Evaluation of the effects of Bamboo on understory plant species diversity and Riverbank erosion control along Nyabarongo River, Ngororero district, Rwanda

Abstract

Bamboo planting in Rwanda has been thought to be effective in restoring degraded riparian zones and controlling erosion along the river banks. However, in order to prevent any adverse effects from this non-native plant species, the ecological repercussion of such intense bamboo planting needs to be evaluated. The main goal of this research was to analyse the effects of bamboo on erosion control understory plant diversity (as an indicator of biodiversity) in riparian ecosystems planted with bamboo as a climate adaptation strategy. Riparian zones planted with bamboo were compared with areas not planted with bamboo (Control sites) along stretches of the Nyabarongo river in Ngororero District, Rwanda. Bamboo and woody stems found in the selected riparian zones were sampled in 100m2 quadrats, with nested 1 m2 subplots used to sample understory vegetation (herbaceous plants, sapling and shrubs). Riverbank erosion was assessed through physical measurements and RUSLE model to depict changes in Nyabarongo river in both C-factor value and annual soil loss from 1995 up to 2023. There was a significant difference in plant species diversity between the two riparian zones, with higher plant diversity found in the control sites. The bamboo-planted riparian zone, had a total of 28 plants species, with 42.9% being native species and 35.7% exotic. Of these exotic plant species, 21.4% were invasive. Control sites had a total of 45 species, 46.7% of these were native species, and 40% exotic, and only 13.3% were invasive. The two communities shared 20 species with four of them being invasive, eight native and the other eight exotics. The results of the study also show a significant difference between land use types and species richness. In non-bamboo riparian zones, Poaceae (41,75% cover) and Fabaceae (38.5% cover) family were most common while Poaceae (41.43% cover) and Asteraceae (18.27% cover) were more common in bamboo riparian zones. There was a significant difference between slope and presence of erosion and no significant difference was found between slope and mean width of the buffer zone. The C-factor changed from 1995 to 2023, with its maximum value changed from 0.320 to 0.1 and the minimum value shifted from 0.226 to 0.05. Results suggest that planting bamboo may have an effect on the diversity of understory plants. However, annual soil loss also had changed, as estimated to 577.182 tons/ha in 1995 and shifted to 55.9972 tons/ha in 2023, these results prove the effectiveness of bamboo in controlling erosion.