Introduction

1More than 50 % of India is severely eroded and degraded, mainly through water erosion, secondarily by wind erosion (Hazta et al, 2001). 60 % of its agricultural land is affected by soil erosion, water logging and salinity (Sivanappan 1999). These figures have to be linked to the fact that most of the Indian sub-continent, and notably peninsular India, is composed of massive ranges of mountains, extended arid and semi-arid areas and long coastal zones, corresponding to fragile ecosystems.

2Thus, through a sectional view of the landscapes of south India (Figure 10.1), one can observe along a west-east transect, from the top of the Western Ghats to the sea:

1. Depleted forests and lands due to rampant deforestation, illicite encroachments and cultivation on steep slopes;
2. More or less degraded lands on middle and lower slopes, depending on climatic and edaphic conditions and largely on natural resources management;
3. Plains and deltas down to the coastal areas, charaterized by huge densities of population and intensive irrigation-based farming systems, and so depending on (upstream) water resources since historical times. This is typically the case of the Cauvery catchment area.

3For the last decades, over-exploitation and mismanagement of natural resources (soil, water, vegetation) have induced excessive erosion in all the catchment areas: land slides, gully erosion, heavy runoff with siltage of rivers and reservoirs, flooding and droughts, salinization, and finally desertification affecting hundreds of millions of inhabitants, their livelihoods and their futures.

4To face such tasks and complex issues, solutions are certainly not only technical. They largely depend on new approaches, able to integrate at various scales the different components, be they bio-physical or socio-economic, and so interdisciplinary-based with the view to meeting the basic needs of the concerned people –notably the farmers– in a sustained manner.

5In this context, social forestry and allied activities such as agroforestry and eco-farming based on the traditional ecological knowledge of local populations may play an important role by contributing similarly at a double level: i) of soil and water conservation, and therefore sustaining foodgrain productivity, ii) increasing and diversifying revenues, notably for the poorest farmers.

The role of vegetation in soil and water conservation

Soil erosion, tree effect in soil and water conservation, sustainability

6Soil erosion results from climatic conditions (erosivity of rainfall, notably), erodibility of the soil (depending on structure, permeability, etc, ), slope (in the case of erosion by water) and vegetation cover such as forests, crops, etc. (Depommier 1985). Erosion is not only quantitative; it also exerts a qualitative action on the soil, in particular on its upper part, and therefore affecting its fertility. For example, it may consist in losses of 20 kg OM, 173 kg N, 48 kg P₂0₅, 30 kg C_aO, 7 kg K₂0 and 3.5 kg MgO per year in a maize plot as measured in Malagasy by Goujon et al (1968). Although the decrease of fertility is not necessarily linked to soil erosion, its maintenance is a primary objective in soil (and water) conservation.

7Soil loss from soil erosion, notably through runoff, has been measured in many sites by various authors comparing different covers of vegetation under different conditions. Thus, the cover factor (c) calculated according to the universal soil loss equation may vary from 1 on a bare soil (reference) to 0.001 in a dense forest, and 0.1 to 0.01 with various cover crops (Roose 1986 from the West African studies). The more the percentage of the ground cover increases, the less is the loss; the better the structure of the vegetation (multi-strata) such as in forests or agroforests integrating woody layer (s) and crops or other undergrowth, the better is the protection of the soil (FAO 1979). Although both parameters are correlated, runoff water loss is rarely calculated along with soil loss.

8Table 10.1, which refers to the effects of regenerating vegetation on hill slopes of Uttar Pradesh at the watershed scale so as to conserve soil and water, clearly shows the link, before and after treatment.

9Finally, as this general reclamation of natural resources was accompanied with an improvement in the physio-chemical properties of the soil –and therefore of its productivity–, it demonstrates that soil and water conservation through appropriate agroforestry technologies may quickly lead to sustainability, viz. productivity cum conservation, of natural resources.

Intrinsic characteristics of woody perennials

In water and nutrients cycles

10Trees take up a large part of the rainfall and evapotranspire a certain volume of it. So they are able to save water which is partially maintained and recycled instead of being lost through the soil profile to the water table at surface level through run-off (Depommier 1985).

11Trees bring up from the lower part of the soil important quantities of minerals: nitrogen, calcium, potassium, magnesium, phosphorus, etc., of which the main part return to the soil through the degradation of the leaf litter and root residues (OM, faunal activity improving both its structure and fertility). As mentioned by Young (1989), of all the effects of trees, that of maintaining soil organic matter levels through the supply of litter and root residues is the major cause of soil fertility improvement.

