Homepage | DESERTLINKS Homepage
| © DESERTLINKS 2004
English-EN | Español-ES | Italiano-I | Ελληνικά-GR | Portuguese-PT
|Desertification Indicator System for Mediterranean Europe|
Lead author: Constantinos Kosmas <email@example.com>
With contributions from: Maria José Roxo and Pedro Cortesao Casimiro <firstname.lastname@example.org>, Jorge García Gómez <email@example.com>, Giovanni Quaranta, Rosanna Salvia <firstname.lastname@example.org>
of reasons for land abandonment and why they are an issue in the context
The extensive deforestation and intensive cultivation of hilly areas around the Mediterranean region that have taken place since ancient times have led to soil erosion and degradation. In some years the prevailing weather conditions, especially early in the growing period for crops, may be so adverse that the soils remain bare, creating favourable conditions for overland flow and erosion. The soils of those areas developed on Tertiary and Quaternary geological formations usually have limiting subsurface layers, such as petrocalcic horizons or bedrock. With high erosion rates under hot, dry climatic conditions, the soils can not economically support rainfed crops, and this leads to land abandonment and desertification.
As the soil is eroded, land use is usually shifted from agriculture to pasture due to low productivity of the various agricultural crops. Pasture-land in the Mediterranean region is often defined as abandoned land today (Martinez-Fernandez et al., 1995; Lopez-Bermudez et al., 1996; Roxo et al., 1996; Puigdefabregas et al., 1996). These authors have used the terms 'abandoned land' and 'grazing land' simultaneously. Grazing or hunting on abandoned land is considered as a traditional use in the Mediterranean region. Only a few areas which have been fenced or strictly controlled by land owners remain free of grazing by nomadic or permanent flocks and herds. Almost all the natural vegetation in the Mediterranean basin, apart from a few forests, is grazed to some extent (Clark, 1996). Therefore, the term 'abandoned land' is used to include areas previously cultivated but where farming has ceased and the natural vegetation has been allowed to grow under various intensities of grazing. Similarly, under adverse climatic conditions, irrigated agricultural land may be abandoned if drought occurs or water supplies are reduced.
The effects of land abandonment on land quality and desertification may be positive or negative depending on the soil and climatic conditions of the area. Soils under good climatic conditions that can sustain plant cover may improve with time by accumulating organic materials, increasing floral and faunal activity, improving soil structure, increasing infiltration capacity, and therefore decreasing the potential for erosion (Trimble, 1990). Martinez-Fernandez et al. (1995) have reported a positive effect of land abandonment. Soils abandoned over a period of ten years approached the characteristics that they had before cultivation. The abandonment of this area resulted in improvement of soil characteristics such as enhanced organic matter content, water retention capacity, aggregation and structural stability. Jaiyeoba (1995) and Unger (1997) reported a deterioration of soil fertility under cropping and concluded that the soils under various types of agricultural land uses contained less organic matter content, total nitrogen, exchangeable bases and CEC than similar soils under natural vegetation. Lopez-Bermudez et al. (1996), in a study along a hillslope under various conditions of abandonment and cultivation in southern Spain, showed that soils on fallow areas or land abandoned for 4-10 years had a progressive recovery of vegetation cover and a significant increase in organic matter content, aggregate stability, water holding capacity, and hydraulic conductivity. However, in the case where vegetation cover remains poor, the erosional processes may be very active and the regeneration of the abandoned lands may be impossible. Many authors have demonstrated that in a wide range of environments both runoff and sediment loss decrease exponentially as the percentage of vegetation cover increases (Elwell and Stocking, 1976; Lee and Skogerboe, 1985; Francis and Thornes, 1990). Kosmas et al (1997) showed that in the hilly Mediterranean shrublands runoff and sediment loss increased with annual rainfall, decreasing down to 280-300 mm, and attributed this to a decrease in vegetation cover. For areas with rainfall below this threshold, erosion decreased with increasing rainfall. Martinez-Fernandez et al. (1996) demonstrated that the post-abandonment land use is of major importance for the evolution of land characteristics after abandonment.
