most cases adversely. Hence, these processes result not only in a loss of biological productivity but also in the degradation of surface microclimates. Phenomena such as global warming and the greenhouse effect, which have their origin in deforestation and desertification, among many other causes, are more serious, global in scope, and therefore potentially more threatening.
Drought tolerance of forests. Forests cover more than 30 % of the Earth's surface. They range from dry scrubby thickets in the African savannah to moist, dense tropical rainforests; from mangroves at the water's edge to hardy conifers on mountain tops. Each forest type has its own kind of trees that are best suited to the local climate. There are three broad types of forests in the world: Boreal, Temperate, and Tropical.
The distribution of vegetation is controlled by several factors (regional climate, topography, soil parent material, time and organisms). However, at the broadest scale, the scale of biomes, global climate patterns are the primary factors determining the structure and distribution of vegetation. Aspects of climate that exert strong control over vegetation are temperature, precipitation, humidity, solar radiation, and wind. Each of these affects plant physiology directly (through effects on photosynthesis and respiration) or indirectly (through effects on nutrient availability via decomposition and soil development).
Drought is a crucial factor in the
survival and growth of trees, especially in dry season areas. In recent
decades, forest decline in several forest tree species has been attributed
to recurrent droughts. Better adaptation of forest trees is important
both for improved growth and because of the danger of climate change (it
is likely that plant communities will - as a result of higher temperatures
- have to face more severe drought conditions in the future).
Most plants, and particularly tree
species, since they are so long-lived, are exposed to drought during their
lifetime. In order to minimize the impact, and during severe drought,
plants must have mechanisms in place to cope with the drought.
The overall ability of a tree to survive a drought depends on many morphological, physiological and phenological characteristics. The mechanisms are categorized here, but it needs to be emphasized that these categories are not mutually exclusive and that the interaction of many factors results in the overall ability to cope with drought. Strategic mechanisms include:
CO2 affects the sensitivity of trees to drought. An increase in CO2 content in the atmosphere reduces stomatal aperture, orifices found on the leaf surface by which plants absorb CO2 and release water vapour. This effect varies greatly from one species to another. In certain species, such as oak, the reduction in stomatal aperture is considerable, and this reduces water losses via leaf transpiration in the tree, and should thus reduce sensitivity to drought. In other species, such as maritime pine or beech, this "anti-transpiration" occurs to a lesser extent, or even not at all.
Forest destruction by fire.
Every year, millions of hectares of the world's forests are being consumed
by fires, resulting in billions of dollars in fire suppression costs as
well as causing tremendous damage to the environment. The ecological and
environmental impacts of forest fires are manifested in the degradation
of the quality of vegetation, erosion of biodiversity, damage to the health
of forest ecosystems, loss of wildlife habitat, air, river and estuarine
pollution and overall ecological retrogression. Forest fires contribute
to global climate change and warming. Biomass burning also destroys an
important sink for atmospheric carbon. So fire is a factor reducing woods
and accelerating soil deterioration, thus increasing hazards of desertification.
On average, only about 8-10% of the heat generated during a forest fire is radiated downward to the forest floor (Wells et al. 1979, DeBano et al. 1976, Raison et al. 1986, Steward 1989, Hungerford 1989). Yet this heating is responsible for all of the direct changes in physical and chemical soil properties caused by forest fire. Organic matter is destructively distilled into hydrocarbons between 175°C and 315°C (DeBano 1976). Hydrocarbons are vaporized at 300°C and move downward precipitating around soil particles at the interface where soil temperatures are cooler. This waxy coating is water repellent, slowing infiltration. Water repellence may last from weeks to some years (Baker 1989, DeBano 1979). Overland flow from these areas can trigger rill erosion and mass erosion, thus inducing degradation and further processes which lead to desertification.