Physical action of the rooting system

12When exploiting the different layers of the soil, tree roots contribute to maintain it against erosion. They also increase its porosity, inducing a better water infiltration and soil accretion which in turn improves the microbiological activity.

Symbiotic associations

13Trees with bacteria and mycorrhizae-associations of nitrogen-fixing tree species with Rhizobium or Frankia bacteria or woody perennials with Endo and Ecto-mycorrhiza fungi are of great interest for soil and water conservation or rehabilitation, notably in arid and semi-arid areas where soils are often poor in nitrogen and phosphorus and water is a limiting factor.

14Tree root systems together with their associated mycorrhizae, improve the efficiency of nutrient cycling, defined as the ratio between plant uptake and losses by leaching and erosion.

Physical action of the canopy

15Tree canopy forms an efficient screen to protect soil against rainfall impact, notably to prevent the structural degradation of its superficial layers. It may also act as a windbreak protecting soils and associated crops, reducing their water requirements and limiting the transportation of the fine (fertile) elements taken away from the topsoil.

16Finally, canopies provide shade resulting in micro-climatic effects at the level of the surface layers (reduction of maximum temperatures and ETP, humidity and biological/faunal activity improved).

17Although trees (agroforestry systems) and tree communities (forests, woodlands, plantations, agroforests, etc.) have an importart potential role to play in soil and water conservation, their efficiency largely depends on their management, which notably includes the choice of the species, their association or combination with other components, and all the silvicultural or horticultural interventions to which they are submitted.

Agroforestry technologies and tree management practices

18Biological and physical (or engineering) measures are complementary tools to conserve soil and water in degraded or fragile areas. Meanwhile, biological measures (trees, shrumbs and grasses for crops) have the advantage i) of being much less costly or labour demanding than terracing, ridging, etc., notably in marginal agro-ecological zones, ii) of being more durable and efficient if properly selected and managed, iii) of being productive and thus offering the farmers additional benefits (wood, fodder, food, etc.) or revenues. This is usually the main motivation for local people (poor farmers) when promoting the plantation and management of trees for soil and water conservation purposes.

Windbreaks and shelterbelts

19Direct use of trees and shrubs in erosion control, notably against dessicating wind in arid and semi-arid areas and cyclones on coastal areas, is well established.

20Windbreaks are living and permeable structures made up of lines of woody or semi-perrenials and sometimes herbaceous plants planted perpendicularly to the dominant wind and aiming at:

1. Reducing wind speed and accordingly wind erosion on the protected area with improvement of cropields;
2. Catching fine elements of soil (nutrients) transported by the wind;
3. Improving microclimatic conditions (increase of air and soil humidity, reduction of maximal temperatures and E. T. P, viz. a better water economy);
4. Providing wood, timber, fodder, foods, etc, according to windbreaks, floristic composition and management.

21To be efficient, such structures have to be designed, built up and managed according to specific characteristics and silvicultural techniques such as the choice of species, their growth and phenology (permeability close to 50 %), the development of their canopy, and their rooting system, (to be respectively wind resistant and low competitive with crops and their ability), in order to support lopping, coppicing, trimming, etc.

22Shelterbelts are similar to windbreaks but applied on much larger scales. This is the case along the Chinese coast where Casuarina equisetifolia has been planted along 3000 km or, on a smaller scale, on the Senegalese coast to protect the soils, stabilize sand dunes and create favourable microclimatic conditions to develop farming systems based on irrigation (vegetable marketing).

Living fences, hedgerows and trees on erosion control structures

23Living fences on boundaries, alignments of trees and shrubs on countour lines (hedgerow intercropping) or planted to stabilize earth or stones structures (bund, banks, terraces) are common ways of controlling soil erosion, to conserve water and maintain soil fertility in sloping and flat lands.

24Living fences usually made of small trees, shrubs and semi-perrenials, often propagated by cuttings at high density, constitute one of the oldest techniques of the rural areas, as one of the main roles of these structures is to delineate land property and defend it against roaming livestock and, at one time, once against men (with non-appetable / spinny species). Hedgerows on countour lines as well as living fences –and more particularly networks of such hedges– may be very efficient in soil and water conservation, notably on slopes, to: i) reduce runoff, increase infiltration and reduce soil loss thanks to their barrier effect, ii) naturally develop terraces through accumulation of soil upslope of the hedgerows.

