Desertification and land degradation: origins, processes and solutions

Table of Contents

1. Authors and acknowledgements
2. Introduction
   1. Aims
   2. Definitions and key concepts
   3. Synthesis of previous and ongoing projects
   4. Key issues and outline
3. Evolution of desertification in the Mediterranean
   1. Climatic fluctuation
   2. Human influence
4. Primary drivers of desertification
   1. Biophysical and socio-economic causes
   2. Pathways of desertification
   3. Driving forces in the Mediterranean
   4. Synthesis of causes for DESIRE hotspots
   5. Conclusions
5. Processes and consequences of desertification in the Mediterranean
   1. Socio-economic and political factors
   2. Biophysical processes
   3. Conclusions
6. Indicators: monitoring desertification
   1. History of monitoring
   2. Indicators
   3. Monitoring and mapping techniques
   4. Concluding remarks
7. Modelling desertification
   1. Definition of modelling
   2. Model types
   3. Biophysical modelling
   4. Socio-economic and participatory modelling
   5. Gaps and progression in modelling
8. Solutions
   1. Biophysical solutions
   2. Political and socio-economic solutions
9. References

Biophysical processes

Erosion

Erosion is a natural phenomenon occurring over much of the Earth's surface, but its extent and intensity have been greatly increased by human activities (UNEP, 1997). It is the detachment, entrainment and transport (and deposition) of soil particles caused by one or more natural or anthropogenic erosive forces (rain, runoff, wind, gravity, tillage, land levelling and crop harvesting) (Boardman and Poesen, 2006). Erosion is subdivided in two main processes: water erosion and wind erosion. Erosion directly affects the area where the process occurs but may also have negative off-site effects in areas that receive the eroded material such as reservoir sedimentation or through dust storms that can travel hundreds of kilometres from their source area (UNEP, 1997). Erosion embraces a complex set of processes and interacting factors. Whether or not erosion takes place, and with what intensity, depends on the balance between erosivity and erodibility. The former variable is the potential ability of rain or wind to cause erosion and it is controlled by factors such as wind strength and rainfall intensity. Erodibility is the vulnerability of the soil to erosion, influenced by physical soil characteristics, land use and management techniques. There are many case studies of soil erosion. A useful textbook is by Morgan (2005). An extensive review of erosion in Europe is given by Boardman and Poesen (2006).

Salinization

Salinization is the concentration of salts in the surface or near-surface zones of the soil and is a major process of land degradation (Thomas and Middleton, 1993) It is a natural process resulting from high levels of salt in the soil, originating from landscape features that allow salts to become mobile (movement of the water table) and from climatic trends in favour of salt accumulation. Alternatively, it may occur resulting from management practices (USDA, 1998). The latter, human-induced, salinization is often referred to as 'secondary salinization' to distinguish it from naturally affected soils (Thomas and Middleton, 1993). Salinization occurs when the following conditions occur together (USDA, 1998):

• presence of soluble salts in the soil
• high water table
• high rate of evaporation
• low annual rainfall

Typical natural spots in semi-arid areas where salinization occurs are areas that receive additional water from below the surface which evaporates, leaving the salts behind, as at the base of hillslopes, the rims of depressions and the edges of drainageways and in flat, low-lying areas surrounding shallow water bodies (USDA, 1998). Human-induced salinization can be due to poor cultivation techniques; the indirect effects of irrigation schemes; vegetation change; sea water intrusion and disposal of saline wastes (Thomas and Middleton, 1993). A well-known example is the construction of the Aswan High Dam after which year-round irrigation was possible and the yearly flushing by the floods was halted (Conacher and Sala, 1998).

