Local-scale tree cover modulates pollination services and crop yield in tropical agroforestry landscapes, Rwanda

Context: Global pollinator decline threatens food security, especially in regions with intensive agriculture and habitat loss. Agroforestry, a sustainable agricultural diversification practice, could mitigate these issues by providing diverse habitats and floral resources for pollinators, yet its effects on pollination services and crop yields in tropical Africa remain undocumented. Objectives: This study examined the impact of tree density and landscape-level tree cover at various spatial scales on pollinator visitation and pollination success of the common bean in Rwandan smallholder agroforestry systems. Methods: We used two bean varieties from 18 farms with varying plot-level tree density and landscape-level tree cover for the study. Bean plants were subjected to two treatments: bagged (pollinator exclusion) and open (natural pollination). Pollinator surveys were conducted in 15-min intervals. Results: We found that open-pollinated flowers had significantly higher yields than bagged flowers, highlighting the critical role of insect-mediated pollination in bean production. Insect pollination increased crop dry matter yield by 120.63%, pods per branch by 163.23%, and beans per pod by 10.81% compared to auto-pollinated flowers. Our findings also show that increased honeybee visitation enhances the number of bean pods, with this effect influenced by local tree density and flower abundance. While higher local flower abundance promoted honeybee activity and supported pollination, increasing tree cover within 500 m of the landscape reduced the number of beans per pod. Conclusions: Our findings underscore the importance of insect-mediated ecosystem services in agroforestry for enhancing crop pollination and this effect was mediated by local scale tree cover. By examining interactions between ecological and management factors, we provide insights into optimizing both productivity and pollination services. The study recommends further research to determine the thresholds at which tree cover shifts from beneficial to detrimental and to further explore which species composition in agroforestry systems influence pollinator support, particularly whether native trees are more effective in promoting pollinator activity within multifunctional landscapes.