Soils will naturally evolve and change over time as a result of such pedogenetic processes as leaching and erosion. In areas of undisturbed natural vegetation, such changes are generally slow and in a historical time frame will have little effect on the ability of the soil to support vegetative growth. However the natural processes may be accelerated in areas exploited by man for agricultural purposes, thereby producing, often within only a few years, major changes in the soil biological, chemical and physical properties. Such a change in soil properties will alter, and usually reduce, the land's ability to sustain a particular quantity and quality of plant growth. From an agricultural point of view any such reduction can be regarded as degradation (Douglas 1994).
The ability of land to support specific forms of agricultural production is finite. Important limits to production are set not only by the soil properties but also by such factors as climate, relief, hydrology, vegetation and land use. Land mismanagement - whether for crop, livestock or tree production purposes - consists usually of removing too much, returning too little and cultivating, grazing or cutting too often. Such "mining" of land beyond its limits results in degradation with decreasing productivity, and is non sustainable. For any given land area there are limits on the types of land use that can be pursued on a sustainable basis. Likewise their potential production will be limited according to the management practices and input levels (FAO 1991b).
In the soil conservation arena the terms soil degradation and land degradation are sometimes incorrectly used interchangeably, with soil erosion regarded as synonymous to both. However there is more to soil degradation than just soil erosion, and land represents a broader concept than simply soil. As with its use in the context of land evaluation (FAO 1976a), the term land refers to all natural resources which contribute to agricultural production, including livestock production and forestry. Land thus covers climate, landforms, water resources, soils and vegetation (including both grassland and forests).
Whereas combating soil degradation is critical to SARM, this cannot be addressed in isolation of other natural resources, as degradation of one can be expected to have an adverse impact on the agricultural productive capacity of the others.
The term desertification is widely used (often inappropriately) when discussing soil and land degradation within the Asia Pacific region. Following the 1992 UN Conference on Environment and Development (UNCED), desertification has been defined as "degradation of land resources in arid, semi-arid and dry sub-humid areas, caused by different factors including climatic variations and human activities resulting from adverse human impact" (in Hurni 1996). It needs to be stated that desertification is not a separate form of land degradation, so much as the end product of a variety of degradation processes that have adversely affected the land within so-called "dry" areas.
Desertification is an emotive term that conjures an image of desert sand dunes advancing over adjacent areas, something that rarely happens in reality. Also, what at first sight might appear to be desertification (e.g. loss of vegetative ground cover) may be a cyclical feature associated with a sequence of drought, or lower-than-average rainfall years. Vegetation recovery can be dramatic once relatively wetter rainfall conditions return. Placing undue emphasis on the threat of desertification can lead to the erroneous assumption that desertification and degradation are one and the same thing. This ignores the fact that land degradation can also occur under humid climatic conditions where the economic impact in terms of lost production greatly exceeds that of drier areas.
Three factors are commonly cited as causes of desertification: overgrazing, inappropriate agricultural practices and overuse of woody biomass. Such factors are not confined to dry areas and can lead to equally severe degradation in humid areas. Land degradation following mismanagement of the natural resources can occur anywhere, irrespective of the prevailing climatic conditions. For this reason it is believed that the emphasis placed on desertification programmes within the Asia Pacific region is misdirected. What is needed is a commitment to a holistic approach to land degradation wherever it might occur.
The word degradation, from its Latin derivation, implies "reduction to a lower rank" (Blaikie and Brookfield 1987). Hence when land is degraded, its productivity is reduced and may continue to decline unless steps are taken to restore the lost productivity and prevent further losses. Unchecked land degradation may result in an almost total loss of the productive land capacity to produce anything of value to humanity. Concern with such an outcome has led to land degradation sometimes being defined as follows:
Land degradation is the loss of the productive capacity of the land to sustain life (IFAD 1992).
However such a definition is perhaps too broad and has somewhat emotive overtones. It ignores the fact that whereas the productive capacity of an area may have been reduced by land degradation, it may still be possible to use the land for productive purposes by adopting alternative land uses, although with an inherent lower productive potential. While land degradation will have taken place, it will not have progressed to the extent that the land can no longer sustain any form of life. It therefore may be more appropriate to define land degradation in a more focused manner as follows:
Land degradation is the reduction in the capability of the land to produce benefits from a particular land use under a specified form of land management (after Blaikie & Brookfield 1987).
Such a definition embraces not only the biophysical factor of land capability, but also such socio-economic considerations as the way the land is used and the products desired (the benefits) from the land.
There are a number of interrelated land degradation components, all of which may contribute to a decline in agricultural production. The most important are (Douglas 1994):
For the purpose of this paper each component is discussed separately. However it needs to be stressed that there are many interactions and interdependencies between them and measures to combat land degradation and promote SARM will commonly address more than one.
A joint FAO, UNEP and UNESCO study (FAO 1979) defined soil degradation as a process which lowers the current and/or the potential capability of the soil to produce (quantitatively and/or qualitatively) goods or services. The study regarded soil degradation as not necessarily continuous but something that could take place over a relatively short period between two states of equilibrium. For instance clearing an area of forest and then using the land continuously for low input maize monoculture result in a rapid decline in the soils humus and nutrient levels. Providing soil erosion does not physically destroy the resource, soil cultivated in this way would not attain or even closely approach a zero humus and nutrient content. Instead it would reach a low-level equilibrium in which humus and nutrient levels remain constant and crop yields are stabilized at a low level (Young 1976). Many of the soils used for low input agricultural production in the Asia Pacific region are believed to have reached this low-level equilibrium.
