Trees in shifting and continuous cultivation farms in Ibadan area, southwestern Nigeria

This study compares the types and density of tree species in continuous cultivation farms at the fringe of Ibadan city, Nigeria, with shifting cultivation farms in a rural district near the city. Both categories of farm are used for cultivating field crops of maize, cassava and yams that are intercropped. Tree density is much higher in continuously farmed plots at the city outskirts, where tree density averaged 511 trees/ha compared to 143 trees/ha in shifting cultivation farms in the countryside. In spite of the general similarity in tree flora between the two categories of farmlands, certain tree species are more abundant in one than the other. Gliricidia sepium, an exotic tree legume that improves soil fertility, is more abundant on the continuous cultivation farms while the oil palm, Elaeis guineensis, is more abundant on the shifting cultivation farms. Certain tree species such as Myrianthus arboreus, Alchornea laxiflora and Blaghia sapida were restricted to the shifting cultivation farms and Trema guineensis to the continuously cultivated plots. It seems that the farmers at the urban fringe, having eliminated the bush fallow period, depend mainly on trees on the farmland, especially G. sepium, for the maintenance of soil fertility, as they do not apply fertilizers, while the shifting cultivators retain mainly trees of economic and utilitarian importance on their farms. It is advocated that the shifting cultivators should preserve more trees as farmers at the urban fringe do. This will not only facilitate tree regeneration, on which restoration of soil fertility during the fallow period largely depends, but also help to conserve soil fertility during cropping and to augment farmers' fuelwood supplies. (C) 2001 published by Elsevier Science B.V.