The relationship between the abundance of non-native tree species and the community structure of resident woody species in tropical montane forest of West Usambara Mountains, Tanzania

Abstract

Non-native tree species are globally claimed as liable to the encroachment of natural forests in their proximity resulting to the loss of biodiversity. This study was conducted in Magamba Nature Forest Reserve (MNFR) in West Usambara Mountains of Tanzania. A total of 96 (20 m x 20 m) plots were established to record all woody plants with Diameter at Breast Height (DBH) ≥ 5 cm while two nested plots of 2 m x 5 m were set to measure any plant with DBH ≥ 1 cm but less than 5 cm. Furthermore, two nested plots of 1 m x 1 m were established within the bigger plot in order to assess and record seedlings (DBH < 1 cm). Generalized Linear Models (GLMs) were used to assess the relationship between total abundance, species diversity, richness, evenness, and biomass of resident species and the abundance of Acacia melanoxylon and Eucalyptus grandis. Findings show a significant (p < 0.05) decrease of abundance of all resident woody species, adults, saplings and seedlings with an increase in the abundance of A. melanoxylon. The study also showed that the species richness and diversity of all resident species decreased with increasing the abundance of non-native species (p < 0.05). The abundance of E. grandis did not show significant (p > 0.05) association when assessed against the total abundance, species diversity, richness and evenness of all the resident woody species. This non-significant result may not necessarily mean no effect. This is because the P-value does not explain the size of the effect in biological data. Further analysis revealed that, the biomass of resident species was negatively related to the abundance of non-native species A. melanoxylon (p = 0.003) and E. grandis (p = 0.038). The negative relationship between the abundance, richness, diversity, evenness, and biomass of the resident woody species and the abundance of non-native species portrayed in this study raise an alarm. Therefore, there is need for setting monitoring and control mechanism to stop further spread of the non-native tree species in MNFR and hence, save the remaining biodiversity of the area.