LAND DEGRADATION AND DESERTIFICATION IN DESERT MARGINS


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LAND DEGRADATION AND DESERTIFICATION IN DESERT MARGINS

Paul Reich ¹, Hari Eswaran ¹, Selim Kapur ², Erhan Akca ²

¹ USDA Natural Resources Conservation Service, Washington DC
² Cukurova University, Adana, Turkey

ABSTRACT

The term 'desert margin' as used here is the transition zone between the typical deserts and regions where there is an adequate supply of moisture for plant growth during the warm season. Low rainfall, high evapotranspiration, and high variability of rainfall characterize the desert margins. The global area of desert margins is 15.1 million km2 or 11.5% of the global ice-free land surface. In a previous study, 'desertification tension zones' where the probability of desertification is maximum were determined and a comparison with the current study, shows that desert margins are most prone to desertification. High human and animal population densities, poor or lack of land management and the low and highly erratic rainfall pattern in combination accentuate the land degradation processes.

The paper considers processes and causes of desertification in the desert margins. The country of Turkey is used as a case study. There is sufficient information to demonstrate that this is the ecosystem within the zone of susceptible dry lands that is most prone to desertification. As such it needs priority attention both from national and international institutions to develop and implement mitigating technologies. In addition, many developing countries would need financial assistance to alleviate the situation, and from a global environmental and socioeconomic point of view, this is a wise investment. Assessment and monitoring of such systems, understanding of the socioeconomic and environmental context of land management and development of national to local policies to facilitate their use are the ingredients for reducing desertification.

INTRODUCTION

The United Nations Convention to Combat Desertification (CCD) defines desertification as "land degradation in arid, semi-arid and dry subhumid areas resulting from various factors, including climatic variations and human activities" (Middleton and Thomas, 1997). The true deserts described as being 'hyper-arid' are excluded in the definition and further, CCD also refers to the arid, semi-arid and subhumid zones collectively as 'susceptible drylands'. An Aridity Index (ratio of precipitation to evapotranspiration) is employed and drylands have a ratio between 0.05-0.65. Despite these politically correct definitions, there is still much divergence in the use of the terms. Mainguet (1999), for example, includes the true deserts in her concept of drylands.

If climate is considered as a major control of desertification processes, such processes intensify in the drier portions of the zone of susceptible drylands. In this paper, the 'desert margins' form the transition between the typical deserts and the climatically better endowed drylands. Low precipitation and high variability of precipitation characterize the desert margins in the zone of susceptible drylands. The risk of drought is probably the highest and communities in such regions are most vulnerable. Thomas and Middleton (1994) made three observations on desertification, which are still valid. First is that available databases are still inadequate to reliably provide information on extent, severity, and changes. Second, drylands are one of the most fragile ecosystems and thus highly susceptible to land degradation and desertification. Third that desertification is a major, if not the primary cause of human suffering and misery in drylands. Some basic questions need further studies and elaboration. These (when the susceptible drylands are considered as a whole) include:

     1. How does fragility of the ecosystem vary?
     2. What are the desertification processes operating in different parts of the zone?
     3. Is land degradation in terms of reduction in land quality a significant process at the drier ends of the zone or is desertification essentially determined by vegetation degradation?
     4. What are the relative intensities and roles of climatic variability and land use pressures in desertification in the desert margins?

The purpose of this paper is to make a global assessment of desert margins, showing their geographic distribution and evaluating some of the land degradation processes operating in this sub-ecosystem of susceptible drylands. Vulnerability to desertification is considered from the point of view of population pressures and their impact on land resource stresses.

METHODS

Reliable global databases are currently inadequate to unequivocally address this issue, however, some preliminary assessments on global and Mediterranean biophysical resources (Eswaran and Reich, 1998, 1999) enables an assessment of vulnerability of the land resource base to land degradation. The actual intensity of the desertification process is then a function of socioeconomic and other factors. A global soil and climate GIS database of Africa developed by Eswaran et al. (1999) was used to provide the biophysical resource characteristics of the desert margins of this continent. For the assessment of vulnerability to desertification, soil units were empirically assigned to vulnerability classes after excluding regions that are cold, dry, or humid as per UNEP's definition of desertification. The procedure of Eswaran and Reich (1998) employing the following considerations are used in this assessment:

     · Coefficient of variability of rainfall - vulnerability increases with increasing coefficient;
     · Depth of soil including presence of impermeable layers;
     · Extreme levels of chemical and physical conditions, such as very high or very low pH (Lal, 1994);
     · Resilience of soil - ability to recover from mismanagement (Brinkman, 1990);
     · Information incorporated in soil classification term (Eswaran, 1992).

