8. SOME AGROFORESTRY PRACTICES IN KENYA                   

8.9 Trees as windbreaks

Spatial arrangement

Windbreaks are lines of trees or shrubs whose main aim is the reduction of wind speed. Well-designed windbreaks, i.e. ones that are not too dense, not only reduce wind speed but may also increase humidity and reduce water loss from the soil. The positive effect of a windbreak is said to be felt up to a distance 20 times the height of the trees in the windbreak.

Areas where the practice is relevant

Large windbreaks are most relevant in large-scale farming areas since otherwise they would extend over many small farms thus causing complicated planning and requiring good co-operation between the farmers. Boundary planting of trees and live fences is usually sufficient as windbreaks in small-scale farming areas. Larger windbreaks are mostly seen in the former White Highlands, e.g. Trans Nzoia, Nakuru, and Uasin Gishu.

Figure 8.11 Multi-storey windbreaks

Establishment and spacing

Any propagation method is suitable, and for large-scale farms planting of seedlings raised in nurseries may be the most common. A windbreak should be planted at right angles to the prevailing wind. It can either consist of a single line of trees with a spacing of 1.5-2.0 m, or two lines with a spacing of 4-5 m within the line and 2-4 m between the lines. In addition to one or two lines of trees, a line of shrubs spaced at approximately 1 m can be planted on the side facing the prevailing wind.

Figure 8.12 Windbreak design

Management aspects

As with any other newly planted trees, initially a windbreak needs to be protected against livestock and fire. Careful weeding and replacement of dead seedlings also need to be carried out. Later the trees in the windbreak may need pruning or pollarding to maintain a suitable density and to minimize the shading effects on crops. Dead trees or trees that have been blown over must be replaced.

Benefits

The environmental benefits, reduced wind and increased retention of moisture, are the most important ones. On the negative side, some land is lost for crop production, but the reduction of wind may well compensate for the loss of land. In addition the windbreak will produce wood.

Examples of species

Acacia albida, Albizia spp., Anacardium occidentale, Annona senegalensis, Azadirachta indica, Balanites aegyptiaca, Calliandra calothyrsus, Calodendrum capense, Cassia siamea, Casuarina spp., Cupressus lusitanica, Ekebergia capensis, Eriobotrya japonica, Eucalyptus spp., Gliricidia sepium, Gmelina arborea, Grevillea robusta, Hakea saligna, Juniperus procera, Macadamia tetraphylla, Mangifera indica, Markhamia lutea, Morus alba, Olea europaea, Prosopis spp., Prunus africanus, Psidium guajava, Spathodea campanulata, Syzygium cuminii, Trichilia emetica, Vitex spp., Ziziphus spp.