After low-severity fires, recovery to pre-fire conditions may take only as long as necessary for soil biota to re-establish and surface organic matter to re-accumulate. The changes listed above - especially the loss of organic matter from above and below ground - affect the chemical and physical nature of forest soil with consequences for post-fire regeneration as well as post fire water yield and soil loss via erosion. Following a severe fire, increased surface runoff during storms enhances the potential for surface erosion and shallow landslides, especially on sites in which roots and other organic structures that hold loose material on slopes were consumed or killed (Bitterroot National Forest 2000). Large, severe fires pose greater risks of erosion until vegetation cover develops and litter begins to accumulate. On slopes with scarce protective vegetation cover and debris after fire, soil can erode even without heavy rains, due to the sheer force of gravity.
Forest productivity. Maintaining the health and productivity of forest ecosystems is an important prerequisite to sound stewardship and the sustainable development of forested lands. Slow productivity of forests means reduced amount of biomass, and consequently a forest cover less resistant and more prone to irreversible transformations. Forests have been, and continue to be, exposed to a broad range of natural and human-caused stressors. Natural stressors include weather extremes, forest insects and pathogens and catastrophic events. Human-caused stressors include activities such as land use change; inappropriate or non-sustainable timber harvesting practices; road building, which reduces the capacity of the site to continue as productive forest land; introduction of exotic pests and air pollution.
Altered ecological conditions caused by human activities or climate change can increase the susceptibility of forests to natural stressors such as drought, changes in the water table, pest epidemics and wildfires. The interaction of human stressors and natural stressors can accelerate the deterioration of forest health. The proximity of settlements and structures to forested land creates management challenges including those related to fire. Forest health problems that reduce the resiliency of forests or that reduce the capacity of forests to support the needs of people and society should be addressed as strategic issues in forest management.
The increasing importance of forests makes forest health efforts more timely and critical than ever before. But for forests to perform their vital environmental functions and to realize their enormous productivity potential, they have to be managed. Silvicultural measures must be taken to improve the health of forests stands, to stimulate their production and to avoid the causes of poor stand health, in order to prevent the action of secondary organisms. The maintenance of species diversity and stand types is equally important for an efficient fight against biotic threats. Proper forest management technologies should be used more widely as the first steps in moving from merely exploitative logging to sustainable forest use. Although there are many unanswered questions about truly sustainable silvicultural systems for forests, there are many well-known practices that can be employed more extensively which could improve forest management and reduce the degradation of forests and their susceptibility to deforestation.
A recent assessment on the forest sector has provided an overview of gaps and priorities for promoting sustainable forest management and development. The following orientations have been highlighted: promotion of multifunctional objectives; strengthening of links between short-term and long-term strategies; monitoring based on criteria and indicators; promoting and motivating forest owners, forest enterprises and forest marketing; improving public administration; support for diversification of forest enterprises activities; strengthening of research, training and communication (EOFM, 2000).
Sustainable forest management involves the enhancement of various aspects of forest functions such as conservation of biodiversity, conservation of soil and water resources, and contributions to the global carbon cycle, as well as wood production.
The impact of grazing. Domestic
animals reduce regeneration through overgrazing, browsing, and trampling.
Large vertebrate herbivores are supposed to be an important structuring
agent in terrestrial food chains, through their impact on plant diversity
and plant nutritional value. In low productive systems, sustained heavy
grazing by large vertebrates may change interplant relationships, e.g.
inter-specific plant competition for light and nutrients and disturb important
ecological processes such as the soil-plant nutrient cycle. Vertebrate
grazing or browsing has, therefore, the potential to change biodiversity.
In more productive systems, grazing may increase plant species richness
by removing strong competitors, thus giving space for the growth of less
competitive species. In contrast, in low productive systems, such as many
alpine areas and on the arctic tundra, intense grazing can reduce plant
species richness and species diversity. The ecological impact of ungulate
grazing is not only determined by the productivity of the system, but
also by the type of grazer, the degree of herbivory and the grazing history.