High levels of salt in the soil affect the ability of plant roots to take-up water, and the effect on plants is similar to that of drought. In the information sheet of the USDA (1998), some indicators of soil salinity are given as well as some suggestions of how to manage salinity problems. Saline soils cover an area of 1900 km² in the Iberian peninsula according to Conacher and Sala (1998). In the eastern Mediterranean and North Africa, there is progressive salinization of soils mainly in irrigated areas and low-lying areas which are subject to strong evaporation and rising groundwater tables (Conacher and Sala, 1998). For an overview of soil salinization in the Mediterranean see Postiglione (2002).

Land use and vegetation change

For the most part, vegetation change is the result of some degradation process, such as salinization or overgrazing or of human action, such as land use change due to the influence of subsidies or market fluctuations. As such, changes in vegetation can be both a cause and consequence of degradation. These changes in vegetation can subsequently lead to (further) desertification. However, changes in vegetation type or cover can also be an efficient remedy against degradation.

Land use changes are the result of environmental factors, but also complex political, social and economic processes play a role (Turner et al., 1995).

Agricultural change

Agricultural change and associated land management techniques can have a large effect on the status of an ecosystem and can be a driver of desertification. Almost all changes in agriculture use of a particular piece of land are driven by economic factors. The change from one particular crop to another brings with it other management and cultivation techniques (e.g. tillage). This change between crops can be induced e.g. by subsidies on certain crops. The change from agriculture to other forms of land use or vice versa can also induce degradation problems. An example of the former is the abandonment of former agricultural areas with the resultant collapse of conservation structures like terraces. Land management includes the conversion of rangeland or forested land to agricultural use. There is a wealth of literature on the causes and effects of land use change (e.g. Lambin et al. 2001; Taylor et al. 2002; Lambin and Geist 2006; Symeonakis et al. 2007).

Overgrazing and overexploitation

Various definitions of overgrazing are used and misused in scientific literature and the term is usually value-laden as it implies grazing at a higher level than desired relative to a specific management objective (Mysterud, 2006). In his paper concerning the role of overgrazing in the management of large herbivores, Mysterud (2006) gives several definitions from the points of view of various ecosystem management options. A general definition is 'an excess of grazing animals that leads to degradation of plant and soil resources'.

According to the Global Assessment of Human-induced Soil Degradation (GLASOD) survey conducted in 1990, overgrazing is the most important cause of degradation in dryland areas of Australia, Africa, Europe and Asia (UNEP, 1997). The reasons for concentrating too many livestock in certain areas, leading to loss of vegetation cover and trampling of the soil surface, may be political, cultural or socio-economic, while they may also result from environmental factors such as drought and the distribution of vector-borne diseases (UNEP, 1997). Overgrazing around settlements in North Africa is often related to the settling of the former nomadic herders.

Deforestation

Little of the indigenous vegetation remains in many parts of the Mediterranean Basin due to its long period of human settlement (Conacher and Sala, 1998). In common with many Mediterranean seasonally arid areas in Portugal, the indigenous mixed oak forest in Spain has been replaced almost entirely by Cistus-dominated matorral on hillslopes and by cultivated dryland farming on the plateaux (Conacher and Sala, 1998). Until the end of the 19^th century, deforestation and exploitation of the residual forest constituted the main forms of degradation in southern France and Corsica (Conacher and Sala, 1998). These problems have been superseded by forest fire, floods, soil erosion and air and soil pollution. Deforestation seems to have caused desertification problems, but as deforestation is no longer a major issue in recent times (rather, reforestation is being done in many areas), this seems not to be a direct problem anymore.