The FAO, UNEP and UNESCO study (FAO 1979) recognised six categories of soil degradation processes:
A more recent study to assess the status of human induced erosion in South and Southeast Asia (van Lynden 1995) has sought to categorise the types of soil degradation in more detail, recognising some 21 types (see box 4).
Water erosion is the most widespread form of degradation within the Asia Pacific region and occurs widely in all agro-climatic zones. This category includes processes such as splash erosion, sheet erosion, rill and gully erosion and mass movement (Douglas 1994).
Splash erosion commonly initiates water erosion and occurs when raindrops fall onto the bare soil surface. The impact of raindrops breaks up the surface soil aggregates and splashes particles into the air. On sloping land relatively more of these will fall downslope resulting in a net downhill soil movement. Some of the soil particles may fall into the voids between the surface aggregates, thereby reducing the amount of rainwater than can infiltrate into the soil and increasing runoff.
Water running over the soil surface has the power to pick up some of the particles released by splash erosion. It can also detach particles from the soil surface. This may result in sheet erosion where soil particles are removed from the whole soil surface on a fairly uniform basis. Where runoff becomes concentrated into channels rill and gully erosion may result. Rills are small rivulets of such a size that they can be worked over with farm machinery. Gullies are much deeper (often being several metres deep and wide) and form a physical impediment to the movement, across the slope, of farm machinery.
On sloping land when soil is saturated, the weight of the soil may exceed the forces holding the soil in place. Under such circumstances mass movement in the form of landslides or mudflows may occur. On steep slopes this mass movement may be very rapid, involving the movement of large volumes of soil, usually on an isolated event and localised basis. In geologically recent and unstable mountain areas, such as the Himalayas, and areas prone to seismic and volcanic activity, such as parts of Papua New Guinea, landslides may be natural phenomena. However their frequency and severity may greatly increase following destruction of the natural vegetative cover by logging and/or clearing for cultivation, as in the Sierra Madre range in the Philippines (AIADP 1990).
The risk of wind erosion is severe in the arid and semi-arid areas of Asia and Australia. It includes both the removal and deposition of soil particles by wind action and the abrasive effects of moving particles as they are transported. It occurs when soil is left bare of vegetation as a result of cultivation, and/or overgrazing following overstocking. Not only can the wind remove topsoil from good farmland; it can result in additional damage by burying land, buildings, machinery and fences with unwanted soil. It is estimated that in Pakistan some 42% of the arable land are affected by wind erosion. In India the figure is 6%, although the total area affected, 11 million ha, is the same as for Pakistan (FAO 1994a). In China there are reports that windblown sand has affected some 2.65 million ha of cultivated land (ESCAP 1995).
Waterlogging is the lowering in land productivity through the rise in groundwater close to the soil surface (FAO 1994a). In its most severe form (termed ponding), the water table rises above the surface. Within the Asia Pacific region the most severely affected countries are India and Pakistan where some 3.08 and 2 million has respectively have been affected (FAO 1994a). Waterlogging is linked with salinisation, both brought about by incorrect irrigation management.
The database for the FAO, UNEP, ISRIC Assessment of the Status of Human-Induced Soil Degradation in South and Southeast Asia recognises 21 categories of soil degradation using a two letter code. The first capital letter giving the major degradation type, the second lower case letter giving the subtype. In some cases a third lower case letter can be used for further specification (see examples below).
Definition: loss of
topsoil by sheet erosion/surface wash
deformation" by gully and/or rill erosion or mass movements
effects of water erosion in up-stream areas
Definition: loss of
topsoil by wind action
Definition: off site
effects of wind erosion
decline and reduced organic matter content
Definition: sealing and
Definition: lowering of
the soil surface
Definition: loss of
Stable under natural conditions; i.e. (near) absence of human influence on soil stability, and largely undisturbed vegetation. NB: some of these areas may be very vulnerable to even small changes in conditions which may disturb the natural equilibrium.
Stable land without vegetation; i.e. (near) absence of human influence on soil stability, e.g. deserts, high mountain zones. Natural soil degradation processes may occur!
Stable under human influence; this influence may be passive, i.e. no special measures had or have to be taken to maintain stability, or active: measures have been taken to prevent or reverse degradation.
Salinisation is used in its broad sense, to refer to all types of soil degradation brought about by the increase of salts in the soil. It thus covers both salinisation in its strict sense, the build-up of free salts; and sodification (also called alkalization), the development of dominance of the exchange complex by sodium. If topsoil becomes too saline or too alkaline its productivity falls. The processes of salinisation can happen when poorly drained land is irrigated in hot climates. The sun evaporates the surface water, leaving behind the salts. At the same time, inadequate drainage causes the water table to rise, bringing saline groundwater into contact with plant roots. It is typically a human-induced process brought about by incorrect planning and management of irrigation schemes.
Saline intrusion is a localised form of salinisation which occurs as the result of the incursion of sea water into coastal soils arising from over-abstraction of groundwater. This is of particular concern to several Pacific nations in the context of rising sea levels. One very serious effect of such a sea level rise is its impact on freshwater lenses underlying atolls. The risk of saltwater intrusion will rise as the sea level rises; lateral leakage will increase, the lenses will become thinner and salt water will move within reach of pump intakes. As the sea level rises salt water will reach the roots of pit grown taro, coconut palms and other tree crops (ADB/SPREP 1992).
