The effects of tropical forest structure and species composition on species diversity were studied using the data of 279 biodiversity inventory plots, in the forest reserves of the East Usambara Mountains (EUM) in the northeastern part of Tanzania, among the 25 global "hot spots" for biodiversity conservation. A standard method of vegetation plot sampling was used, based on a 450 m grid system constructed throughout the forest on transect lines. A plot of 50 m x 20 m was sampled in each grid square and all trees with a diameter at breast height (dbh) (greater than or equal to10 cm) were measured and identified. The results revealed that the tree species richness and abundance in the EUM were comparable to the reported maximum richness and abundance in the Neotropics and in Southeast Asia. No significant difference in species numbers between intact mature natural forest and former logged or former fired forest or former cultivated area was observed, but size inequality in species richness and stem density in the intact mature forest was significantly less than that in disturbed forests. This implied that biodiversity of intact mature forest was more stable and frequent human disturbances would threaten the stability of biodiversity. The number of species and stems decreased from the smallest trees to emergent trees, while the occurrence rate of species (species number/stem) increased with tree size. Significant negative relationship between relative species richness (species number/1000 stems) and tree densities in all tested size classes suggested a global negative power-law in the relationship between them. The results of the present study, in summary revealed that species diversity in Tanzanian tropical forests was significantly influenced by structure and composition of forests. (C) 2002 Elsevier Science B.V. All rights reserved.
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