A major aim in studying the ability of an ecosystem to return to the original state after disturbance (resilience) is the prediction of the response of such a system to a variety of natural and human-induced disturbances (Dell et al., 1986; Westman, 1986), such as drought, fire, grazing, vegetation clearance and cultivation. Moderate grazing pressure of land abandoned following agricultural use may lead to partial rejuvenation of vegetal communities with a high diversity index (Fox and Fox, 1986; Martinez-Fernandez et al., 1996). The decline in vegetation due to overgrazing can result in a loss of herbaceous plant families (Leguminosae, Gramineae), plants whose structures help to maintain soil structure. These plant families can both protect the soil surface from raindrop splash effects and reduce erosion rates by increasing soil aggregate stability. Disturbance by grazing does not result in the complete removal of vegetation in the same way as an intense fire does. The impact of fire is greatest in those areas with the lowest fire frequencies (Fox and Fox, 1986). An increase in fire frequency leads to fewer plant species, caused by the loss of those that cannot persist when fires are too frequent (Fox and Fox, 1986; Grove and Rackham, 1996).
The Lower Inner Alentejo, Portugal - left
bank of the Guadiana River
This area is located in the South-Alentejo peneplain, a polygenic erosion surface, last carved in the beginning of the Quaternary period. The present landscape, rolling topography, evolved with the river incision, related to the regressive erosion from the main river, the Guadiana, and the highly impermeable characteristics of the lithology, corresponding to Paleozoic formations. Consequently, there is a remarkable hill top iso-altitude, at around 180-220 m.
The landscape is characterized by flattened summits and gentle slopes, and simple slope profiles never exceed 25 %. However, there are in some places steep slope angle sectors, due to lithology contrasts (bedrock outcrops) and where streams are deeply incised along the Guadiana, in many cases following fault and fracture structures. Morphology and soils are a consequence of the metamorphic nature of the parent material, where Lower Devonian red schists dominate, overlaid by an extremely dense drainage network. The landscape is often characterized by vast flat surfaces, on Carbonic yellow schists.
Soils are Mediterranean red soils (Vx), shallow 10 to 30cm, or very shallow, less than 5cm in depth. In the Vale Formoso area, the observation of some open profiles revealed that the Ap horizon lies normally on a transition CB horizon, made of reasonably weathered bedrock (Vacca and Roxo 1994). Recent textural analysis indicates for the Ap horizon 26-34 % of coarse sand, 28-32 % of fine sand, 12-24 % of silt and 12-24 % of clay ("franc soil"). Soil organic matter contents are extremely low, averaging 2 %.
The climate is Mediterranean, continentally enhanced, rainfall being concentrated in the autumn and winter (67 % of the total annual rainfall on average), followed by a long hot and dry season from May to September, that represents a severe hydric stress for vegetation, which consequently has sub-xerophytic characteristics. Rainfall variability is extreme, annually (ranging from 1041.4mm in 1989/90, to 236.4mm in 1980/81) and monthly (total annual average, for the 1931/90 period was 562mm), and sometimes concentrated in a few, very violent, events with thunderstorms. Periods of drought, sometimes lasting several years (e.g. 1980-82, 1990-93) have a critical socio-economic impact, as do the less common extremely wet years. The average annual air temperature is 16.3°C with no significant gradient in the target area. Nonetheless average minimum and maximum temperatures frequently rise above 30ºC in July and August (maxima around 43-44ºC several days each year), and stay as low as 4ºC in January and February.
The analysis of land use evolution in the lower inner Alentejo (Casimiro 1993, Roxo 1994), over the last 300 years showed a dramatic increase in agricultural land by replacing forest and shrubland. In the beginning of the 19th century agriculture was already widespread, with cereal cropping and grazing. The intensity of man-induced environmental degradation, especially on areas that were not suitable for uses either than low density grazing, started back in the Middle Ages.
Between 1900 and 1950, almost all the remaining natural vegetation areas (hill tops, steepest slopes) were transformed into cereal fields, as colonization was encouraged by communal land division and donation. Most of the Inner Lower Alentejo became a tree-less area of cereal monoculture, with some patches of Quercus (oak) and shrubland. This was further stimulated by a governmental policy called the Wheat Campaign, that supported farmers (with seed, fertiliser, machinery and subsidies). By the 1950s there was an official realisation that soil degradation had reached serious proportions.