In addition to salinisation other processes may adversely affect the chemical properties of soil, notably (after FAO 1994b):
Of particular concern for SARM is the reduction in the reserves of soil nutrients. When soils are used for agricultural purposes significant quantities of nutrients are removed from the harvested products. With the present level of world yields the annual amount of harvested plant nutrients in three major cereals (rice, wheat, maize) is estimated at 40 million tons of N, 15 million tons of P2O5, and 28 million tons of K2O (FAO 1991b). If nutrients removed are not replaced (in the form of chemical fertilisers, organic manure, by natural fixation from the air or by weathering of rock minerals), then there will be a net decline in soil nutrient levels.
Plant growth may be adversely affected by the build-up of particular metals or salts to toxic levels within the soil. Aluminium toxicities may be a problem in strongly acid soils. The higher aluminium levels found in the subsoils of some tropical soils may be an additional factor in yield decline following loss of topsoil from erosion (Stocking personal communication). Calcium carbonate and gypsum (when present as calcrete or gypsum horizons) may cause nutrient imbalance. Manganese toxicity is most likely in acid soils with impeded drainage. (FAO 1979, FAO 1983 and Young 1976).
Several countries in Asia have a significant proportion of their land area with acid sulphate soil limitations, notably Cambodia (1.2%), Malaysia (2%), Thailand (2%) and Vietnam (4.6%). Such soils are commonly developed on estuarine and marine alluvium (mangrove swamps) and contain considerable amounts of sulphides. Chemical degradation can be rapid once such areas are "reclaimed." Once drained, the sulphides oxidise to form sulphuric acid; and the pH, which is around neutral prior to drainage, drops below 3.5. The acid attacks the clay minerals causing the liberation of aluminium ions in amounts toxic for plant roots and micro-organisms (Dent 1990).
Toxicities can also result from the presence of municipal, industrial, radioactive or oily wastes such as occur mainly around towns, industrial areas and mines (FAO 1979). Such toxicities are usually of only local extent but in some countries they may lead to significant problems, for instance the extensive areas of tin tailings from former tin mining operations in Malaysia. In Vietnam (and possibly Laos and Cambodia) there are areas where the soils contain residues of the chemical defoliants used between the 1960s and 1975, during the American phase of the Indochina war (Vietnam FARM CCC 1996).
Both crop and livestock production can lead to a deterioration in the physical condition of the soil. This degradation can take many forms, and has a variety of consequences. Deterioration of soil structure is the most common form of physical degradation, and involves (FAO 1994b):
Physical degradation is of concern because soil structure and its stability governs soil-water relationships, aeration, crusting, infiltration, permeability, runoff, interflow, root penetration, leaching losses of plant nutrients and therefore ultimately the productive potential of a soil (Lal 1979a).
Topsoil degradation occurs when an open structure of soil aggregates is broken down by excessive tillage. Also, the impact of raindrops and/or livestock hooves may produce a continuous compacted layer or crust at the surface. Reduction in topsoil porosity, and particularly surface crusting, will result in decreased water infiltration, increased runoff, poorer seedling emergence and often increased erosion (Ibid).
Physical degradation of the subsoil may be in the form of a distinct pan, or a more general compaction, i.e. loss of the original subsoil structure and increasing bulk density with reduction in size and quantity of pore spaces. The physical effects may be decreased water storage capacity, loss of aeration and reduced soil permeability. Waterlogging may occur in the soil above the compacted horizon and the absorptive capacity of the subsoil will be reduced, thereby increasing the amount of rainfall going as surface runoff. Plant root development will be hindered in the subsoil because compacted horizons are physically difficult to penetrate. Plant growth will be restricted because of the lower availability of air and water, and therefore nutrients, in the root zone. (Lal 1979a, 1979b, Young 1976)
Some of the blame for increased soil degradation in the region has been ascribed to a specific change in farming practice, namely the replacement of the traditional digging sticks and hoes with the plough. In part, this has facilitated the expansion of the area under cultivation thereby exposing more land to the risk of accelerated erosion. The main reason is that ploughing, using techniques imported from temperate regions in Europe and America, increases the disturbance of the topsoil, breaking up its structure and making it less resistant to erosion. Traditional shifting cultivation systems rely on the presence of tree stumps and roots, not only for rapid restoration of the vegetation during the fallow period, but also for reducing runoff velocity and holding the soil in situ during cultivation. The conservation benefits of the traditional land clearing techniques are lost once the plough has been adopted, as ploughing is difficult, if not impossible, unless the stumps and roots are first removed from the field (Douglas 1994).
Subsoil compaction, particularly the formation of "plough pans," is normally associated with commercial agriculture and the use of heavy machinery such as tractors. However, evidence suggests that a cultivation pan, immediately below the cultivated layer, can occur even under traditional hand cultivation systems from the pressure of the tiller's feet and the practice of cultivating to the same depth each year (Shaxson personal communication). Although the low yields under low input systems may largely be because soil nutrient and organic matter levels have reached low-level equilibrium, physical degradation in the form of a cultivation pan may also be a contributory factor.
