Effects of A. sieberiana trees on soil properties and plant diversity were investigated in Songa pastures, Rwanda. Tree characteristics and crown architecture of A. sieberiana were studied. Soil properties were assessed and plants were identified under and away from tree crowns. Counts of individual plants/species were done only under tree crowns. Nitrogen, P, and K were analysed in the soil, grass, and A. sieberiana leaves. Plant diversity was determined using Simpson's diversity index. Data were subjected to ANOVA. Soil organic carbon (SOC), cation exchange capacity (CEC), Ca2+, N and pH, and plant diversity were higher in soils under tree canopies than in open areas. Tree leaves were significantly richer in N and poorer in P and K as compared to grasses. Tree crowns grew wider and horizontal and developed intertwined secondary branching, reducing light intensity to as low as 38% under tree canopies compared to the open pasture. At 3 trees/ha stocking, A. sieberiana trees shaded 0.18？ha and herbaceous plants and grasses unpalatable to livestock dominated under tree canopies. A tradeoff of A. sieberiana tree value versus the loss of palatable grass due to tree presence needs to be assessed to decide whether the trees should be included in pastures and if yes, the apporpriate stocking identified. 1. Introduction The incorporation or retention of trees into farming systems is an old tradition in Africa. This makes a basis of what is often referred to as agroforestry [1]. Trees in such systems fulfil a lot of functions and services [2]. They may provide shelter to soils, plants, and livestock and may increase soil organic matter and sometimes soil nitrogen content. Being perennial, tree functions and their influence on the site may be cumulative [3]. A few or a group of trees are usually left deliberately or planted on land under some other main use in which case trees fulfil a secondary role [4]. It is well known that some trees may deplete soil resources and this is more severe with some exotic species, for example, Eucalyptus [5], which compete with other plants for soil moisture [5–7] and for soil nutrients [8–10]. More often, some tree species negatively interfere with biodiversity, leading to a decline in plant diversity [11]. This tendency is sometimes attributed to allelopathy [12–14]. Some plant species may improve soil properties [15] and yet deplete biodiversity. Even the best trees in soil improvement may be detrimental to biodiversity, especially when planted at a narrow spacing where the reduction in the amount of light reaching the ground
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