A Structural and Floristic Analysis of Woody Fallow Vegetation Near Ibadan, Nigeria

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Abstract
The structure and floristics of 8 yr old secondary forest regrowth covering 40 ha of abandoned farmland near Ibadan in Nigeria were studied, with the aim of improving the basis for the use of such vegetation as an index of site conditions. All vascular species occurring in each of 50 randomly-located 25 .times. 25 m plots were recorded. Woody stems .gtoreq. 10 cm girth at breast height (1.3 m) were enumerated in each plot. Vertical structure was examined by constructing a profile diagram for a 10 .times. 100 m transect. The general features of the vegetation were dominance by phanerophytes, a rich flora (approximately 150 species on 0.25 ha), and a high stem diversity. The families represented by the greatest number of species were Papilionaceae, Euphorbiaceae, Rubiaceae and Gramineae. The Apocynaceae, Euphorbiaceae, Mimosaceae, Moraceae and Sapindaceae were each represented by at least 3 spp. with frequencies exceeding 60%. The abundance of Holarrhena floribunda and Newbouldia laevis was the most distinctive floristic feature of the vegetation as a whole. The vegetation profile showed a patchy upper canopy-layer at 20 m and a more continuous one at 5 m. Ordination by principal components analysis showed that there were detectable gradients in both structure and floristic composition of the vegetation, reflecting variations in stem density and the abundance of certain species. The gradients were interpretable in terms of soil type, topography and previous land use. The absence or rarity of species (e.g., Acioa barteri) typical of fallows on soils derived from sedimentary rocks, and of Musanga cercopioides (abundant in wetter areas), was noteworthy in the drier Ibadan site, located on metamorphic Basement Complex rocks. Intensive land use in the Ibadan area was indicated by the abundance of Allophylus africanus, Millettia thonningii and Phyllanthus discoideus. Attention is drawn to the implications of these findings for the assessment of site conditions in the Ibadan area.