Whereas conservation measures can be adopted to control soil erosion, and chemical fertilisers can be used to replace soil nutrients, physical degradation, particularly in the subsoil, is less easily overcome. Subsoil compaction is regarded as a form of soil degradation as it markedly reduces the yields of dryland crops and may be expensive to overcome, requiring deep ripping with a tractor. However in areas where paddy rice is grown, farmers adopt soil compaction techniques, such as puddling, to reduce subsoil permeability thereby raising the capacity of the soil to produce rice. In such a situation soil compaction is regarded as land improvement rather than degradation.
Soils used for agricultural purposes are often deficient in the biological processes which both maintain their physical structure and their ability to supply essential chemical elements to plants (Swift and Sanchez 1984). Of particular concern is the decline in soil organic matter or humus content following cultivation. In part this is because large amounts of biomass are harvested and removed from the site. The actual humus mineralisation rates may increase due to soil temperature changes following the removal of a protective vegetative cover.
The agricultural significance of soil organic matter is greater than that of any other property with the exception of soil moisture. It improves soil structure, and thereby root penetration and erosion resistance; augments cation exchange capacity; and acts as a store of nutrients, slowly converted to forms available to plants (Young 1976). It is possible to obtain an overall balance of soil organic matter with shifting cultivation under conditions of low population density. However for most of the Asia Pacific region shortage of suitable land means that shifting cultivation is no longer a viable option and most of the land is cultivated each year. Under such permanent agricultural systems decline in organic matter can be severe and rapid. Typical values of the organic matter. status of tropical soils that have been under cultivation for two or more years are 30-60% of the corresponding values under natural vegetation. Values below 50% are considered to represent an undesirable situation calling for remedial measures (Young 1976).
Biological degradation is usually synonymous with decline in organic matter. Yet it also applies when there is a decline in the beneficial soil fauna (organisms). Certain soil organisms have the ability to influence the physical structure of soils. Some species of termites annually transport large amounts of soil through the soil profile (Lee and Wood 1971). In so doing they achieve a mixing of the organic and mineral components of soil and alter the porosity of the soil at the micro- and macropore levels. This can result in increased surface infiltration of rainwater and a consequent reduction in runoff and erosion, which (together with the stimulation to soil aeration) contribute to maintaining soil fertility (Swift and Sanchez 1984). Earthworms, while important for the soils of the temperate parts of the Asia Pacific region, may play a similar role as termites in some soils in the tropics, although not in comparable numbers or biomass (Young 1976).
Most mechanical forms of soil tillage bring about marked changes in the qualitative and quantitative characters of the soil fauna community. The consequences of such biological degradation are poorly understood and merit further study. However the improved soil structure and enhanced efficiency of nutrient return from crop residues to plant, associated with conservation (minimum) tillage techniques are believed to be associated with the maintenance of a healthy soil fauna, particularly of termites and earthworms (Swift and Sanchez 1984).
Whereas the deterioration in the chemical, physical and biological properties of the soil is discussed separately in the previous three sections their combined effects can be considered under the heading of soil fertility decline (FAO 1994a). Decline in fertility is a major effect of erosion and the term is best used to describe the combined effect of processes other than erosion. The main processes involved are:
Such a decline in fertility has occurred in some of Tonga's rich volcanic soils, where the large-scale expansion of squash cultivation has led to unprecedented tree removal and the indiscriminate use of fertilisers and pesticides. It has been reported (Ofa Fakalata in Clarke and Thaman 1997) that an increasing number of growers are finding that no matter how much fertiliser or pesticide they apply their yields are dropping. The damaged areas are referred to as "Hot spots ... big areas of land that have been cleared with hardly any trees left, and where the land has been farmed continuously for a number of years ... so that the structure of the soil in these areas has been destroyed and the soil no longer can absorb water to feed the plants".
A study of land degradation in South Asia (FAO 1994a) noted that an interrelated set of soil fertility problems had been reported, directly or indirectly associated with fertiliser application.
Although the study focused specifically on countries in South Asia similar problems associated with the Green Revolution emphasis on the use of chemical fertilisers for increasing crop production can be expected to occur in most if not all the Asia Pacific countries.
A particular problem associated with the increased use of nitrogenous fertilisers is that, irrespective of how efficiently it is applied, nitrogen recovery efficiencies greater than 50% are rarely achieved (FAO 1994b). Much of the unrecovered nitrogen ends up in groundwater in the form of nitrate,13 or in the wetlands as ammonia in the atmosphere. When it is washed off the soil surface (or through extremely sandy soils) into waterways it may cause eutrophication problems, proliferation of algal growth and exhaustion of dissolved oxygen with consequences for fish and other aquatic species.
Vegetation degradation is usually regarded as a reduction in the available biomass, and decline in the vegetative ground cover, as a result of deforestation and overgrazing. Such degradation is a major contributory factor to soil degradation particularly with regard to soil erosion and loss of soil organic matter. The term also applies in situations where the reduction is not in the quantity of biomass but in quality - for instance bush encroachment into rangelands, and the loss of palatable pasture grasses and their replacement with nonpalatable species. In such a situation the value of the land will have declined from an agricultural point of view with a decline in its livestock carrying capacity. However the degraded vegetation may still be contributing to the soil in terms of ground cover and organic matter. In practice soil degradation usually accompanies rangeland degradation but it may not be such a clearcut relationship as that associated with deforestation.