25In hedgerow intercropping, fertility and soil moisture can be maintained by lopping or coppicing tree and shrub and laying the prunings on the ground surface in the cropped alleys, leading to the progressive stabilization of the risers through stems, roots and litters on slopes (acting as biological bunds). Association with grasses may consolidate the efficiency and durability of the structure contributing to a better stabilization and acting as a filter to catch the smallest particles of soil.

26The ad hoc plantation of nitrogen-fixing (trees or crops) such as Leucaena leucocephala -one of the most widely used species in hedgerows-is indeed an additional factor in soil fertility improvement.

27Finally, and as for the windbreaks, trees which constitute hedgerows or fences may contribute to the supply of wood, fodder and food and to the diversification of farmers’ revenues, as observed in the Bamiléké traditional agroforestry system in the Uplands of Western Cameroun (Depommier 1983).

Agroforestry parklands and other tree-based systems

28Homegardens -which can be considered as agroforests- are among the commonest tropical agroforestry systems of the humid zone. They typically are multi-strata systems, characterized by high density and diversity of trees and shrubs and so very similar –at least from a structural point of view– to dense forests from which they usually derived from (Depommier 2001). Apart from providing food, energy and building materials –this productive role being the first one of the homegardens such structures provide a very efficient cover to the soil.

29Accordingly, soil erosion levels are remarkably low, almost nil, even on steep slopes like in the Western Ghats of Kerala, where they constitute the main farming system. From a water conservation point of view, homegardens and other tree-based systems like the coffee-based system in Coorg, Karnataka, associating coffee to shading trees, operate as a forest cover, substantially reducing run-off and, moreover, storing water at the root-soil interface and regulating the water cycle.

30Scattered trees and shrubs planted or kept in combination with crops and/or livestock, or pure plantations of trees (in woodlots, fodder banks, etc) in judicious parts of the agricultural landscape is another way to contribute to soil and water conservation. Many traditional agroforestry systems still prevail, such as the Acacia (syn. Faidherbia) albida parklands of Sahelian Africa or Prosopis cineraria in Rajasthan. In both regions, which are ecologically very similar, the species (nitrogen-fixing trees) are maintaining soil fertility, the parklands they constitute acting as permanent fallows. Effects on soils vary according to the density and the size of the trees, but also according to the site conditions (edaphic, notably) and the management of the system, in particular of the trees (lopping to feed the livestock which in return contribute to enriching the soil in OM through their droppings). Acacia albida, due to its reverse phenology (unlike the other species, it is leafy during the dry season, getting water from the water table thanks to its long tap root), has no water need during the rainy season and, as a matter of fact, does not compete with the associated crops (millet, sorghum, maize). Its effect on grain yields is highly significan, serving food subsistence systems (Depommier 1999). Finally, on a larger scale, Acacia albida parklands, which often cover full (micro)-watershed areas around the villages or occur on bottom lands along rivers, are interesting models to be studied and developed elsewhere, for example in the drylands of peninsular India.

31But, whether this species or any other multipurpose tree, shrub or palm, as mentioned by Young (1989), the essence of agroforestry in watershed management is "to apply sound land-use planning to the whole of the catchement, with particular attention to erosion control and water management. Adequate mechanisms for control of land use and management practices are essential, combined with the cooperation of the land users".

Conclusion

32Appropriate agroforestry systems and the management of tree control erosion maintain soil organic matter and physical properties and promote efficient water and nutrient cycling.

33Potential roles in this matter are considerable and may apply to a wide range of agro-ecological zones and socio-economic situations in the mountain areas, as well as in the drylands and coastal areas of peninsular India. In this perspective, needs and priorities should focus on:

1. Operational participative and integrated approaches where agroforestry and social forestry components may play a major role for a sustainable management of the water resources;
2. Acquiring traditional as well as scientific knowledge about the contribution of vegetation and tree-based technologies for water management and conservation in rural areas. Development of comprehensive information systems and diffusion of ad hoc information to be promoted among the different stakeholders;
3. Applied and multidisciplinary research, notably to assess at on-farm level the effects of trees, in terms of soil and water conservation systems, and to develop and model water economy-tree-based systems for the rehabilitation of degraded landscape and watershed areas.