Overgrazing is often blamed for worldwide desertification, which is partly true and partly false, depending on the situation. It depends certainly on overgrazing intensity (such as slight, heavy, very heavy and destructive) and also overgrazing duration, which can be measured in months, years, decades or even centuries. Apart from prolonged droughts, acceptable and fixed stocking rates will cause temporary overgrazing in some years because of grazing capacity fluctuations. Damage to the vegetation, if any, is usually repaired by natural processes. Stocking rates and managerial systems that result in continual destructive grazing are a major cause of desertification on rangelands. The desertification process is accelerated when these practices are maintained during drought and certain seasons where plants are highly vulnerable to abuse. While it takes a long time, this will eventually result in desertified rangelands. Destructive grazing also causes poor livestock performance and many private landowners have learned this lesson. It is, therefore, not as common on private lands as it is on public or community grazing lands, where central control is lacking.
Role of forest management. The role of forest management is to maintain an appropriate enhancement of forest resources and encouragement of productive functions of forests (wood and non-wood). Forest management practices safeguard the quantity and quality of the forest resources in the medium and long term by balancing harvesting and growth rates and by promoting techniques that minimise direct or indirect damage to forest, soil or water resources. Appropriate silvicultural measures maintain the growing stock of resources at - or near to - a level that is economically, ecologically and socially desirable. Forest management practices make the best use of natural structures and processes and use preventive biological measures wherever and as far as economically feasible to maintain and enhance the health and vitality of forests. Adequate genetic species and structural diversity should be encouraged and/or maintained to enhance stability, vitality and resistance capacity of the forests to adverse environmental factors, and to strengthen natural regulation mechanisms.
Forest management practices promote a diversity of both horizontal and vertical structures, such as uneven-aged stands and the diversity of species such as mixed stands. Where appropriate, the practices should also aim to maintain and restore landscape diversity. Tending and harvesting operations should be conducted in a way that does not cause lasting damage to ecosystems. Wherever possible, practical measures should be taken to improve or maintain biological diversity.
The principal aim of forest management planning is:
Things going on in the wider world. Deforestation is the product of the interaction of the many environmental, social, economic, cultural, and political forces at work in any given region. In most cases, deforestation is a process that involves a competition amongst different land users for scarce resources, a process exacerbated by counter-productive policies and weak institutions. It creates wealth for some, causes hardships for others, and almost always brings serious consequences for the environment. Deforestation is an important contributor to global warming, but its contribution relative to the other factors is not precisely known. The principal cause of global warming is the excessive discharges in industrialised countries of greenhouse gases, mostly from the burning of fossil fuels. It is thought that an additional 2,000 million tons or about 25 % of the total carbon dioxide emissions are a consequence of deforestation and forest fires (WCFSD, 1997). At the regional level, deforestation disrupts normal weather patterns, creating hotter and drier weather.
Based on the most recent estimates of the rates of deforestation, and assuming that 75 % of forest losses are attributable to agricultural expansion, it is estimated that over the next 25 years the agriculture sector will require an additional 250 to 300 million hectares of new land to accommodate the demands of commercial farming, subsistence cropping, pasture and range development. Most of this increase in land area will come at the expense of tropical forests. The agriculture sector must be challenged to find appropriate solutions.
Any effort to combat deforestation must be based on a complete understanding of what the agents of deforestation are and what its direct and underlying causes are. While forests will continue to be lost for decades to come, it is critically important that the fight against deforestation is done in the most rational way possible. Through improved protection and management of the remaining forests, through well-targeted socio-economic development programs, and through policy and institutional reforms; deforestation can be brought under control (http://www.rcfa-cfan.org/english/issues.12-9.html).