Between 1950 and 1985 agriculture started to decline, people started emigrating to the main cities and abroad, and depopulation began. Land abandonment became a reality. After Portugal joined the EU in 1986, wheat production costs were 3 to 4 times higher than elsewhere in northern Europe. Currently the tendency is towards land-abandonment or to a conversion of agricultural land, through reforestation with endogenous species (Quercus suber, Quercus ilex), or in most cases pine trees (Pinus pinea). There is, however very recently a new interest in cattle: cows mostly, in addition to sheep already there for a long time, which has been further supported by the CAP subsidies.
In this way vast areas that had been abandoned are being reconverted into pastures, both natural and improved. However, this process has critical implementation phases, as the soil is subject to practices favouring the degradation of their physical and chemical proprieties, such as when shrubs are cut and destroyed, or when ploughing to prepare the pastures for seeding is carried out too deeply.
Nonetheless, according to the edapho-climatic characteristics of the Lower Inner Alentejo, land abandonment allows an improvement of soil conditions, and favours the appearance of natural vegetation species (annuals and perennials), that minimize and tend to almost neutralize the soil erosion processes. The experimental results obtained, surveying 20 x 20 metres plots in abandoned areas of different ages (between less than 5 years and more than 25 years), carried out within the framework of the MEDALUS II Project, revealed (in terms of soil properties) a clear recovery in organic matter content and drainage conditions, as well as a better development of the soil vertical profile. The vegetation cover became more dense, with an increasing number of species, the longer the time since abandonment.
It is important to note, or bear in
mind, that the degree of recovery relies and depends on the degradation
status in the beginning. Even in situations where abandonment happened
a long time ago, there are situations where the high degree of degradation
and desertification are only associated with an extremely slow or almost
impossible regeneration process.
The relationship between soil erosion by water and land abandonment has been tested in the Vale Formoso Erosion Centre, with one Wischmeier erosion plot (20 x 8.33 metres) that was abandoned in 1989. The results clearly show a decline in erosion rates.
Land abandonment processes are similar in many areas across Mediterranean Europe. In the Guadalentín basin in southeast Spain, the main factors leading to land abandonment, especially in dry farming areas, are socio-economic. The land abandonment processes are most likely to happen in dry farming areas because irrigation activities, which require smaller land area and yield high profits, are less likely to be abandoned.
In general, dry farming is becoming increasingly marginal. EU subsidies are necessary to assure sufficient farm income, so changes in EU regulations affect farming decisions. The farmer's age is another important factor. New generations are reluctant to carry on the activity as incomes are not assured and they prefer to work in other economic sectors, often out of the rural areas. Fragmentation of land parcels is also important. Larger areas are needed for a dry farming parcel to be profitable but land partition processes (from fathers to sons) makes it more difficult.
In the specific case of the Guadalentín area in Murcia, parcels of land under almonds are the most likely to be abandoned. This is the most important cultivation in dry farming areas, commonly in stepped areas or marginal soils. Some of the areas have changed to olive cultivation because more EU subsidies are available for olives. Low profitability due to low market prices, climatic events such as drought and frost, competition from markets in other countries all lead to land abandonment.
Land abandonment may lead to deteriorating or improving conditions of plant cover depending on the soil and climate conditions of the area. It is postulated that a natural soil restoration process may occur in well-vegetated environments, although its occurrence may be delayed because of adverse conditions related to soil conditions at the time of agricultural set-aside, and periodic wildfires. In contrast, cultivated fields with low soil fertility are prone to enhanced soil degradation processes, either if inadequate or no management practices are applied, or if they are abandoned at this stage.
The loss of vegetation and the progressive inability of soils to regenerate adequate vegetation cover due to water erosion after repeated wildfires have already led to severe degradation and desertification of extensive hilly areas in the Mediterranean region. Furthermore, in areas affected by wildfires or abandoned agriculture, secondary succession has become dominant, resulting in a less diverse shrubland and decrease in species richness and cover. If almonds are abandoned there is a particulartly high risk of soil erosion and wildfires.
