5. INTERACTION BETWEEN TREES AND CROPS

5.5  Shade

Beside competition for moisture and nutrients between trees and crops, shade is another factor of great importance. Shade has positive effects on the soil due to lower temperatures and hence a reduced rate of decomposition of organic matter resulting in improved soil structure, better water-holding capacity and less erosion.

Some trees, e.g. Grevillea, were introduced to Kenya primarily as shade trees for coffee. This use of Grevillea is an example of shade from a particular species being well tolerated which may lead to an overall increase in production as compared to coffee grown without shade.

The shade effect is, however, often negative on crops, and it becomes more significant the more light-demanding the crops are. The negative effects of shade can easily be reduced by managing the trees to reduce the degree of shade, e.g. by pruning or pollarding. Such management not only reduces shade but is also a way of harvesting useful products from the trees.

Sometimes the negative effects of shade on crop yields can be tolerated. If the value of production from the trees is high enough, it may outweigh the value of the crop loss and thus the overall benefit increases in spite of lower crop yields.