Impact of human population. Rapid growth of the population and societies and increased demand for higher standards of living has brought about increased exploitation of the earths natural resources, which in turn has led to rapid destruction of the landscape. It is evident that through history mankind has put added stress on the land, starting from use of fire by prehistoric man in Palaeolithic times, accidentally and intentionally burning large areas of forest for settlement and agriculture. (Pickering and Owen, 1997). Human impact on the land including clearance of natural vegetation, for settlement, urbanisation, reservoirs, mineral extraction, agriculture and tourism has led to a series of problems including: global climate change, desertification, food shortage, competition for land and space and most importantly a reduction in biodiversity. Agricultural intensification, throughout the twentieth century, brought about increased grazing pressure and the domestication of animals, both of which have led to vegetation clearance, meaning the exposure of top soil to wind and rain. Soil erosion has become a major issue as it causes leaching of nutrients, the possibility of natural disasters during periods of heavy rain or even desertification (World Resource Institute, 2002). Domestication of animals has many knock-on effects especially when animals are non-native to the area of introduction. Human activities such as deliberate land clearance for resources and settlement have brought about global deforestation problems.
Like deforestation, desertification
is exacerbated by an expanding population (http://www.munfw.org/archive/45th/csd1.htm).
Deforestation is due to man's decisions to expand cultivation and ranching
to meet an increasing demand for land, forced by an ever increasing population.
Forest degradation and loss induced by the spontaneous expansion of people's
activities into forest lands is notoriously difficult to quantify. Traditional
agricultural productivity crisis and then abandonment of wide areas, excessive
use of water and concentration of economic activities on coastal zones,
new urban areas, intensive tourism and agriculture bring negative contributions
to the desertification process. Forest destruction to meet the agricultural
productive land requirement of the steadily growing population is perhaps
the most important deforestation threat in the developing countries. The
growing population pressure for food and space is pushing some of the
remaining specialized and sensitive flora and fauna of poor countries
to local extinction. The exploitation of the worlds forest resources
to meet the basic needs of it's population for minor and major products
is also a major contributing factor to forest degradation and destruction.
More serious in some tropical countries is the trading of virgin land
to concessions for foreign exchange.
Deforestation and a change in erosion risk. Deforestation in hilly areas often leads to decreased infiltration of water and consequently higher runoff and increased peak water discharges following rainfall events and reduced runoff during dry seasons. Most importantly, it results in increased soil erosion, gully and ravine formation, flooding risk, and siltation of reservoirs and irrigation schemes. For instance, deforestation in the Himalayas has been associated with a doubling in torrent width since 1990-and a downstream cost of more than US$1 billion (Government of India, 1983). Afforestation of denuded hilly land could reduce peak runoff; lessen the risk of flooding; and conserve soils and prevent severe siltation, gully formation, and landslides. In experimental work, up to 500-fold differences in erosion rates between forest and cultivated cropland have been recorded (Maass et al., 1988). The presence of forest litter is critically important in facilitating infiltration rates and preventing soil/sediment movement by overland flow. Forests generally are expected to use more water (the sum of transpiration and evaporation of water intercepted by tree canopies) than crops, grass or natural short cycle vegetation. This effect may be related to increased interception loss - especially if tree canopies are wet for a large proportion of the year (Calder, 1990)- or, in drier regions, to trees' greater root system, which allows water extraction and use during prolonged dry seasons.
Lower Inner Alentejo, Portugal
The descriptions left by travellers and chronicle writers during past centuries reveal that Mértola municipality was covered by large areas of scrubland, together with open forest dominated by Quercus ilex and Quercus suber.
The analysis of historic documents reveals various phases of deforestation. One of the most important was during the seventeenth century when forests were cleared for cereal cultivation. This was as a result of a significant population increase resulting from the settlement of the border areas of inner Portugal by Jews, escaping persecution by the Spanish inquisition.
Another period of intense destruction of the vegetation cover coincided with the peaks in labour and extraction at the São Domingos mine. The shrubs and trees were used to produce charcoal. Similar situations happened during both world wars when, because of fuel shortage, charcoal production was of utmost importance. This led to the almost complete destruction of the natural arboreal vegetation.
Currently the most common deforestation activity is the destruction of the shrub vegetation for cereal cultivation and pasture. It is important to note that clearing of trees (Quercus ilex and Q. suber) is strictly forbidden by law.