It is important to know not only the present characteristics of the area that is abandoned but also the history of land use (or land use evolution) in the area. These factors will give an idea about the processes that can occur after abandonment. In fact, the areas that are abandoned first are those less profitable or more difficult to be cultivated. These are the marginal and steeply sloping areas, and abandonment has relevant consequences for several degradation processes. In these areas natural restoration is very difficult, and high aridity and soil water stress make it even more difficult. The low organic matter content, low vegetation cover, soil slope conditions and climatic conditions (less than 300 mm of rainfall/year, with torrential rain, drought events and strong insolation of nearly 3000 hours/year), result in a high moisture deficit and give an extremely arid character to the basin. These conditions make erosion processes common and intense. The natural restoration processes in these areas are really slow, almost impossible, and persistent erosion processes can lead to severe soil degradation and desertification.
To understand the reasons for land abandonment in the Agri Basin it is necessary to look at changes in land use, especially agricultural land use.
Major changes took place in the landscape of the Agri basin in southern Italy from the last century (Morano, 1994, Storia di una società rurale: la Basilicata nell'ottocento, Laterza, Bari), when massive deforestation took place (Tichy, 1962). In comparison to the available 290,000 ha of forest cover for the whole of Basilicata at the beginning of the last century, it is estimated that 17% was destroyed from 1800 to 1860, 20% from 1860 to 1908, and 19% from 1908 to 1930. Despite measures adopted by the French and then the Bourbonic authorities, more than half of the forest cover of Basilicata was destroyed during this period. This was principally due to population growth, but also to major social transformations during the last century. Indeed the transformation of the feudal system into a new bourgeoisie of land owners dominated by the latifundium system was also responsible for land clearance, and for short term interests. After this period of intense degradation, the forest cover stabilised. First, a special law was adopted in 1923 (Law n. 3267, Dec. 20, 1923) to stop the destruction of forest and promote rehabilitation measures. However at the same time a "wheat battle" was engaged in to counterbalance the prohibition of emigration and supply the growing needs of the population, resulting in expansion of the cultivated area and reduction in the impact of soil conservation measures.
Despite the "wheat battle", the forest cover was stabilised from the 1920s until the 1980s when a noticeable increase occurred as a consequence of regional incentives to reafforest erosion prone areas, supported by the Italian government and regional authorities. Parallel to the re-afforestation process which affected mainly public lands, deforestation continued on private lands, leading to an increase in the area under cultivation. This was a consequence of EU measures in the agricultural sector and has led, since the fascist period, to a continuous increase of arable lands for cereal cultivation.
In the last decade the Agri Basin, following the same trend as the rest of Basilicata, has suffered a strong contraction in the values of the Total Agricultural Area (TAA) and also in those of the Utilised Agricultural Area (UAA).
Such reduction in agricultural area affects the types and distribution of crops. Considering the type of cultivations that requires more consistent intervention both in intensity and frequency of action (i.e. sown and arboreal crops) and looking at changes in this value within the total surface, it can be seen that in the whole Agri Basin, there is a progressive decrease in the degree of agricultural use of the territory.
A comparison between this information and the trends also in sown and arboreal crops in relation to Utilised Agricultural Area allows a clearer interpretation of what has happened in the different sections of the basin.
In the Lower Agri Basin, although there has been a contraction of about 10% in agricultural land use, there has been an intensification in the primary sector. 91% of the UAA is in intensive crops. In reality there is strong competition for the use of soil resources between the agricultural sector and the other economic sectors, especially tourism, and this explains the progressive reduction in areas shown.
In the Middle Agri Basin both the values show negative signs underlining extensive agricultural activity and progressive abandonment of the land. For the Upper Agri Basin there is similar behaviour, although less marked in comparison to the Lower Agri Basin.
A further confirmation of the degree of marginalization of the territory of the Middle Agri Basin comes from evidence of the presence or absence of agricultural families living on the farms. The data from the last census shows that only 39% of the farms in this area with residences are in fact inhabited. This is extremely worrying if the maintenance of the landscape is related to the presence of the farmers on their land.
