|SPORE Bulletin of the CTA No. 14 (CTA Spore, 1988, 16 p.)|
|Growing vegetables and hope|
Agroforestry is a term which has only recently come into general use. It is used - and practised - most widely in tropical countries because of its advantages in optimizing production while reducing soil erosion. While the name may be new, the technique has long been practised in some countries.
A groforestry covers all systems where trees are deliberately left or planted on land where crops are grown or animals grazed, and so includes pratices as diverse as shifting cultivation, taungya, the growing of shade trees in cash crops such as coffee, and the use of living fences to contain or exclude animals.
The upsurge of interest in these practices over the last ten years stems from the increasing population pressures and shortages of food and fuel among poor people in many developing countries. To meet their needs, those living in the more marginal areas have been forced to shorten the time the land is left fallow, to encroach further into forested land, and to overgraze pasture. As a result, the natural soil cover has been removed and the recycling of nutrients prevented leading to soil erosion and rapid decline in crop yields.
Agroforestry is seen as one way of solving these problems, since by including trees in their farming systems, farmers not only benefit from a supply of wood and other tree products but help ensure their land remains fertile and productive as well
Leaf litter from the trees adds organic matter to the soil and acts as a mulch to retain soil moisture and prevent soil erosion. Deep-rooted trees may tap sources of nutrients which are out of the reach of annual crops, making them available once the leaves fall. Leguminous trees such as leucaena can also improve soil fertility more directly by nitrogen fixation. Tree roots can help bind the soil and also create channels which improve its aeration and permeability to water.
A mixture of tree and annual crops of different heights provides a more complete ground cover which again helps protect the soil from erosion and makes maximum use of available sunlight. The tree cover helps moderate extremes of temperature preventing rapid heat loss from the soil at night and protecting crops from excessive heat during the day.
While these environmental benefits are of great importance to the conservation of fragile ecosystems in the long term, farmers are more likely to appreciate the value of trees in their farming system if they can see more immediate rewards. Multi-purpose trees which provide fodder for stock and edible fruits or nuts, as well as fuel, timber and support for climbing vegetables are those most likely to be used.
Traditional agroforestry systems differ in the extent of the association between tree and crop or livestock components. Amongst the most closely integrated agroforestry systems are home gardens such as those commonly found in West Java and the more humid parts of West Africa. These gardens contain an intensive mix of crops of all types so that their structure resembles the natural forest. Annuals such as rice, maize and sweet potato are among perennials such as fruit, nuts and fibres and forest trees retained to provide fuel, timber, tannins, gums and medicinal products. Fodder for small stock comes from tree leaves and the grasses which grow under the trees.
In the agroforestry system known as taungya, crops are grown among young trees only until the tree canopy closes over (one to three years), so that in this case the tree-crop association is temporary. The system was developed in Burma in the 1 850s, when foresters needed labour to help establish new plantations as teak was felled. By allowing farmers to grow crops among the trees, weed growth was controlled among crop plants and trees.
This system has since spread through Asia to Africa and Latin America. In West Africa it has been used to help establish plantations of Gmelina arborea. But although the system benefits poorer farmers as well as the forester, farmers sometimes resent the restrictions imposed and the constant need to move on. As a result they have been known to damage the trees deliberately and so postpone the time when they shade out the crone.
In the case of shifting cultivation, or bush fallow, trees and crops occupy the same ground in turn. Although the more valued trees may be retained during the years when crops are being grown, and the trees are never completely cleared, the association between trees and annual crops is less close than it is in other agroforestry systems. Even so in the past it shared many of the advantages of more closely integrated systems.
It is only now, when increasing demands o the land have led to shorter and shorter fallow periods, and in some cases eliminated them altogether, that soil erosion and reduced fertility have become a serious problem. Because bush fallow is the dominant form of farming in Africa as well as parts of the Pacific, it is this more than anything else which has promoted the search for alternative and more sustainable agroforestry systems.