In the Murcia Region 45% of the land is forest. This is an area of 509.000 ha, with about 300.000 ha of the forest area with trees (190.000 ha of Pinus halepensis) and about 200.000 ha of forest area with only shrubs (Mediterranean macchia).
Deforestation processes in this area are mainly the result of wildfires and clearance to change to agricultural or urban uses. Fires have become frequent in the pine-dominated forests during the last fifty years. Pine forests are not the natural evolution of forest in these areas. They were introduced as an afforestation choice, since pines grow faster than "natural" forest and the trees can be used for timber. The problem with pine forest is the low biodiversity in the understorey and the recurrence of fires. Nowadays in forestry activities, mixed Mediterranean macchia (sometimes mixed with evergreen forest such as Quercus ilex) is preferred, but in many cases pines are still used.
Clearing forest land to change to agricultural uses has occurred in two different periods. The first period was when forest was cleared to cultivate cereal, almond and pastures for cattle. This was the main income in some dry farming and mountain areas, and started in the nineteenth century finishing in the 1950s and 1960s. The process stopped gradually when dry farming became less profitable and migration away from rural areas began. The second period is more recent and results from the conversion of non-cultivated areas to mainly irrigated but also some dry farming. It started in the late 1970s and, even though it is a more marginal process now, has not stopped. The reasons for this are as follows:
The Agri Basin, Italy
The Agri valley is one of the biggest hydrographic basins in the Basilicata Region of southern Italy. It follows a NW-SE direction extending from the eastern sector of the Lucanian Apennine Chain to the Ionian Sea (Ferrara et al., 1996). The Agri basin can be divided into three homogenous sub-areas (upper, middle and lower valley). These areas were identified by their physical-environmental characteristics as well as their socio-economic and demographic features. This division was also confirmed by the pattern of human settlements and land use.
The upper part of the Agri Basin is
characterized by numerous slopes and cultivated terraces. It is well endowed
with woods and vegetation that prevent and reduce the risk of erosion.
Therefore, landslides are quite limited and localized (e.g. the municipality
of Montemurro). Agriculture is characterized by specialized crops, such
as fruit orchards and corn.
Water erosion, with gullies and badlands,
is particularly evident in the Middle Agri Basin, which is dominated by
flysch and sandstone. This phenomenon, however, is due not only to the
nature of the soil, which is composed of clay and marl: deforestation
too, over the past century, has also played a role. Land instability is
particularly evident during short periods of heavy and strong rain. The
hills are covered with pastureland, groves and vineyards, while olive
trees are grown in the lower parts.
A key to the interpretation of land use change may be provided by the analysis of the interventions carried out on the territory in the last 60 years. The 1950s were characterised by the first initiatives of the special intervention for the South of Italy achieved by the CASMEZ (National Economic Body for the promotion of a huge programme of public works in the so-called "Mezzogiorno" of Italy). It was regarded necessary to "settle the southern regions, by means of land transformation, the realisation of public works, land reclamation and irrigation and protection in order to reduce the hydro-geologic instability". In this regard a strong point of the CASMEZ activity during these years was forestry, aimed at achieving water and soil protection and afforestation, favouring employment as well.
Up to the 1960s this policy granted "sustainable and balanced land management", which summoned forces that achieved a significant development project. However, land reclamation and transformation were not always positive. In some cases, such as at Policoro (Lower Agri Basin), the intensity of this reclamation, in association with the development of the coastal plains, coincided with the excessive deforestation of these areas for land conversion, so much that today only 500 ha of wood remain (Pantano Sottano Wood, the last remaining natural lowland forest in Southern Italy at sea level included in the list of Natura 2000 sites). Concerning deforestation, in the lower and middle part of the valley, where forest cover is already less widespread and the climatic conditions less favourable, it faces a slow and continuous reduction in extent. Forest is being replaced by farmland and is therefore becoming limited to a marginal area and role.
