LEDD issues in cropland: Greece and Messara Valley

Editor's note 30 Apr 2012: Text source D111.

Greece has undergone important transformations since the middle of the nineteenth century, when intensification of agriculture really began. Land mismanagement stimulated by demographic dynamics resulted in shifting of the agricultural population (and activities) to marginal areas unsuitable for agriculture. Human impact on the landscape was increasingly negative through conventional large-scale extensive agriculture, negatively affecting soil properties and enhancing erosion processes. The extension of cultivated areas at the expense of forest land has resulted in high ecological alterations due to deforestation and the break-up of the original equilibrium between cultivation, grazing and forestry (Yassoglou 1989).

Crop production in Greece changed significantly after accession to the European Union in 1981. After that, agricultural development focused on maximisation of fodder and cash crop production, which resulted in intensive arable cropping on all fertile, irrigable lands. Further mechanisation and expansion of the irrigated area to 1 million hectares were realized soon after the country became a full member of the European Union (Boyatzoglou 1983). National production targets for major crops (maize, cotton, sugar-beet, etc.) were achieved as early as 1985. Furthermore, the development of fast transportation means and the availability of cheap holidays have encouraged the expansion of domestic and international tourism over the last 50 years. The rapid expansion of tourism in coastal areas has resulted, in recent decades, in the intensification of agriculture on low lands, abandonment of agricultural terraced land on slopes, and an increase in the number and frequency of fires. Demands for water consumption have increased and have affected water availability and quality. Irrigation using water with high salt concentrations has increased the salinity of the soil, rendering land unproductive, abandoned and desertified, particularly in the plains located along the coast.

The concentration of tourism in narrow strips along the coast or around major cities has resulted in the loss of fertile agriculture soils, over-exploitation of natural resources, soil and water pollution and the loss of valuable coastal habitats.  Increasing tourism exerts a significant impact on the environment and results in changes in land-use patterns and resource availability. The most immediate changes in land-use are: (a) Shifts in crop production to meet tourist requirements; (b) a change from traditional to modern crops; and (c) abandonment of low quality land. The need for intensification of agriculture to meet the increasing costs of production, poor quality irrigation water (sea water intrusion) and the lack of proper drainage systems are in many cases responsible for soil degradation resulting from water-logging, salinisation, alkalinisation, and soil erosion.

One of the main effects of land degradation in Greece is the reduction in the area of productive cropland. As good quality soils become scarce, agriculture becomes concentrated in areas with rich soils. This process aggravates the problem of land degradation by increasing inputs in these richer areas, over-loading the land system and creating further problems of land degradation and loss of cropland. The main LEDD issues in cropland in Greece are therefore soil erosion, soil salinisation, and land desertification.  

Soil erosion

Soil erosion in Greece has proceeded at a rapid rate over the past 50 years, following the intensification and mechanisation of cultivation. One of the most spectacular examples of severe soil erosion is the complete removal of the thick dark surface soil horizon that occurred in the hilly Tertiary landscapes of central Greece at rates exceeding 1 cm yr-¹ (Danalatos 1993).  Erosion processes mainly responsible for land degradation in Greece are related to water, tillage, and wind erosion.

As can be seen in Figure 1 below, extensive areas of Greece are at high risk of erosion (Kosmas et al. 2006a). Water erosion is attributed to climatic conditions, vegetation cover and land use management practices. The large scale deforestation of hilly areas which has occurred in recent decades, accompanied by intensive cultivation and overgrazing has resulted in accelerated erosion and the formation of badlands with very shallow soils. Extensively eroded areas are confined to rock formations primarily of Mesozoic limestone and secondarily of acid igneous and metamorphic rocks (Kosmas et al. 2006a).

Figure 1. Areas of high potential erosion risk in Greece. Source: (Kosmas et al. 2006a)

Erosion rates measured on different types of land use, such as cereals, vines, olives, bare land and shrubby vegetation show wide variation depending on the type of vegetation.  Rain-fed cereals cover a large part of the country’s uplands. The most crucial period for soil erosion under rain-fed cereals is from early October to late February, when the soils are almost bare or partially covered by the growing crop. Erosion rates measured in hilly areas located in Thiva (central Greece) and Petralona (northern Greece) ranged from 0 to 52t km-²yr-¹ (Kosmas et al. 1996). Today, production of rain-fed cereals in the hilly areas has declined due to degradation of the soil and cultivation has become more concentrated in the lowlands.

Perennial crops such as almonds and vines occupy extensive hilly areas, although the area under vines has declined during the last decades. These areas require frequent removal of annual vegetation using pesticides (weed control) or ploughing the soil. Such soils remain almost bare during the whole year and the frequent use of heavy machinery negatively affects aggregate stability and organic matter content, creating favourable conditions for overland flow and soil erosion. Soil erosion rates measured in vineyards in the Attica area ranged from 15 to 252t km-²yr-¹ (Kosmas et al. 1996).