A particular form of vegetation degradation affecting significant areas within some Asia Pacific countries, notably Indonesia, Papua New Guinea, the Philippines and Vietnam, is the replacement of tropical forest with Imperata cylindrica grasslands (Scherr and Yadav 1996). This typically follows forest clearing for agriculture. Poor soil management practices then lead to the onset of soil degradation with the Imperata invasion of the farmland. It is an extremely difficult weed to control by hand labour alone and has an adverse effect on crop yields often resulting in the abandonment of the field.
Natural ecosystems are exceptionally important for island societies, even though their total area may be minuscule. Small island forests, for example, can be absolutely critical for life support systems, especially for water supplies, subsistence wood, food and medicine, and soil stabilisation. The watersheds of small islands are far smaller than those of the Asian continent and smaller than those on the larger islands in the Asian Archipelago nations. On these smaller islands (in both Asia and the Pacific) even slight forest degradation can destroy watershed functions (Bass and Dalal-Clayton 1995).
Deforestation as a result of timber extraction and the establishment of plantation crops has led to significant anthropogenic environmental transformation within the small islands of many Asian and Pacific nations. The typical process has included (Bass & Dalal-Clayton 1995):
As a result, biological productivity, diversity and resilience have diminished, and much land in many islands now lies derelict (Bass and Dalal-Clayton 1995).
An environmental report on the Pacific (ADB/SPREP 1992) described agrodeforestation as a major threat to sustainable development. The term deforestation is well known in the context of the destruction or removal of forests. The term agrodeforestation covers the loss of trees from within existing agricultural landscapes, an issue that has received far less attention (Clarke and Thaman 1997). Trees are an important component of many Asian and Pacific farming systems. With a combination of both cultivated and wild species there is considerable bio-diversity to be found amongst the tree species within individual traditional farming systems. The different tree species may be:
In the Pacific on the larger Melanesian islands over 100 tree species are commonly integral parts of rural and urban agricultural systems, whereas atoll systems have up to 30-40 different species. Such trees serve at least 12 distinct ecological functions, have over 70 cultural and economic uses (see box 5), and provide up to 70% of the real income and production of rural Pacific people. Replacing the products from these trees with imported substitutes would either be impossible or too expensive. Elimination of these trees thus constitutes a major ecological, cultural and economic disaster which would undermine self-reliance and sustainability in the Pacific islands.
Agrodeforestation is a problem in Asia and the Pacific as agriculture becomes more intensive and specialised. When cultivation is undertaken by hand, retaining or planting trees within fields is not a problem, but once tractors are used, then trees hinder tillage operations and are usually removed. Within a traditional mixed biodiverse farming system the number of trees per unit area may be high, whereas the numbers of an individual species may be low, given that a farm family may be able to meet its subsistence needs for a particular product from just one or two plants.
As farm families become more commercially oriented there is a tendency to specialise in the production of a more limited number of cash crops (annuals and/or perennials). This results in the removal of many existing trees as they are perceived as occupying land that could be used for more "valuable" purposes. If trees are planted then they tend to be in monocultural plantations (coffee, cocoa, rubber, coconut etc) with a resulting loss in bio-diversity and the ecological benefits of the traditional mixed home garden/farm forest.
Examples of agrodeforestation can be drawn from Fiji, where trees of cultural and ecological value that were traditionally almost always protected when clearing fallow land for new gardens are now disappearing from the agricultural landscape. As reported in Clarke and Thaman 1997:
Instead of being protected or pruned and pollarded, they are now bulldozed, uprooted, ringbarked, or burn-girdled at the base to maximise monoculture , often plow-culture, of sugarcane, taro, sweet potato, cassava, kava, ginger, or cocoa for export or local sale.
Land degradation, particularly soil and vegetation degradation, has resulted in a deterioration in the quantity and quality of both surface and groundwater resources over much of the Asia Pacific region. With less vegetative cover to protect against the impact of raindrops causing surface sealing, a decline in pore spaces resulting from loss of organic matter and loss of structural stability following cultivation, less rain infiltrates the soil. Runoff increases, stream flows fluctuate more than before (in particular stream flow storm hydrographs are likely to have sharper and greater peaks), flooding becomes more frequent and extensive. Groundwater recharge decreases, streams and springs may cease and the water table is likely to drop so that wells and boreholes may dry up.
Cultural and economic functions
Prop or nurse plants
Source ADB/SPREP 1992
Increased runoff encourages upland erosion while an increase in severity of flash flooding encourages stream bank erosion. As a result sediment loads in rivers are increasing. The storage capacity of dams and weirs is reduced by siltation, lowland irrigation schemes are affected by silted up canals and sediment deposited in the fields, hydro-electric schemes are damaged, navigable waterways are blocked and water quality is deteriorating. High silt loads reduce the fish catch not only in inland waters but also where silt-laden rivers discharge into coastal waters.
In Thailand measurement of the suspended sediment and bedload transported by the Ping, Wang, Yom and Nan rivers suggests that each year some 27.5 million tons of soil are removed from a total catchment area of about 70,000 km2. The Harbour Authority of Thailand reportedly dredges over 19 million m3 of sediment every year out of the last 18 km of the Chao Phya River at a total cost of 424 million baht in order to keep the channel open and to control floods.