The main cause of deforestation and forest degradation in Agri Basin is forest fires. Fires, in terms of both frequency of occurrence and areal extent, are a serious problem in that environment, already subject to degradation and desertification phenomena. Just to give some information about the size of forest fires occurrence in the Agri Basin, in the period 1990-1995 about 1327 hectares of forest, mainly represented by high stands of broad-leaved, were affected by 304 fires. This means 3,88% of the total forested area of the valley and 13,41% of the total number of fires registered in the Region. Fires in the Agri Basin follow a clear pattern which is found in many other Mediterranean Regions and specifically, the highest number of fires are found in areas having the lowest forest index. In the Agri Basin fires occur mainly in areas having forest indices up to 20%, corresponding seemingly to those areas dominated by coppices, i.e. a forest area of rather poor productivity.
The island of Lesvos, located in the Aegean sea and covering an area of 163,429 hectares, can be used as a typical example to analyse the land use evolution and the environmental changes occurring in the last five thousands of years in the Mediterranean region. This island has a long documented history but the first traces of changes are lost in the past, or are hidden behind myths. The first written documents referring to Lesvos prove that the island was covered by dense forests, but after more than 4000 years of human pressure on land serious degradation of natural resources occurred, and in certain places there has been severe desertification.
Lesvos was fully covered by dense
forests in pre-historical periods. This is also revealed by the first
name of the island 'Lasia' which means 'place full of dense forests'.
Lesvos was probably vegetated by the same plant species as nowadays, but
their distribution throughout the island was different. In the ancient
period, pine trees occupied the central and southern-west part of the
island, with oaks the northern-west and the southern part (Tsimis et al,
Information on vegetation, derived from Sapporo's poems (612 BC), illustrates that oak trees were widespread around the Eressos area, in the western part of the island. Alkeos (630 BC) complained in his poems about the long period he was exiled in the dense forest in the area of Messa (south-western part of the island), which was full of wild animals such as wolves. Today this area is highly degraded with only few remnants of forests remaining.
During the Roman period the main economic activities were based on agriculture and commerce. The higher demands for agricultural products from an increasing population caused over-cultivation and overgrazing of the land. Extensive forest clearing took place during this period using wood as fuel and construction material for ships. The analysis of historical documents shows that mountains were still covered with dense forests, full of wild animals. Pastures were extended in hilly areas, and houses were surrounded by vineyards and orchards. Pastures were mainly extended in the western part of the island after clearing the existing pine or oak forests. Olive plantations appeared mainly in the lowland or on slightly sloping areas in the eastern part of the island.
During the Byzantine period (2nd century to 15th century AD), the economy of the island largely depended on agriculture, resulting in significant changes in the types of land use and great exploitation of land resources. Pastures increased significantly by restricting (mainly) oak and pine forests. Oak forests were dramatically reduced by expanding cereal or grass production for stockbreeding. Olive groves were widely expanded, mainly by replacing pine forests. Vineyards were also expanded producing famous wine with a very high reputation in the Byzantine empire.
Summarising travellers' descriptions for the period 15th century to 19th century AD, pastures increased, while pine forests were reduced in order to satisfy the needs of stockbreeding, or to meet ship construction needs. The pine forest located in the western part of the island was transformed into mixed forest and a pastoral zone, while the pine forest located in the eastern part diminished due to the expansion of olive groves. In the northern part, stockbreeding was an important economic activity and pastures increased by removing oak forests.
Olive groves were significantly expanded in the eastern and southern part of the island, mainly by removing pine forests and pastoral land. Inhabitants used to engraft upon the wild olive trees found in pine forests and gradually the olive belt of the island was developed.
A comparative analysis of land use evolution from the ancient period to the 19th century has shown that dramatic changes in land use occurred this periods. In the ancient period forested and pasture land covered 50% and 23% of the total area, respectively, while the agricultural land was restricted to about one quarter of the area (22%). In contrast, forested land has greatly decreased to less than half (23.9%), while pastures increased from 23% to 37.2% in 1886. Of course, pasture land has increased on the island by replacing the low profit agricultural land.
