When soils are exhausted, this allows the growth of certain weeds. A practice similar to this has been described in a part of south-western Nigeria to raise shade-tolerant crops such as Dioscorea spp and cocoyam in, essentially, a permanent forest setting (Adesina, 1988). In addition to the fact that agroforestry techniques can be perfected to cope with the new conditions that are anticipated in a drier climate and with a higher population density, they lead to an increase in the amount of organic matter in the soil, thereby improving agricultural productivity and reducing the pressure exerted on forests.