The implications of water degradation for sustainable agriculture are serious. With less water entering the soil and stored for use during dry periods, crop yields are falling. In the drier agroclimatic zones of the Asia Pacific region this may mean the difference between success or failure in producing a worthwhile crop. In Asia a significant amount of the staple food crop production (rice and wheat) depends on irrigation, with water being supplied from dams, run of the river offtakes and boreholes. In these areas widespread catchment degradation has affected both surface and groundwater resources. With less water available for irrigation, less land can be irrigated and less water used in individual fields with the consequence that crop yields and total production is declining. Many farm households are unable to meet their subsistence consumption needs from the production of their lowland irrigated plots. Frequently they grow a mixture of dryland crops, and/or gather forest products for sale in adjacent upland areas, to supplement their lowland farm production. The result is further degradation of the upper catchment areas contributing to yet further decline in the quantity and quality of the water resource.
A recent environmental study (van Gils and Baig 1992) has identified groundwater depletion as the most ominous component of the land degradation process in Baluchistan Province, Pakistan. This is because the vast majority of both the rural and urban population of the province depend on groundwater, for which no economic alternative is available. Current levels of groundwater pumping are unsustainable with the level of the watertable dropping in some places by as much as 3 metres per year. New tubewells are continuing to be sunk leading to increased exploitation of a declining resource. If this present situation continues the study concludes that in the next 10-20 years loss of the groundwater will lead to a decline of the province's largest economic sector, irrigated agriculture, as well as the collapse of the Quetta City water supply.
In some areas the rising use of fertilisers and pesticides has led to a deterioration in water quality due to increasing contamination from a range of agrochemicals. This, combined with bacterial and chemical contamination from industrial, urban and rural (humans and livestock) sources has affected drinking water supplies. Ill health arising from water borne diseases will seriously affect the agricultural productivity of farm households.
Although the short-term effects of land degradation are serious, evidence suggests that loss of vegetative cover and soil degradation may also be disrupting long-term rainfall patterns and increasing the likelihood of drought (Cook 1992). For instance, the climate in the agricultural upland areas of the Northern region of Thailand appears to have become much drier following the clearing of the mixed deciduous dipterocarp forest for cultivation. Given the climatic fluctuations that occur naturally this has been difficult to prove from meteorological records.
Computer models suggest three ways in which deforestation and soil degradation may reduce rainfall (after Timberlake 1985, Harrison 1987, Milner & Douglas 1989):
Although none of the models conclusively proves the link, it is necessary to consider the possibility that (under the stresses imposed by growing population) land degradation and climatic deterioration reinforce each other (Cook 1992).
It is theoretically possible that SARM programmes, which tackle the problems of deforestation and soil degradation, could have a positive impact on the local climate. Irrespective of possible macro climatic changes, considerable scope exists for ameliorating microclimatic conditions to improve soil conditions for the benefit of crop production. For instance the use of windbreaks, shade trees, and mulch will reduce soil surface temperatures and conserve moisture.
It has been suggested (FAO 1991b) that there is a link between soil degradation and the threat of global warming from increasing atmospheric concentrations of CO2. Terrestrial ecosystems play an important role in the global carbon budget. In addition to the effects of deforestation, world soils also have a significant impact on the global carbon budget. Rapid depletion of soil organic matter through the use of non-sustainable land use practises, may lead to emission of greenhouse gases into the atmosphere. One estimate suggests that reduction of about 1% in organic carbon content of the top 15 cm layer of soils of the tropics could lead to an annual emission of about 128 billion tons of carbon into the atmosphere (Lal 1990). Others question the evidence for global warming, and its possible effects on food production, believing that much of it is speculative and based on massive extrapolation and doubtful assumptions (see Hudson 1992).
Should global warming prove a reality, then the expected result would be a rise in sea level and increased storm (typhoon) frequency. Some reports indicate that this is already happening in the Pacific (Commonwealth Secretariat 1996c). A 30 cm sea rise is possible by the year 2030 as is a rise of 1.5oC in the temperature of the sea surface. This could result in an increase in the frequency of typhoons and their wind strength, and an increase in wave energy and destructive power, with the following physical effects (Bass and Dalal-Clayton 1995):
The lowlying island nations in the Asia Pacific region, notably Maldives, Kiribati, Marshall Islands, Tokelau and Tuvalu could be almost totally inundated. The coastal lands of many other Asia and Pacific countries are highly productive (for crop and aquaculture production) as well as the location of important infrastructure (ports, airports, roads, urban and industrial areas) and are highly vulnerable to rising sea levels and storm surges. A consequence of the threat of global warming may be the increase in the comparative advantage of areas which are less sensitive to climate change, and particularly to sea level rise. This will affect land prices and alter land use patterns, reversing the current trend towards investment and settlement in the coastal zones. Population groups can also be expected to shift to `safer' upland areas increasing pressure on areas where the risk of land degradation is higher (Bass and Dalal Clayton 1995).