The lowest rates of runoff and sediment loss usually occur in olive groves under semi-natural conditions, i.e. maintaining understory vegetation of annual plants, which in combination with the dense leaf canopy of the trees efficiently protects the soil surface from raindrop impact. Under such conditions, water runoff and sediment loss is highly restricted. Soil erosion rates measured in the Attica area and Zakynthos island show that erosion may range from nil to 5.63t km-²yr-¹. Of course higher erosion rates are expected if the soil is cultivated and the annual vegetation is removed.

Tillage erosion is considered one of the most important processes of land degradation in hilly cultivated areas in Greece. It is estimated that 8 percent of the hilly agricultural land in Greece has been abandoned in the last decades due to diminished productivity caused by soil erosion (Kosmas 1999). Tillage erosion exposes subsoil, which may be highly erodible by wind or water, and fills in ephemeral flow areas, acting as a delivery mechanism for water erosion.  Studies conducted over long periods in areas cultivated with cereals, such as in Thessaly (Tsara 2001) have clearly demonstrated that tillage - rather than water erosion - is the most important factor controlling land degradation in hilly cultivated areas. Water erosion in areas cultivated with cereals, vines or olives is responsible for a loss of a few millimetres (1-3) of soil per year or even less (Kosmas et al. 1996). The estimated total annual soil loss in the same areas cultivated mainly with cereals reaches 4-14 mm per year (Kosmas et al. 2000). Soil studies in the Thessaly plain show that soil depth has been reduced by 24-30cm in a period of only 63 years.

Wind erosion is another erosive process, particularly in the semiarid areas of Greece. However, information on the extent of wind erosion in Greece is rather limited. Areas more vulnerable to wind erosion are the islands of the Aegean Sea (Figure 1) and the north-eastern part of the mainland. Strong north or north-easterly winds prevail during the dry period in Greece, creating favourable conditions for wind erosion. The main factors controlling wind erosion are vegetation cover, slope exposure, soil water deficit, grazing, and fires. Mainly steep slopes with shallow soils and semi-arid climatic conditions characterize the Aegean islands and Crete.  The vegetation cover may range from bare to fully covered depending on slope gradient, slope exposure, soil depth, parent material and grazing intensity. Land usually remains bare when soil depth is less than 20 cm. Under dry climatic conditions, perennial vegetation cannot grow, and only annual vegetation is present during the wet period. If the land is grazed, soils remain virtually bare during the summer period, favouring conditions for wind erosion. Fires destroy the existing vegetative cover and contribute to wind erosion by exposing the soil surface to wind action (Kosmas et al. 2006a).

One of the study sites investigating LEDD issues in cropland is located in Crete (the Messara Valley), the largest island of Greece located in the south part of the country.  Soil erosion in Crete is a major land degradation issue. Areas vulnerable to erosion are located in the upper hilly and mountainous areas.  Such areas are characterised by (Kosmas et al. 2006a):

• Steep slopes,
• Relatively shallow soils
• High soil erodibility especially in soils formed on shale parent materials
• Low infiltration rates in soils formed on marl and conglomerate deposits
• Moderate to poor  plant cover
• Removal of understory annual vegetation in olive groves
• Burning and overgrazing of natural vegetation
• Clearing of natural vegetation and planting olives without actions to prevent soil erosion
• Heavy rainstorms occurring frequently in the area

The effects of soil erosion caused by surface water runoff, tillage or wind are either on- or off-site.  On-site effects can be either long-term, such as the progressive degradation of soil or short- to medium term such as the effect on crop production. The main on-site effects in Greece are the removal of fertile topsoil, removal of organic matter, leaching of plant nutrients, exposure of large amounts of rock fragments or bedrock on the surface, reduction of crop production, desertification and land abandonment.  Off-site effects can be either short- or medium- to long term such as damage to crops and infrastructure from uncontrolled runoff and flooding, siltation of channels and reservoirs, environmental alterations of wetlands, lakes and estuaries, decline of the economy of local communities, and migration of local people (Kosmas et al. 2000; Tsara et al. 2001; Kosmas et al. 2006a).

Soil salinisation

Salinisation is an important process of land degradation and desertification in Greece, especially in irrigated lowlands with poor drainage conditions. The basic conditions that promote salt concentration in soils are: irrigation with low quality water, poor drainage, and dry climatic conditions favouring a negative water balance. Due to warmer and drier conditions in the last few decades, aridity and drought hazards for growing plants have increased (Sala et al. 1998). Therefore, irrigation has been extended over large areas for efficient agricultural production to meet increasing market demands. According to expert assessments, about 15 percent of the present irrigated lands face salinity/alkalinity problems. Problems of salinisation in Greece are expected to become more severe in the future if: (a) the area of irrigated land expands, (b) new more productive and water consuming plant varieties are introduced into cultivation, and (c) the climate becomes warmer and drier (Yassoglou 1989).

