Tree occurrence in agricultural landscapes of peri-urban Kigali in semi-arid context of Rwanda

Abstract

Abstract In tropical Africa, a wide range of agroforestry practices exist, resulting in various configurations of trees in the landscape, with an unknown impact on ecosystem services. We evaluated tree occurrence, structure, and composition in agroforestry systems within four contrasting landscapes representing different levels of tree cover in peri-urban Kigali and in the eastern province of Rwanda. We used a stratified random sampling procedure, based on soil depth and slope, to allocate 60 plots of 0.26 ha in each of the four landscapes. We conducted an extensive field inventory to provide deeper insight into the dendrometric characteristics of woody vegetation and compare landscapes. We counted all woody plants with ≥ 4 cm diameter at breast height (DBH). Over 70% of the individual trees were exotic tree species across the four agricultural landscapes. The three most common and dominant tree species across the landscapes were Grevillea robusta, a tree used mainly for timber which represents 42.7% of all individuals encountered, followed by Mangifera indica (12.34%) mainly used for fruit production, and Euphorbia turcalii (11.04%) mainly used for hedgerows. Most of the agroforestry trees encountered were small, scattered individuals, with almost 80% < 15 cm DBH bordering the farmlands and cut before reaching bigger sizes. The occurrence of trees varied considerably within and between landscapes, suggesting the effect of agroecological systems. Here, future studies may potentially address this link more explicitly. We showed that it is important to consider native tree species in landscape restoration initiatives given a high adoption rate of exotics.