Wind protection in a hedged agroforestry system in semiarid Kenya

In semiarid Laikipia (Kenya) severe crop damage and loss of mulch material may be caused by south to south-easterly winds from June to September. Demo nstration agroforestry systems which surround farms with live fences had so me success in protecting crops, mulch and soil, but great care must be take n, because air may be channelled through or over them. For demonstration pu rposes, a deliberate gap was made in a two meter high Coleus barbatus live fence to study its effect on wind speed and damage to crops. The effectiven ess of protection given by this hedge together with intercropped Grevillea robusta trees was quantified using electrical cup anemometers. The combinat ion of hedges and trees gave protection to a maize/bean intercrop but the b iomass distribution was not the most suitable one. This picture was complic ated by variable wind direction and interactions between wind and the bioma ss of hedges and trees. The lowest efficiency of wind reduction occurred cl osest to the deliberate gap, where also the lowest protection by the Grevil lea trees was found. South to south-easterly winds increased the gap effect and caused gradients in tree protection perpendicular to the southern hedg e. This gave at times wind speeds even higher than outside the system. Only visually wind effects could be detected. Trees and hedges strongly compete d with the crops if not root pruned.