Typical management regimes for protecting areas from salinisation are surface drainage and irrigation of soils with good quality water. Surface drainage is mainly achieved by the construction of surface ditches. In the semi-arid regions of Greece under irrigation, drainage ditches are necessary for removing excess water, and required for leaching of undesirable salts from the soil and disposing of excess rainfall. Ground water recharge is another management practice to improve ground water quality and to avoid soil salinisation. For example, on the Argolis plain, which is facing severe problems of intrusion of brackish water and soil salinisation, recharge of the aquifers is achieved by supplying good quality spring water through wells during the winter period (Yassoglou 1989).

Soil salinisation risk in Crete is mainly confined in the lower plain areas along the coastal line, such as the Messara valley, Kissamos and Kolimbari plain areas and the Ierapetra plain.  The soils of these areas are mainly characterized as poorly to very poorly drained with ground water tables fluctuating between 30 to 150cm during the year. Other areas are sensitive to salinisation only if poor quality water is used for irrigation. Studies carried out in previous EU research projects, such as DESERTLINKS (contract No: EVK2-CT-2001-00109) and DESIRE (contract No: 037046) have shown that important indicators related to salinisation risk and which are affected by land management characteristics are; frequency of flooding, land use type, and efficacy of reclamation. As the frequency of flooding increases, salinisation risk also increases. Salinisation risk decreases as land use type changes from pasture, wetland, recreation area, and agriculture (DIS4ME, 2005). Reclamation of salt-affected areas was mainly related to the presence of a drainage network. As the efficacy of reclamation increased due to lowering of ground water, desertification risk decreased. Other important indicators related to salinisation risk are; distance from seashore, elevation, water quality, ground water depth, drainage and rainfall.

Land desertification

Desertification of land in Greece is a phenomenon that has been taking place for three millennia and is causing loss of land productivity and available water reserves. Such extreme degradation of these two important resources occurs in hilly areas cultivated with olive trees, vines and cereals.  Areas threatened by desertification cover approximately 34 percent of the total area of Greece (Figure 2); 49 percent is moderately affected; whilst 17 percent is at low risk. Areas particularly vulnerable to desertification are the eastern Peloponnese, Sterea Hellas and Thessaly; central and southern areas of Macedonia; central and eastern Crete; and the Cyclades islands in the Aegean. In spite of the adverse physical conditions observed in Greece, desertification proceeds only if land is not managed appropriately.

Figure 2. Potential desertification risk map of Greece. Source: (Greek National Committee for Combating Desertification 2001)

Processes of desertification in Greece are either physical or chemical. The dominant physical process is soil erosion, which is initiated by the destruction of the vegetative cover and affects marginal sloping lands. Soil salinisation and sodification is the dominant chemical process. It is localized but affects valuable low lands and is the result of poor irrigation practices. Unsustainable human actions can easily trigger desertification in the semi-arid and in the dry sub-humid zones of Greece, because several land parameters are unfavourable and/or sensitive to such actions (Yassoglou and Kosmas 2000):

• Climate and bioclimate is characterized by large moisture deficits, temporal variability and frequent extreme events.
• Landscapes are rugged, with steep slopes, large elevation differences and are highly dissected by torrential steams.
• Surface geology favours formation of soils which are sensitive to drought and erosion.
• Hydrology is characterised by the scarcity of surface and ground waters, and by the need to bring water from elsewhere to satisfy demand.
• Soil formation rates are much slower than soil loss, resulting in inadequate rootable depth and water storage capacity on sloping land.
• Out of phase rainfall and vegetative periods.
• Four millenniums of human land use and frequent abuses of land.

The natural resources of Crete have been overexploited for many years. Large scale deforestation of sloping lands accompanied by intensive cultivation and overgrazing have resulted in accelerated erosion and the formation of badlands with very shallow soils through the progressive inability of the vegetation and soils to regenerate themselves. Based on the land desertification risk map of Greece (Greek National Committee for Combating Desertification 2001), more than 50 percent of the island of Crete is characterized by high desertification risk.  The high erosion rates occurring in the island are attributed to the climatic conditions, to topographic characteristics and to the generally poor vegetation cover. Semi-arid landscapes by definition are water-limited and therefore are potentially sensitive to environmental change, with associated effects on biomass production. These areas become vulnerable to erosion because of the reduced protection of the ground surface by vegetation from heavy rains with high intensity.

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