Economic development within the Asia Pacific region has led to expansion of urban and industrial land. This has been particularly rapid over the last few years in many of the newly emergent countries of Asia. Much of the present urban and industrial development has taken place on what was formerly good agricultural land. The expansion of such cities as Bangkok, Jakarta and Metro Manila has resulted in the loss of considerable areas of good quality paddy rice land. Given the present shortage of such land, in Thailand, Java and the Philippines respectively, these losses increase the pressure on the remaining areas. Being so mountainous, with only 10% being arable land, and a population in excess of 30 million Fujian province has possibly the most acute land shortage problem in China. The uncontrolled urban and industrial development in the coastal zone (following the new policy of economic liberalisation) has seen the loss of much prime agricultural land to new roads and buildings.14
The expansion of urban settlements is a particularly acute problem in the small Pacific countries with little arable land. Population pressure is most acute in the capitals of the atoll nations (e.g. Tarawa in Kiribati) but is of concern even in those countries with larger land areas (e.g. Port Moresby in Papua New Guinea). Throughout the Pacific, people are gravitating from the mountains to the coastal cities, from the outer islands to the provincial or national seats of government, and from scattered rural hamlets to larger villages and towns. Under customary ownership laws, land is not readily available for housing estates. Hence squatter towns develop with health problems from overcrowding, unsanitary conditions and water pollution; valuable agricultural resources are lost; and forests, lagoons and reefs are degraded. (ADB/SPREP 1992)
Farm households affected by urban expansion may be forced to use their remaining plots more intensively or to seek land elsewhere, which in a land scarcity situation usually means moving into marginal upland areas. Hence urban and industrial expansion may be a contributory factor to soil degradation elsewhere.
The causes of land degradation can be divided into natural hazards, direct causes and underlying causes (FAO 1994a). Natural hazards relate to those factors of the bio-physical environment that increase the risk of land degradation taking place e.g. steep slopes are a hazard for water erosion. Direct causes are unsuitable land use and inappropriate land management practices. Underlying causes are the reasons why inappropriate types of land use and management are practised and usually relate to the socio-economic circumstances of the land users and/or the social, cultural, economic and policy environment in which they operate.
The major natural hazards in the Asia Pacific region, bio-physical conditions which act as predisposing factors for land degradation are (after FAO 1994):
For water erosion:
For wind erosion:
For soil fertility decline:
For lowering of the water table:
In some cases these natural hazards are of sufficient intensity to give rise to unproductive land without human interference. Examples are the naturally saline soils which occur in some interior basins of dry regions, or areas of natural gullying (badlands). With respect to land degradation, the key feature is that land shortage within the Asia Pacific region has led to the widespread use for agricultural purposes of areas with natural hazards. These are the passive, or predisposing, conditions for land degradation (FAO 1994a).
Erosion is a natural process. The conventional wisdom has been that soil erosion, following the growing of crops and/or grazing of livestock in upland and highland areas, is the primary cause of high river sediment levels. However, a considerable proportion of the eroded sediment found in river systems can be attributed to natural causes such as mass wasting (e.g. landslides) and various on-going geomorphological processes associated with the shaping of upland landscapes (e.g. Carson 1985). Hence when looking for the cause of land degradation a key question that has to be asked is, what proportion of the present erosion and river sediment levels is attributable to on-going natural processes, and what proportion is largely the result of `accelerated erosion' because of inappropriate land use?
High annual rainfall totals are a feature of many highland areas, much of which may fall within a limited portion of the year (the rainy season) and often as isolated heavy storm events. Even with excellent forest cover the soil can become totally saturated during periods of heavy and prolonged rainfall. With high levels of natural runoff, often concentrated into a single channel, flooding associated with high volume stream flows (with the ability to transport large quantities of sediment) can be expected to occur on a periodic basis within many parts of the Asia Pacific region. It is worth remembering that the floodplains of the Indo-Gangetic river systems were developed by inundation from forest-covered mountains long before watershed damage by man had become a significant factor.
SARM in geologically recent hill and mountain landscapes has to recognise that various natural denudation processes are at work even in areas where there has been no human disturbance. Such processes have to be considered as natural hazards, and therefore fixed design constraints when seeking to develop improved land use management practices. As one recent UNESCO publication states "It is often conveniently forgotten that floods are a natural hazard in areas with heavy rainfall" (Bruijnzeel & Critchley 1994).
There is a distinction, although they overlap, between unsuitable land use and inappropriate land management practices (FAO 1994a).
Unsuitable land use is the use of land for purposes for which it is bio-physically unsuited on a sustainable basis. An example would be the growing of annual crops on steep hillsides with shallow soils.
Inappropriate land management practices refer to the use of land in ways which could be sustainable if properly managed, but where the necessary practices are not adopted. For instance the failure to adopt soil conservation measures on sloping land or to replenish soil nutrients removed in harvested products. It can also refer to land use which is ecologically sustainable when the intensity of use is low due to an abundance of land but becomes inappropriate when land scarcity leads to higher intensity of use. Examples are shifting cultivation and the grazing of semi-arid rangelands.
Various types of human activity can be identified as direct causes of land degradation. These can be considered under the following headings (after van Lynden 1995, FAO 1994):
Poor agricultural activities: defined as the improper management of cultivated arable land. It includes a wide variety of practices, such as insufficient or excessive use of fertilisers, shortening of the fallow period in shifting cultivation, use of poor quality irrigation water, absence or bad maintenance of erosion control measures, untimely or too frequent use of heavy machinery, improper crop rotations etc. This category would also include the extension of cultivation onto lands of lower potential and/or high natural hazards. Degradation types commonly linked to this causative factor are erosion (water or wind), compaction, loss of nutrients, salinisation, pollution (by pesticides, fertilisers).
Deforestation and removal of natural vegetation: defined as the near complete removal of natural vegetation (usually primary or secondary forest) from large stretches of land, for example by converting forest into agricultural land, large scale commercial forestry, road construction, urban development, etc. Deforestation often leads to erosion and loss of nutrients.