The Greek island of Lesvos is located in the north-east part of the Aegean sea covering an area of 163,429 hectares. It is characterized by a variety of landscapes, lithological units and climatic conditions. The land is covered by a number of land-uses representative of the Mediterranean region e.g. semi-natural forests and shrubland, and agricultural land that is now largely being abandoned. The climate of the area is characterized by strong seasonal and spatial variations of rainfall, and large oscillations between minimum and maximum daily temperatures, typical of Mediterranean climatic conditions. A gradient in rainfall occurs across the island with the average annual rainfall in the study sites ranging from 677 mm (eastern part) to 415 mm (western part). The average air temperature is 17.7°C without any significant gradient across the island. The greater part is already badly degraded and desertified, and the rest is experiencing a slow but constant deterioration of its natural resources.
The analysis of land use evolution
on Lesvos over the last 4000 years has showed a dramatic increase in agricultural
land replacing forested land. Many of the areas that once supported forests
were cleared in order to sustain agriculture, but since measures for soil
conservation were insufficient these areas were severely eroded and consequently
abandoned. Overgrazing and fires further destroyed the natural vegetation
cover and prevented its regeneration. Now these areas are mainly unproductive,
sparsely populated and desertified. The socio-economic and political background
determined human impact on the environment and it was increasingly negative,
stimulating desertification. About 45-50 years ago areas extensively cultivated
with cereals, vines and olives were abandoned due to low productivity.
After the abandonment, the area was moderately grazed and the growing
shrubs were occasionally cleared by setting fires.
Detailed studies have been conducted on the island of Lesvos on the effect of land use change from agricultural arable to grazing non-arable (abandoned) on soil properties and vegetation establishment. Measurements related to land protection after abandonment, such as fertility status (organic matter content, pH, cation exchange capacity, exchangeable potassium and sodium), water storage capacity (soil water retention characteristics, soil depth), erosion resistance (soil aggregate stability), and vegetation characteristics (plant species, plant cover) were conducted (Kosmas et al., 2000). The obtained data indicated that soil pH, and cation exchange capacity were slightly affected after abandonment as compared to the cultivated soils. The amount of exchangeable sodium and potassium was higher in cultivated soils. Organic matter content and soil aggregate stability were greatly enhanced in most of the abandoned soils. Physical characteristics of the parent material greatly influenced the establishment of the natural vegetation. A critical soil depth of 25-30 cm was measured, below which the natural perennial vegetation cover was rapidly reduced under the prevailing climatic conditions of the study area. The rate of reduction was related to the parent material. Perennial vegetation may not be supported if the soil is eroded below a crucial depth, ranging from 4-10 cm, but depending on the parent material. The study showed that soil depth is the most important parameter which has to be considered in planning land use change from agricultural arable to grazing non-arable land or to abandoned land.5 top
Overview of how the indicators inter-relate
The process of land abandonment can be affected by various factors related to the physical environment, and management and socio-economics characteristics of an area. Land abandonment from agriculture can be predicted by assessing various indicators related to land productivity and farmer income, such as soil depth, parent material, slope gradient, amount and distribution of rainfall, existing subsidies, population migration, water availability and accessibility. Several of these indicators are interrelated and depend on local conditions.
By definition, semi-arid landscapes are water-limited and are therefore potentially sensitive to environmental change and effects on plant growth. The water available for plants to grow depends on the climatic conditions (rainfall, evapotranspiration) and the soil water storage capacity. The water storage capacity of a soil is defined by the water holding capacity (WHC) of each soil layer and this is related to soil texture, soil depth, amount of rock fragments and parent material. As shown in the figure below, wheat biomass production measured in hilly areas of Greece is clearly related to soil depth. Under dry climatic conditions, which generally prevail in Greece, production of rainfed wheat rapidly declines, cultivation is no longer profitable where the soil depth is less than 30 cm and land has to be abandoned. It can be inferred from this figure that the depth of 25-30 cm must be considered as a critical depth, under the existing soil and climate conditions of the study area in which hilly cultivated areas must be abandoned allowing the natural vegetation to grow for adequate protection of the land. It must be emphasized that this critical depth can be used for land use planning and protection of the environment only under the prevailing soil and climate characteristics of the area. Generalization of this conclusion may be risky for other areas.
