7.  THE TREE COMPONENT

7.2 Criteria for a good agroforestry tree

Main criteria

The main criterion for selection of a tree species is that it be liked by the farmer. A well-known tree is better than an unknown tree, but when a new species is introduced it is, of course, necessary to work with an unknown tree. A tree that is disliked by the farmer, for whatever reason, is always a non-starter in extension.

Figure 7.1 A tree with most of these characteristics is likely to be highly valued by farmers

A farmer is likely to appreciate a tree that gives a high net value, i.e. high values for the positive factors (= high "income") and low values for the negative factors (= low costs).

High "income"

The perceived value for the farmer of the direct and indirect production should be high. Which product or service is seen as being most important may vary from one area to another and from one farmer to another according to his needs. In fact the values attached to various products or services may often vary even within the family, e.g. the wife's preferences may sometimes be quite different from those of her husband. Thus it is desirable to involve both wife and husband in discussions on tree species.

Low cost

The major cost is competition with other components, which should be as little as possible. Land holdings are often small, so very competitive trees (e.g. Eucalyptus spp., Acacia mearnsii) may not be accepted by the farmers even if they are fast growing and have valuable production.

Trees with a deep root system are usually less competitive with crops than those with many shallow roots. Most indigenous trees in Kenya have deep roots, e.g. Acacia spp. and Albizia spp. Grevillea is also deep rooted. Shallow root systems may be desirable if the trees are intended to stabilize soil and the question of competition with crops is less important. Examples of trees with shallow roots are Casuarina spp., Leucaena leucocephala, Cupressus lusitanica, and Sesbania sesban, although the latter does not compete with crops since its overall root system is small and this species fixes its own nitrogen. Eucalyptus may represent a special category which has roots distributed both near the soil surface and deeper down in the soil profile.

Trees with dense shade compete with light-demanding crops such as cereals. Shading can be reduced through management. Other trees naturally have a light shade, e.g. Acacia spp., Sesbania sesban, Casuarina spp., Entada abyssinica and Eucalyptus spp. Such trees may either have small leaves, or vertically oriented leaves or be bare or partly bare during the crop-growing season.

Figure 7.2 Acacia albida over sorghum and maize, with pod basket

Some trees which naturally have rather dense shade but that can be easily managed to reduce the shade are, for example, Grevillea robusta, Markhamia lutea, Cordia abyssinica, Croton spp., Leucaena leucocephala and Calliandra calothyrsus.

Figure 7.3 Management can reduce the shade effect of trees

Other factors that determine the degree of competition are ability to fix nitrogen and the chemical composition of the litter.

Easy methods of propagation also contribute to low costs. On-farm methods are usually cheaper than seedlings produced in central nurseries, and use of cuttings, wildings or direct seeding are the cheapest. A long life span reduces the relative cost of propagation. For instance, coppicing of trees once they are established makes it possible to harvest several times from just one establishment procedure.

The risks involved in agroforestry may be related either to failures of one component, e.g. the trees do not perform well, or to failures in the interaction between components, e.g. introduction of pests or diseases. Other risks may be that trees turn out to be weeds requiring more labour, or that they prove to be a nuisance in some other way, e.g. they are poisonous. The risks are usually fewer with indigenous and well-known species than with exotic or unknown species.

Finally, trees are more likely to be appreciated by farmers if there are no negative taboos or beliefs associated with the particular species.

Figure 7.4 A shady tree provides a pleasant community meeting place

How to select tree species for extension

If extension is geared towards promotion of the growing of more trees in an area, selection of species should normally be done primarily by the farmer with technical support from the extension worker. A basic requirement for the extension worker to be effective in this process is for him or her to know the common and important tree species suitable for different situations and uses, preferably including knowledge of their vernacular names in the area concerned. Refer to the ICRAF book A Selection of Useful Trees and Shrubs for Kenya which was developed primarily to help extension workers acquire such knowledge.

If you are looking for species in addition to those already known to and grown by farmers in the area, the following are some useful hints:

• Identification of trees in urban areas

• Identification of trees near old settlements, mission centres, institutions

• Contact central institutions where databases, e.g. the ICRAF multipurpose tree (MPT) database, are available for search of suitable species.

Exotic tree species, some of which are potentially very useful in rural areas, have often been introduced in urban areas but have not spread further. Investigation of such species may often be fruitful, and the same applies to trees at mission centres or other places where foreigners or people from other areas in Kenya have lived. The ICRAF MPT database has information on a large number of species and searches can be made based on locality, ecological and land-use conditions and desired uses. The results from such searches need to be used with caution, but they can provide useful hints on potential species. The major source of information on useful trees will always be the local people, and final decisions should also always be made by them.