Employing the procedures indicated earlier, thematic maps (at scale 1:30 million) depicting land quality, land degradation, and desertification assessments are made of the desert margin zones of the world. Published reports of degradation (Oldeman et al., 1992) were used to validate specific locations on the map. To evaluate the number of people affected, the map of vulnerability to desertification was superimposed on an interpolated population density map developed by Deichmann (1994). Other overlays such as land cover or land use could be used to further characterize the desert margins.

Mechanisms in desert margins . Land degradation, being the loss in land quality or the reduced ability of the land for biomass production, results from a number of processes. The processes are generally self-feeding with one being the consequence of another or accentuating a related process.

1. Anthropogenic pressures result from overgrazing, over-cultivation of marginal land, removal of biomass for fuel, and mismanagement of irrigated land. Collectively these processes strip the land of the protective vegetative cover that protects the land from wind and water erosion and crusting. A major characteristic of the desert margin is its inability to recover from such shocks in a short time frame; its resilience or the ability to restore the original biomass when conditions revert to the normal is low. Once the land cover is removed, the land is at the mercy of degradation processes. Local conditions of landform and climate determine the kind and intensity of these processes.
2. Loss of vegetation, which results in reduced evapotranspiration and an increase in albedo or the amount of radiation reflected back to the atmosphere, accelerates land surface-atmosphere feedback. Absence of cloud formation and thus rain causes a positive feedback further reducing the ability to regenerate vegetation. Progressive decline in land cover (banded, patchy, stippled patterns) is an indicator of desertification.
3. Hydrological feedback occurs when the reduced ground cover results in greater runoff and decreased soil storage of moisture. Rainfall efficiency in terms of fostering vegetation production, which is typically low in these areas, is further hampered. In early stages of desertification when the shrubs are present as bands, the individual clumps may have a high biomass (Valentin and d'Herbes, 1999) but a definite decline on an area basis may be measured.
4. Climate variations, caused by large scale deforestation at distant sites or by surface temperature anomalies, also affect the above processes. A characteristic feature of rain in desert margins is that it comes in a few storms of high intensity and regional climatic variations accentuate storm intensities. In the absence of adequate ground cover, high storm intensities are very erosive. Monitoring the extent, severity, and expansion of desertification is the most urgent and most difficult area of degradation research. This requires large investments that would be rewarded by our ability to predict and control desertification processes.

Both the vegetation and land use in the countries bordering the Mediterranean Sea clearly indicate the continuous influence of human activity for several thousand years. Some believe that deforestation and improper land use since the Classical times have led to extensive erosion and land degradation. Modern pressures on land and influx of urban migration required political action in the latter half of the last Century and the European Union responded with several projects, the most important of which is the MEDALUS or the Mediterranean Desertification and Land Use project (Brandt and Thorne, 1996). The principal goals include the understanding, prediction, and mitigation of desertification. The emphasis is on providing a methodology for addressing the desertification problem in Europe.

In the lands bordering actual deserts, such as the Sahel or the periphery of the Central Asian Deserts, the fluctuation of the climate is from arid to less arid conditions. The flux appears to have no predictable periodicity. Conditions of aridity are determined by global climate patterns. In such situations, land degradation results from humans attempting to utilize the small amounts of biomass available and from a point of view of intensity of land degradation processes, rates are low. This is in contrast with the Mediterranean or more humid semi-arid regions, where the iterative processes of land degradation contribute to aridity. In the lands bordering actual deserts, when more favorable moisture conditions prevail, vegetation re-establishes quickly or the resilience is high. In contrast, the land degradation induced conditions in the semi-arid parts of the world are different. The resilience of both the soils and vegetation is low and thus recovery is poor.