The great change in agricultural land has mainly occurred in the area occupied by olive groves. This area has increased from 2% in the ancient period to 26.9% in 1886. Olive groves have mainly expanded by clearing oak or pine forests.
As in the previous periods, great changes in land use have been observed on the island during the last century (Kosmas et al., 2000). Olive groves were expanded, changing from 26.9% to 41.2% of the total area. Additionally, deciduous oak forests have been significantly expanded from 2.2% to 7.1% of the area due to production of oil used in the island's leather industry. The total area occupied by pine forests remains almost unchanged, even though the geographical distribution has changed. Pine forests have replaced deciduous oak forests or pastures due to their higher ability to recover after fire. Their expansion was influenced by soil depth and parent material. The increase of the area occupied by the olives, oak and pine has been partially compensated by the reduction of areas used for pastures (area decreased from 37.2% to 22.6%.).
One of the most important land degradation
factors in Lesvos is forest fire. Fires have become frequent in the pine-dominated
forests during the last fifty years. Of course large fires have occurred
in the past. Most of the fires can be attributed to people's carelessness.
The majority of fires occur in areas with a high xerothermic index and
moisture deficit. Soil dryness and wind speed are the principal factors
of fire evolution.
Overview of how the indicators inter-relate
The environmental hazards of desertification and deforestation, though distinct, provide mutual feedbacks and are far from being independent of each other. They consequently have similar implications and solutions.
Forests have always been crucial to human subsistence, food security, economic development and well-being. Today, the needs and demands facing the forest environment are more pressing than ever due to demographic growth, environmental concerns, socio-economic development, and appreciation of cultural and spiritual values. As the most important terrestrial ecosystem, forests sustain life through biodiversity, climate regulation (climate changes), water and soil conservation, and more. They have been deeply scarred by human activity during the last millennia, particularly by agriculture, urbanization and natural resource use. The processes of deforestation can therefore be affected by various factors related to physical environment, management and socio-economics characteristics of an area.
There are many causes for deforestation.
The first and most important cause is wood extraction. Wood has always
been a primary forest product for human populations and industrial interests.
Since wood is an important structural component of any forest, its removal
has immediate implications on forest health. Intensive, non sustainable
harvest can lead to severe degradation, even beyond a forests capacity
to recover. Forest fires too contribute to reduce the forest cover land
and induce different ecological and environmental impacts, such as degradation
of the quality of vegetation, erosion of biodiversity, damage to the health
of forest ecosystems, loss of wildlife habitat, air, river and estuarine
pollution and overall ecological retrogression.
Deforestation has many devastating
effects. It affects climate significantly, in part because the forest
plays a major role in the water cycle, recycling rain back into the clouds
as it receives rainfall. As a result, when the land is cleared, flooding
and drought become serious problems, as rainwater travels quickly through
the ground without the forest to regulate it, thereby causing increased
runoff and flooding in the wet season and water shortages in the dry season.
Burning and felling of the forests
is also exacerbating the Greenhouse Effect. Burning releases carbon dioxide
(CO2) into the atmosphere, fostering global warming.
In addition, fewer trees means a lessening of the global forest's ability
to absorb CO2.
Deforestation robs the world of countless species, destroying crucial biodiversity and losing species with potential uses in medicine, agriculture and industry. Biodiversity is important because it contributes to resilience. A world without biodiversity would be more fragile and likely to amplify disturbance into catastrophe through the collapse of ecosystems that had lost keystone species. Thus, biodiversity reduction, combined with climate change, has the potential to spin out of control and to threaten the prosperity of human habitats.
The global effects of deforestation are the main reasons for worrying environmental processes associated with desertification. Among the main causes of soil degradation are: deforestation, overgrazing, over cultivation, water logging and salinization. Whether land will be deforested by natural or human causes can be predicted by assessing various indicators related to reduced vegetation cover and soil degradation, such as clearing forest land, forest fire, pastoral and agriculture activity, level of forest productivity, change in climatic condition etc. Several of these indicators are interrelated and depend on local conditions.