Secondary Vegetation and the Transition to Savanna Near Ibadan, Nigeria

The fproblem. Ibadan lies about 30 miles (48 km.) inside the northern boundary of the Lowland Rain Forest zone of Western Nigeria. This type of vegetation has been described and discussed by a number of writers including Ainslie (1926), Keay (1948, 1953), MacGregor (1937), Richards (1939, 1952) and E. W. Jones (1955, 1956) in Nigeria. It is generally conceded that the northern part of this zone, in which Ibadan lies (variously termed Mixed Deciduous or Semi-deciduous Forest, Dry Forest or Foret Mesophile), differs from the wetter forests to the south both in the higher proportion of deciduous trees, and in the frequency of such species as Triplochiton scleroxylon and Celtis spp. There is, however, little agreement as to what criteria should be used for distinguishing these forest types, or indeed whether it is possible to find any satisfactory basis for defining communities in the Lowland Rain Forest. Most of the attention has been directed towards forest vegetation, and there has been comparatively little detailed work on the secondary vegetation of agricultural land (previous work is discussed in Section V). This is no doubt because of the great complexity of a tropical ecosystem in which there is no clear distinction between natural vegetation and farmland; for the peasant farmer never completely clears his land before planting his crops, and when it is exhausted he restores its fertility by letting it revert to bush for several years. At. first sight the farm land around Ibadan appears to be a chaotic patchwork of food crops, cacao, oil palm and bush fallow, in all possible combinations. Indeed, the average size of 'field' is only about one-tenth of an acre (0 05 ha.), so that there is usually a recognizable change in the vegetation every twenty paces. The object of this paper is to show that, although we may be unable to define clear-cut associations in secondary bush, we can distinguish certain 'nodal' communities akin to the 'noda' of Poore (1955,1956). Even these are necessarily diffuse, for uniform stands 'at least as large as the minimal area' seldom occur in this type of vegetation, and the concept of a community must be built up from the observation of a large number of fragments. Never-theless, they enable us to discern some order in the apparently continuous network of variation, and can be used as a reference to trace certain trends of succession. B J.E. 217