Global distribution : The term 'desert margin' as used here is the transition zone between the typical deserts and regions where there is an adequate supply of moisture for plant growth during the warm season. At the periphery of deserts, there is a flux in the climatic conditions; at the borders of the Sahara, the desert conditions vary annually. In some years there may be adequate soil moisture for biomass production while in other years, drought conditions prevail. In these desert rims, there is a probability of 60% or more that strong moisture stress prevails in most years. Away from this zone, the amount of soil moisture increases permitting one grain crop cultivation in most years. In addition to the desert rim, there are three other types of transitions. First, where the desert merges with areas with winter rainfall areas or a Mediterranean type of climate and these are termed Dry Xeric soil moisture regimes (SMR) in this paper. Second, where the merger is with semi-arid regions in tropical areas (Aridic Tropustic SMR) and thirdly, to temperate areas (Xeric Tempustic) with semi-arid climates. Within the zone of susceptible drylands the desert margins occupy the 'arid areas' and part of the 'semiarid areas' as defined by Middleton and Thomas (1997). These areas are defined on the ratio of precipitation to evapotranspiration; the arid areas have a ratio of 0.05 to 0.20 and the semi-arid, 0.20 to 0.50. The desert margins as defined in this paper occupy the arid areas and part of the semi-arid areas and table 1 provides the ratio and soil moisture regime term for some selected stations. The ratio is a simplified manner of expressing the susceptible drylands and will not equate to the soil moisture regimes employed here.

A combination of soil moisture and temperature regimes of Soil Taxonomy (Soil Survey Staff, 1999) characterizes the soil climate and table 2 provides the area occupied by the susceptible drylands combinations. The desert margins at the periphery of the true deserts or the soils with weak aridic SMR (Table 2, Fig. 1) is about 7.93 million km2 or 20.6% of the desert areas. The Mediterranean region (or soils with xeric SMR) occupies 4.42 million km2 and the desert margin of this ecological area where it abuts soils with aridic SMR occupies 1.98 million km2 or 44.5%. The semi-arid areas of the tropics (Tropustic) occupy 20.35 million km2 and the desert margins in this zone (table 2) occupy 6.30 million km2 or 31% of the zone. The temperate and boreal semi-arid zones (Tempustic) occupy 10.91 million km2 and the desert margin component of this is about 0.31 million km2 or 5.0%. In total, the global area of desert margins is 15.1 million km2 or 11.5% of the global ice-free land surface.

The differences in the causes, intensities, and consequences of land degradation processes in the different desert margins of the world must be understood to better manage them. In addition to soil moisture conditions, soil temperature also influences land degradation through its influence on land use and land cover. Desert margins (DM) can be differentiated through their soil climate attributes as shown in table 3.

The arid DM borders the true deserts. The Mediterranean DMs have winter rains while the semi-arid DMs have rains during the warm periods when temperature is conducive to plant growth. In the tropical DMs, the difference in the soil temperature between winter and summer equivalent months is <5oC; the temperate regions have a mean annual soil temperature >8oC while in the boreal regions, the mean annual soil temperature is less than the biological zero of 5oC. By this approach, the DMs occupy about 15 million km2 or 11.5% of the earth's ice-free land surface. The region around the Mediterranean Sea has extensive areas of desert margins. Despite the exploitation of these lands for several centuries, their fragility has become of concern only recently. Table 4 shows the extent of DMs in the countries of the Mediterranean region. Iraq, Lebanon, and Syria have large areas of DMs. The North African countries have much smaller extents due to the large areas of true deserts. About half the land mass of Turkey qualifies as desert margins and the advent of desertification is very prominent there.

Dominant processes : There are many processes that initiate or are responsible for land degradation and these can operate individually, simultaneously, successively, or in varying combinations. One set of processes or conditions, unless checked, frequently triggers others. To implement mitigating actions, it is important to evaluate the processes and address each, or a combination, in an organized manner. A clear understanding of processes, interactions and consequences is necessary to have meaningful results.

