5. INTERACTION BETWEEN TREES AND CROPS

5.4  Competition for moisture by the roots

Some of the positive effects attributed to trees with regard to soil improvement have a direct link to soil-moisture retention. Successful soil conservation contributes to infiltration of water into the soil and reduces water runoff. A high organic-matter content increases the water-holding capacity of the soil, and thus also contributes to reduction in runoff. Roots in the soil also increase the infiltration, as do remnants of dead roots. Litter or mulch on the ground reduces evaporation, and reduced wind speeds lessen drying of the soil.

All these positive factors have, in each situation, to be weighed against the increased competition for moisture that follows the introduction of trees or shrubs. Recent findings indicate that in alley-cropping systems the negative effects on crops of competition for moisture increase more rapidly than the benefits from having shrubs in the system the drier the area is. In other words, in dry climates (rainfall below 800 mm) the introduction of alley cropping may increase moisture stress so much that the net effect on crop yield is negative.

A tentative conclusion is that it may be extremely difficult to extend technologies that involve close interaction of fast-growing trees and shrubs with crops to dry areas (Akyeampong et al, 1992). It is doubtful if such technologies will be relevant in areas receiving less than 800 mm rainfall annually and in areas with acidic soils.

Fast-growing trees normally consume more water than slow-growing ones. However, some fast-growing trees are efficient water users, e.g. Eucalyptus have been reported to use less water per unit of biomass produced than many other tree species (Davidson, 1989). So, the severe competition that can be observed near fast-growing trees should be attributed to their fast growth not to their "inefficient" use of water.