The Effects of Small-Scale Agricultural Expansion on Tree Species Composition and Diversity along Elevation Gradients in Tanzania’s Dry Sub-Montane Tropical Forest Ecosystem

African forest ecosystems harbour significant biodiversity, yet face constant pressure from neighbouring communities, primarily due to selective harvesting and expansion of agricultural farming practices. Studies on how the forest structure changes because of small-scale farming within the landscape are limited. This study evaluated the effects of agricultural expansion on tree species composition and diversity along elevation gradients in a dry tropical forest mountain ecosystem. Vegetation data were collected from 60 sample plots established in disturbed areas affected by small-scale agriculture and another 60 plots from relatively undisturbed areas, across different elevations. We used agglomerative hierarchical clustering to identify tree species communities and an indicator species analysis to determine species significantly associated with each community. Species richness, evenness and Shannon-Wiener diversity indices were calculated using the “vegan” package in R software and compared between disturbed and undisturbed areas using Generalised Linear Models (GLMs). We recorded 1576 individual trees from 64 species and 27 families. Most of the identified tree species were shared among the three species communities. Agricultural practices were significantly linked to lower tree species richness and altered community composition compared to undisturbed areas, with varying effects along elevation gradients. Higher elevations exhibited low species diversity and composition, while mid-elevations showed more diversity in both disturbed and undisturbed zones. Disturbance from small-scale agricultural practices had a pronounced impact on species diversity at lower elevations. Our findings highlight the importance of considering topographic heterogeneity in conservation planning and sustainable land management strategies, emphasising strict regulations and effective measures to mitigate small-scale agricultural practices adjacent to and within protected forest ecosystems.