Table 5 is an outline of dominant causes and their relative consequences in the three kinds of desert margins. The desert margins of the Sahel, the Mediterranean region, and the Asian (particularly Central Asia) are distinct entities in the context of their socioeconomic status, land use systems, and land degradation pressures. For example, land resource impacts of tourism is a major determinant of the quality of land and water resources in most of the Mediterranean countries. Overgrazing perhaps is an immediate problem to reckon with in the Sahel while, subsistence farming hurts much of Asia. Even if the land users recognize that their land is being degraded, poverty prevents them from addressing the conditions in most of the countries and particularly in the Sahel and in Asia. It is perhaps true to suggest that unless poverty is addressed land degradation would remain the character of subsistence farming. In the poorer countries, demands on the constrained national budgets reduce the political will to initiate or participate in any programs. This is also a poverty-related issue. Feeding and clothing the population and ensuring minimal civil strife is the challenge in developing countries, which prevents them from mounting conservation programs.

In low-income countries, poverty feeds on population growth. Eswaran et al., (1999, 2000) and Beinroth et al., (2000) have demonstrated the population supporting capacity globally and for some countries of Asia. Many of the countries require large investments with modern technologies to produce the food for their populations. A combination of the small farm size and the resource poor conditions of the farmers, are the basic ingredients for land degradation. Further, as much of the land of reasonable quality is already used, sloping lands and swamps are under onslaught with predictable consequences.

Desert margins of Turkey : Desert margins occupy about 54.4% of the land area of Turkey (Table 4). Figure 2 shows the distribution of the desert margins in Turkey. According to the definitions, the desert margins occupy much of the Anatolian Plateau, the Mediterranean coast and the GAP area extending into Iran on the east and Syria in the south. The Konya Basin, being aridic, is excluded. The Black Sea coastal zone has high rainfall and is also excluded from the region considered as desert margins. Dogan (1998) estimates that about 83% of the whole country suffer from wind or water erosion problems. In addition, there are other soil-related constraints, such as salinity/alkalinity and hydromorphism that reduce the agricultural producing capacity. Most of these constraints are in the desert margin zones. Productivity is further hampered by the fact that the rural poor do not have the means or the technology to appropriately manage such systems in a sustainable manner.

The low-input agricultural systems in the desert margins promote land degradation and desertification. The issue is land management or the lack of appropriate management systems. Where irrigation facilities have been developed, productivity is significantly enhanced during an initial phase. With time, there is a slow but systematic build-up of salinity due to inadequate drainage systems. Productivity decline then sets in. When irrigation is not available, grazing with small ruminants is the traditional land use. In good years, when rainfall is adequate, one crop of low-yielding grain is obtained. Managed pastures are few. Thus, over-grazing leads to erosion and a general change in vegetation with hardy weeds replacing edible shrubs. All the causes of desertification (Table 5) operate in the desert margins of Turkey and good examples of each can be documented. As this eco-zone occupies more than half the country, policies based on detailed assessments are urgently needed to reduce land degradation pressures and bring some semblance of sustainability.

Although soil surveys exist in many of the areas of the desert margins in Turkey, the information in combination with information of other resources is not widely used for resource assessment. The condition of land resources is not monitored, as in most countries of the world, and consequently there is no appreciation of both the state and the degradation of the resource base. The norm is to try to respond when there are desperate calls for help from land users. Preliminary studies (Cangir, et. Al., 2000) clearly demonstrate the intensity of land degradation processes in the country.

CONCLUSION

Desert margins, from a biophysical and a socioeconomic point of view, form an unique ecosystem which has not been the subject of detailed studies largely because they occur in the developing countries of the world. However, due to population increases and demands for agricultural land, this last frontier of land is invaded and currently being stressed. This is the ecosystem within the zone of susceptible dry lands that is most prone to desertification. Eswaran et al. (2000) identified 'tension zones', which are ecosystems that are most prone to desertification largely due to high population density. Most of the tension zones occur in the desert margins. Identification and location of desert margins in countries, if followed-up with appropriate policy decisions and action plans, will help to:

1. Enable the judicious use of land resources through protection and preservation of fragile systems, sustainable production on the better endowed areas, and targeting of research and development; ensure a balanced land use through appropriate land allocation for forestry, wild-life, agriculture, and urban use; and promote a more rational use of the scarce water resources;
2. Buffer the socioeconomic stresses and reduce economic instability and political unrest in a country as a whole; reduce pressures on affected areas and promote sustainable development outside the affected areas;
3. Alleviate pressures on biodiversity and promote environmental integrity;
4. Help reduce the iterative processes leading to global climate change through increased land cover and as a result, enhance carbon sequestration; and
5. Assure food security and a better quality of life for most of the people.

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