In the last four decades,
favorable soil and climatic conditions and the availability of ground
or surface water have resulted in intensive farming of the lowlands of
the Mediterranean. The development of high input agriculture in the plains
provided much higher net outputs than those obtained from hilly areas
or terracing agriculture. Furthermore, the developments of fast means
of transportation and the availability of cheap holiday offers have encouraged
the expansion of domestic and international mass tourism over the last
30 years. The rapid expansion of tourism throughout Mediterranean Europe
especially along the coastline has resulted in the intensification of
agriculture on the low lands, abandonment of agricultural terraced land
on the slopes, and increase in the number and frequency of fires. The
high demands for water consumption or other economic activities have increased
the price of water and the cost of agricultural production, while in many
cases water of low quality is used for irrigation (water with high electrical
conductivity). Irrigation using water with high salt concentrations increases
the salinity of the soil, resulting in an unproductive abandoned and desertified
land, especially in plain areas located along the coast.
Soil erosion due to surface
water runoff, winds and tillage operations is a serious threat to soil
quality and productivity. Tillage operations transport large amounts of
soil from convex slopes and deposit it in concavities in hilly cultivated
areas. The effects of soil erosion on productivity depend largely on the
thickness and quality of the topsoil and on the nature of the subsoil
(Frye et al., 1985; Acton and Padbury, 1993). Productivity of deep soils
with thick topsoils and excellent subsoil properties may be virtually
unaffected by erosion. However, most hilly soils are shallow or have some
undesirable properties in the subsoil, such as a petrocalcic horizon,
or bedrock that adversely affects yields. In either case, productivity
will decrease as the topsoil gets thinner and undesirable subsoil is mixed
into the surface horizon by tillage, or as water-storage capacity and
effective rooting depth are decreased.
Many uplands have been
terraced for cultivating cereals, vines, olives and other crops. In many
cases the terraces, which have been constructed with stones, are hundreds
or even thousands of years old. Individual crescentric terraces have been
carefully constructed for individual trees. The soil was removed from
other places to fill these terraces. This conservation management requires
high labour costs to maintain the terraces. In the last few decades the
value of such terraces has decline markedly because of the difficulty
of access and because they cannot easily be cultivated with tractors.
At present most of these areas have been abandoned, and many terraces
have collapsed causing a rapid removal of the soil by runoff water, except
in some cases where the stone walls are protected by the roots of fast
growing shrubs and trees. Maintaining such terraces appears a very expensive
practice compared to most other alternatives for soil erosion control.
The process of land degradation can be greatly accelerated by high densities of livestock which lead to vegetation degradation and, in turn, to soil compaction. An obvious consequence of overgrazing is the increase in soil erosion, since the gradual denudation of the landscape exposes the soil to water and wind erosion. Under such management conditions, soils of these areas usually have limiting subsurface layers, such as petrocalcic horizons or bedrock, and under high erosion rates and hot and dry climatic conditions, the soils cannot economically support sufficient crop production, leading to desertification and abandonment of the land. Overgrazing of these climatically and topographically marginal areas, especially when affected by fires, constitutes a desertification-promoting land use, further deteriorating the existing land resources.
Low land productivity combined with small farm size has resulted in massive migration of people from rural to urban areas. The land was abandoned from agriculture or was used in some way by the remaining farmers. Subsidies are allocated today for specific types of crops or land uses (such as olives, cereals and pastures) greatly affecting the intensity of the land use, the land use decision-making, and the income of the farmers. For example, in some cases the productivity of hilly areas with shallow soils and semiarid climatic conditions cultivated with cereals is very low, and not economically feasible without support. In some cases such land must be abandoned or used as pasture if subsidies allocated per hectare are withdrawn. On pastures the number of animals has increased significantly in the last few decades due to allocation of subsidies per animal. In some cases subsidies have definitely adversely affected land degradation and desertification of abandoned land.