8. SOME AGROFORESTRY PRACTICES IN KENYA                   

8.5 Trees on soil-conservation structures

Spatial arrangement

Trees can be planted as biological soil-conservation measures (tree strips), either in combination with grasses or alone. Trees alone are effective enough only on gentle slopes (less than 8%). On steeper slopes combinations with grasses are desirable.

Figure 8.5 Siting trees on terraces

Trees can also be planted on terrace edges in order to stabilize the structure and make maximum use of the land. Trees planted on the edge of a terrace will not have ideal moisture conditions but are effective for stabilizing the soil. Fruit trees, which need the best moisture conditions, can be planted just below the edge of the terrace where they can benefit from run-off and more moisture generally.

Trees can be planted on many other types of soil-conservation structures to stabilize the soil and to make optimal use of the land.

Figure 8.6 Trees on contour ridges

Areas where the practice is relevant

These technologies are relevant in all areas where cultivation takes place on sloping land.

Establishment and spacing

To be effective as tree strips, trees should be planted in lines along the contours. The spacing between the contour "tree strips" is determined in the same way as for other soil-conservation structures. Within rows spacing can be 0.1-0.2 m for shrubs and 0.5-1.0 m for trees. If the trees are combined with grasses or shrubs the spacing can be wider, and the spacing should also be wider in dry areas because of moisture competition between the trees.

The spacing for trees and shrubs intended to stabilize terraces can be 1.5-3.0 m for trees (including banana, pawpaw and guava), and 0.3-0.5 m or wider for shrubs if the farmer so wishes. Initial establishment can always be more dense and thinning can be carried out later to obtain the final spacing. On steeper slopes an option is to grow trees on every second terrace to avoid having too great a density of trees.

Management aspects

The trees are managed in the same way as trees in fields. Shrubs must be trimmed almost as though they were in a live fence in order not to occupy too much land.

Benefits

Which of the many possible products and services can be obtained from the trees will depend on the choice of species. The role of stabilizing structures is important. If perennials are grown on bunds, for example, the structures will become more permanent and will be respected by tractor drivers when they are ploughing.

Examples of species

Basically the same as for trees in fields, but more fruit trees can be added, e.g. bananas, Citrus spp., Psidium guajava, Cyphomandra betacea, and Carica papaya. On steeper slopes, preference should be given to trees which either will not grow very big or to trees that can be pollarded since it is desirable to have a high density of trees. The steeper the slope, the shorter the horizontal interval between the soil-conservation structures, so if trees are grown on all the structures more trees are required per hectare to stabilize them.