8. SOME AGROFORESTRY PRACTICES IN KENYA                   

8.2 Trees dispersed in cropland

Spatial arrangement

In most farming areas there are scattered trees growing in the fields. Often such trees were left when the land was cleared for agriculture, or naturally dispersed seeds may have germinated and the seedlings been deliberately protected during farming operations. In such cases, the spatial arrangement is a random one. In other situations, trees may have been planted or sown, and then they are often in lines. In areas where farming is mechanized, a system with trees in lines rather than at random will facilitate operations.

Figure 8.1 Trees dispersed in cropland

Areas where the practice is relevant

In most situations, it is feasible to have trees in cropland but it is particularly relevant in areas with shade-tolerant crops like coffee or beans. There are limited possibilities in irrigation schemes. The benefits to be obtained from the trees, in terms of soil fertility and soil structure, are normally more clearly seen in areas where little or no inorganic fertilizer is used.

Establishment and spacing

Many methods of tree propagation are possible. Which one is chosen will depend largely on which species are desired. Protection of wildings is a good option which should always be considered before recommending nursery production of seedlings. Direct sowing may be feasible if trees are to be grown in lines, otherwise it is difficult to know where the seeds will germinate.

A population of up to 100 trees per hectare, corresponding to a spacing of 10 m x 10 m, is appropriate in high-potential areas if the crop is light demanding, e.g. maize. If trees are to be planted, a better option than square spacing may be to plant trees in lines with close spacing in the rows and a wider between-row space resulting in a similar overall density. A spacing of 5 m within rows and 20 m between rows can be suggested. The tree and crop species and management methods chosen are factors that will influence decisions on spacing. With shade-tolerant crops a greater density of trees can be tolerated, as can a certain loss in yield if the tree products are in high demand. If the trees are of a species which has very light shade, e.g. some Acacia, more trees can be grown per hectare, and the same applies if the trees will be intensively pollarded. Root pruning can also be done to minimize competition.

In ASAL areas, lower tree populations are recommended, e.g. 40-70 trees per hectare, corresponding to a square spacing of 12-15 m. If trees in cropland are to be established from seedlings, a greater number than that ultimately required need to be planted since there will always be some mortality.

Management aspects

Tree-management practices will depend on the tree and crop species concerned and the need for tree products. With a light-demanding crop, pollarding or pruning is essential, but this does not apply to shade-tolerant crops or to trees that naturally only have light shade. Species that need and tolerate pollarding are Albizia spp., Balanites aegyptiaca, Bauhinia spp., Bridelia micrantha, Cassia siamea, Commiphora eminii subsp. zimmermanni, Cordia abyssinica, Croton macrostachyus, Ficus sycamorus, Grevillea robusta, Markhamia lutea, Morus spp., Piliostigma thonningii, Spathodea nilotica, Syzygium spp. and Terminalia spp.. Timber production and light reduction both call for pollarding high up, at 12-15 m, but shade tolerance and pole or fito production call for lower and less frequent pollarding. It is important to consider the land-use pattern throughout the year before recommending tree planting in cropland. Post-harvest grazing and burning of crop residues are other factors that need to be taken into account.

Benefits and risks

Establishment of trees is easy since the tree seedlings will benefit from weeding and protection of the crop. A variety of products can be harvested from the trees, and among service functions soil improvement, wind-speed reduction and improved microclimate are important. Too many trees can cause reduced yields due to shade, and trees may attract birds which may be a threat to crops. Competitive trees with shallow roots, such as Eucalypts, pines and Acacia mearnsii should be avoided. Trees that cannot be pollarded and which grow to a considerable size producing dense shade should also be avoided, e.g. Acrocarpus fraxinifolius. Other trees to avoid are Croton megalocarpus, Cupressus lusitanica and Ficus benjamina. Nematode build-up has been reported under Sesbania sesban.

Examples of species

Albizia spp., Cassia siamea, Commiphora eminii subsp. zimmermanni, Grevillea robusta, Sesbania sesban, Ficus sycomorus, Bridelia micrantha, Croton macrostachyus, Cordia abyssinica, Jacaranda mimosifolia (only in high-rainfall areas), Spathodea campanulata, Markhamia lutea, Acacia spp., Combretum spp., Terminalia spp., Piliostigma thonningii, Erythrina abyssinica, Entada abyssinica, Balanites aegyptiaca, Morus spp., Faurea saligna, Ficus natalensis, Grewia spp., Melia volkensii, Moringa oleifera, Psidium guajava, Calliandra calothyrsus, Flemingia macrophylla, Gliricidia sepium.