Population Structure and Exploitation of Three Commercial Tree Species in Nguru ya Ndege Forest Reserve, Morogoro – Tanzania

Abstract

Forests in tropical environments are unstable and hardly maintain fixed climax species composition over long periods. They are victims of modifications by variety of factors including human influences that result in long lasting disturbances. Having observed this, we investigated the population structure and rate of exploitation for three commercial tree species namely, Dalbergia melanoxylon, Pterocarpus angolensis and Afzelia quanzensis in Nguru ya Ndege Forest Reserve. Objectives were to determine the density, DBH distribution and the harvesting rate. We employed simple random procedure to select sampling points, where a map of the reserve was used to divide the study area into 100 equal grids. Fifty grids were then picked randomly, and within these grids, 20 by 20 m quadrats were established for counting stumps and trees with DBH ≥ 4 cm. In addition, saplings with DBH of ≤ 4 cm were counted inside 5 by 5 m quadrats placed within the 20 by 20 m quadrats. Results showed that, the density of P. angolensis was 19 stems/ha, A. quanzensis 8 stems/ha and D. melanoxylon 3.1 stems/ha. DBH graphs showed characteristic inversed J shapes for P. angolensis and A. quanzensis, but broken pattern for D. melanoxylon. Harvesting rates (stumps/ha) was 4.5 for A. quanzensis, 4.0 for P. angolensis and 1 for D. melanoxylon. We concluded that, all target species were found facing harvesting pressure, with the population structure of D. melanoxylon being disturbed. We recommended that harvest of trees in the reserve should be controlled. Keywords: Density, DBH distribution, Illegal harvest, Tropics