Temporal and Spatial dynamics of native and exotic trees in southwestern Rwanda: case study of Nkungu and Nzahaha sectors of Rusizi district

Abstract

This study assessed the spatial dynamics of native and exotic agroforestry tree species in Nkungu and Nzahaha sectors of Rusizi District, Southwestern Rwanda, an area increasingly affected by land degradation, biodiversity loss, and declining agricultural productivity. Understanding how farmers choose and distribute tree species in such vulnerable landscapes is essential for promoting sustainable agroforestry. However, empirical data on the balance between native and exotic species, their spatial arrangement, and the motivations behind species selection remain scarce in Rwanda’s southwestern highlands. To address this gap, 100 agroforestry-practicing households were selected using Cochran’s formula, as adapted by Gahutu Mbabarira and Nahayo (2020), ensuring statistical rigor with a 95% confidence level and 5% margin of error. Data were collected through structured household questionnaires, direct tree counts, and field observations to evaluate species composition, planting history, spatial patterns, and socio-economic drivers behind species preferences. The findings revealed a dominance of exotic tree species, with a native-to-exotic ratio of approximately 1:1.92. Grevillea robusta and Markhamia lutea emerged as the most commonly planted exotic and native species, respectively. Mann–Whitney U tests indicated a statistically significant difference in planting frequency between these categories (p < 0.001). Exotic species were often planted near homesteads and accessible zones, while native species were dispersed across marginal and less-managed areas. Farmers’ tree selection was heavily influenced by institutional support (68%), economic profitability, and seedling availability. While native species were valued for their ecological functions, market pressures and limited propagation resources reinforced the preference for exotics. The study was conducted to fill a critical information gap regarding the ecological and socio-economic dimensions of tree species selection in agroforestry. By generating localized evidence on species use, distribution, and farmer preferences, the research provides practical guidance for policymakers, extension services, and restoration programs seeking to align agroforestry interventions with both productivity goals and biodiversity conservation in Rwanda and similar landscapes.