Forest fires

Major wildfires commonly occur every 20-30 years in natural Mediterranean-type ecosystems, assisted by high air temperatures, low summer rainfall, fire-prone vegetation and dry fuel loads (Margaris and Koutsidou 2002). This vegetation is naturally adapted to fire which can be beneficial to physical, chemical and biological attributes of the landscape at low intensity, provided any grazing is controlled. However, widespread introduction of highly flammable fast-growing tree species (poplar, eucalyptus and pine) has not only reduced biodiversity, but also led during the 1990s to 600 000 ha of forest burning annually (FAO 2001), which is likely to rise in the future through global warming (McCarthy et al. 2001; Scholze et al. 2006). In Portugal, Spain and Italy, >3% of forests were burnt annually during this period (Scarascia-Mugnozza et al. 2000). Wildfires not only lead to landscape degradation through the temporary biomass loss, but also, and arguably more importantly, by affecting the physical and chemical properties of the soil and its structure, the nutrient status, and by causing a considerable increase in runoff and soil erosion during the post-fire 'window of disturbance', which can last for several years (Shakesby et al. 1993; Ferreira et al. 2000; Shakesby and Doerr 2006). Important off-site impacts include increased flooding and reduced water quality.

Flooding

Flooding as a desertification-related process may seem paradoxical. It is a secondary problem, as it is the consequence of other desertification-inducing processes, mainly water erosion and urbanization. Its effects are mostly outside the area (a so-called offsite effect) that is identified as a 'desertification hotspot'. In these areas, as soils have become thin or even absent, water from torrential rainstorms is transported downstream quickly and in large quantities, leading to flooding of downstream areas. Urbanization leads to an increase of impervious surfaces, which also leads to the quick transport of water and flooding downslope. For example, streams draining the Catalan Coastal Ranges suffer from increased urban use of their watersheds and streambeds for housing, car parks and roads, as a result of which human and economic losses caused by flooding are often high (Conacher and Sala, 1998). However, floods are also associated with the Mediterranean area, because of the climatic characteristics of that area (i.e. torrential and very variable rainstorms). Floods constitute the second form of land degradation in the south of France and Corsica, examples of violent and sudden floods include that of 1940, 1986 and September 1992 (Conacher and Sala, 1998). See Sala (2003) for a study on (the increase of) flooding in a typical Mediterranean area.

Sedimentation and siltation

Like flooding, sedimentation and siltation (of reservoirs) are off-site effects of desertification through erosion. Sedimentation can harm existing crops but can eventually lead to an increase in productivity due to increase in soil thickness and quality. Siltation of reservoirs is mentioned in many papers (e.g. Symeonakis et al., 2007; Liquete et al., 2005, Mtimet et al., 2002) as an off-site effect of other desertification processes mainly erosion and land use change. The capacity of reservoirs has decreased significantly as a consequence (e.g. in Spain, Morocco, Algeria and Tunisia).

Loss of biodiversity

Land degradation affects biodiversity both directly and indirectly. In terrestrial land systems, physical and chemical processes of land degradation can destroy soil biota (earthworms, rhizobia, mycorrhizae) and alter and/or reduce vegetative cover. In aquatic and coastal systems, land degradation can affect the sediment flow and can thus indirectly affect the biodiversity of these systems, especially of coral reefs, mangroves and sea grasses. In some cases, this effect is exacerbated by the pollutants, including POPs, that might be absorbed to soil particles. There are also further feedbacks. For example, decreased productivity on farmlands due to land degradation can force farmers to clear additional areas of natural habitats to maintain production. Conversely, changes in biodiversity (e.g. introduction of exotic species, or of species that become invasive) can contribute to further land degradation. (Gitay, 2004).

Biological diversity is involved in most services provided by dryland ecosystems and is adversely affected by desertification. Most important, vegetation and its diversity of physical structure are instrumental in soil conservation and in the regulation of rainfall infiltration, surface runoff, and local climate. It is the disruption of the interlinked services jointly provided by dryland plant biodiversity that is a key trigger for desertification and its various manifestations, including the loss of habitats for biodiversity (See Fig. 4.1 ) (Millenium Ecosystem Assessment, 2005). The major components of biodiversity loss (in green) directly affect major dryland services (in bold). The inner loops connect desertification to biodiversity loss and climate change through soil erosion. The outer loop interrelates biodiversity loss and climate change.

Fig 4.1 Linkages between Desertification, Global Climate Change, and Biodiversity Loss (from: Millenium Ecosystem Assessment, 2005).