Overexploitation of vegetation for domestic use: contrary to "deforestation and removal of natural vegetation", this causative factor does not necessarily involve the (near) complete removal of the "natural" vegetation, but rather a degeneration of the remaining vegetation, thus offering insufficient protection against erosion. It includes activities such as excessive gathering of fuelwood, fodder, (local) timber, etc.
Overgrazing: besides actual overgrazing of the vegetation by livestock, other phenomena of excessive livestock amounts can be considered under this heading, such as trampling. The effect of overgrazing usually is soil compaction and/or a decrease of plant cover, both of which may in turn give rise to water or wind erosion.
Overexploitation of surface and groundwater resources: In areas of non-saline groundwater, the technology of tubewells has led to abstraction of water in excess of natural recharge by rainfall and river seepage and a progressive lowering of the water table. In some parts of the region the over-extraction of water (for irrigation, urban and industrial use) from rivers and other surface water sources has led to reduced downstream availability. Where water is returned after use it may have a higher salt content and/or be polluted from agro/industrial-chemicals and human wastes.
Industrial activities: includes all human activities of a (bio)industrial nature: industries, power generation, infrastructure and urbanization, waste handling, traffic, etc. It is most often linked to pollution of different kinds (either point source or diffuse).
The nature, extent and risk of land degradation, and the potential sustainable yield of individual crop, tree and livestock enterprises, will ultimately be determined by the prevailing biophysical conditions within a specific area. Decisions as to what their landholdings are actually used for, and the management practices to be followed, will be influenced primarily by the socio-economic circumstances in which individual rural households operate. While current land use enterprises and management practices may accelerate land degradation, technical remedies will only succeed if they can function within, and address, local socio-economic constraints.
In the past too much emphasis has been given to assessing what is happening rather than why it is happening. Priority has wrongly been given to tackling the visual symptoms of land degradation (e.g. soil erosion control, gully plugging, reforestation etc), whereas the first step should have been to analyze why undesirable land uses and poor management practices were being followed (Sanders 1992a). Attention should be directed to identifying the ultimate cause, which in the case of accelerated (as opposed to natural) erosion, more often than not, will have a socio-economic origin.
Failure to consider the socio-economic dimension may result in the underlying causes of land degradation being overlooked and much time, effort and money spent in dealing with the symptoms of a problem rather than with the problem itself (Douglas 1994). SARM therefore requires that the issue be looked at, not just from a biophysical perspective, but in terms of the economic, social and political environment of those directly affected. These issues will be addressed in subsequent chapters.
Downstream sedimentation problems are attributed to erosion in the fields of small-scale farmers who grow annual crops within the upper reaches of a watershed. This is often backed up with experimental data from small research-managed runoff plots showing high rates of soil loss under so-called farmers' traditional practices. The true source of eroded material may lie elsewhere. What is needed is for more extensive field assessments to determine what the reality is on the ground.
The initial results from the IBSRAM PACIFICLAND trial sites15 indicate that rates of soil loss and runoff are much less than suggested by previous assessments (Howlett 1995). For instance one of the highest annual rates of soil loss from one of the Fiji plots was only 2.6 tonnes per hectare, whereas an earlier study had suggested a rate of 60-66 tonnes/ha/yr. In Western Samoa infiltration rates were found to be high, often over 90% even on 25o slopes with over 4,000mm annual rainfall. The trial plot under the farmers' traditional practice produced virtually no soil loss or runoff. Such results beg the question as to the source of the observed downstream sedimentation problems in the country. If it is not from the fields of the small-scale farmers then it has to becoming from elsewhere, e.g. roadside cuttings, stream bank erosion or forestland.
Qualitative techniques exist for the visual field assessment of erosion (see Douglas 1995 and Herweg 1996 for examples) and these should be used to determine whether erosion is taking place, and if so is it associated with:
In many cases the worst downstream sedimentation problems may be attributable to one or more of the following:
The effects of water and wind erosion are largely irreversible. Plant nutrients and soil organic matter may be replaced, but given the slow rate of natural soil formation, replacing the actual loss of soil material would require taking the soil out of use for many thousands of years, an impractical course of action.
In other cases, land degradation is reversible: soils with reduced organic matter can be restored by adding plant residues, and degraded pastures may recover under improved range management. Salinised soils can be restored to productive use, although at a high cost, through salinity control and reclamation projects.
Land reclamation frequently requires inputs which are costly or labour-demanding or both. The reclamation projects in salinised and waterlogged irrigated areas demonstrate this fact clearly. In other cases, the land can only be restored by taking it out of productive use for some years, as in reclamation forestry. The cost of reclamation, or restoration to productive use, of degraded soils is invariably more than the cost of preventing degradation before it occurs. (FAO 1994a)
13 Problems could arise if the groundwater is tapped by wells and boreholes for drinking water. The WHO upper limit for acceptable drinking water is 10 ppm of nitrate nitrogen, a level which it is believed is rarely found in the Asia Pacific region except where excessively high amounts of nitrogen fertilisers are being used. Such a situation is only likely to apply with high input large-scale intensive commercial farming.
14 In 1993 when travelling on the main road from Fuzhou to Xiamen one passed through long stretches of agricultural land. In late 1996 urban and industrial development had expanded to such an extent that at least 70% of the route was now flanked by factories, shops, houses and offices.
15 The IBSRAM PACIFICLAND Network currently has trials in Papua New Guinea, Vanuatu, Western Samoa and Fiji.