For those involved in improving and developing existing systems, agroforestry demands an all-round approach very different from that found among conventionally-trained agriculturalists and foresters, so justifying the need for a new discipline with a new name. Researchers, extension workers, and policy makers concerned with agroforestry have to consider not only how these improvements will affect the sustainability of the system: they also have to take account of local laws and customs concerning the ownership of trees and land, since, for example, farmers may be reluctant to plant trees if this calls into question their rights to crop a particular area.
Although agriculturists can provide information about many of the crops found in agroforestry systems, little is known about most of the trees, with the exception of those such as leucaena, some acacias, glyricidia and calliandra. Even less is know about interaction between the trees and the crop. The research possibilities seem limitless.
Diagnosis and design
The International Centre for Research in Agroforestry (ICRAF) has played a key role in developing new research approaches to cope with the complexity of the subject. Established as recently as 1978, and based in Nairobi, its main aims are to initiate, promote and support research, and to increase awareness of the benefits which agroforestry brings. With only a limited number of staff, it is fulfilling these aims admirably, acting as a catalyst for agroforestry research worldwide.
To help researchers investigate existing systems, see how they can be improved, and set priorities for research, ICRAF has developed an approach it calls D & D - diagnosis and design. In the diagnosis stage researchers discuss with farmers their basic needs, such as those for food, fuel, shelter and a cash income. They examine the existing system in terms of both its productivity and sustainability. In the design stage they use their knowledge of agroforestry systems elsewhere to suggest improvements which the farmers might adopt straight away.
At the same time they identify more serious problems which might be resolved by research and the development of new technology.
ICRAF has also proposed that an agroforestry research network for Africa (AFRENA) be set up, linking researchers in five widely occurring agro-ecological zones, so that those with common problems can share information. Two such networks have so far been established. One links Malawi, Tanzania, Zambia and Zimbabwe which all have highland areas characterised by a hot or warm climate and a single rainy season, where the natural vegetation is miombo or savanna woodland. The second is in the hot humid lowlands of West Africa and initially involves only Cameroon.
ICRAF is supporting the programme by first promoting the network approach among government departments and research institutions in the countries concerned and then offering training in appropriate research methods. The networks should help researchers establish which problems are best solved on a regional basis and which should be tackled at the local level.
However ICRAF is by no means the only international centre concerned with agroforestry. The International Institute of Tropical Agriculture (IITA) based in Nigeria and the International Livestock Centre for Africa (ILCA) based in Ethiopia, and with an outstation in Nigeria, have for some years been testing and developing the agroforestry system known as alley cropping.
Originally from Asia, this is one of the innovative approaches adopted to tackle the problems caused by reduced bush fallows. Food crops are grown in the alleys between the lines of trees or shrubs. The trees are cut back like a hedge while the crop is grown, but allowed to grow out and cover the soil once it is harvested. The benefits are similar to those in a traditional bush fallow, but in this case cropping and fallowing occur side by side, so allowing continuous use of the same piece of land.
The researchers have confirmed that alley cropping can be adapted to Nigerian conditions, and have tested several tree species, examining how they can be established, at what height they should be pruned, and the best distance between rows. The system has now been adopted by several villages in southwest Nigeria, although researchers were horrified to discover farmers growing highly competitive crops such as yam, melon and cassava between the young trees instead of the recommended maize. However, the trees survived and the researchers realised that what mattered was that the trees could be adapted to suit the farmers' existing system, not whether they took six months or two years to establish.
This example highlights the importance of considering the farmers' needs in order to simplify the research that needs to be done. Once it is established that a system works, and has advantages over the existing one, it is more appropriate to consider the range of conditions which component species can tolerate rather than under what conditions they grow best. For farmers with limited resources farming under highly variable conditions, it is the flexibility and diversity of traditional agroforestry systems which give them their strength.
Steppler, H A and P K R NAIR 1987 Agroforestry a Decade of Development ICRAF. Nairobi
Beets W. 1986 The Potential Pole of Agroforestry in ACP States. A State-of-the-Art Study. CTA. Wageningen
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