Tree-root influence on soil physical conditions, seedling establishment and natural thinning of Acacia seyal var. seyal on clays of Central Sudan

Abstract

Acacia seyal is an important component of the agrosilvopastoral systems of the Sahelian zone of Africa. The relationships between lateral spreading of the tree roots and the establishment of the understorey vegetation are, however, little understood. The effect of the tree roots on soil bulk density, soil moisture content and understorey vegetation was studied in the Central Clay Plains of the Sudan. Three study sites were included: a forest, a-two-year-old logged-over area and a-one-year-old abandoned farm, and these sites were distributed over three different geographical locations. Roots of trees, tree seedlings and annual herbs were concentrated in the top 70 cm of soil, possibly due to compacted soil and the confinement of rain water in this layer. Soil bulk density at the 60 to90 cm layer was high in forest, low in logged-over and intermediate in farm. Soil moisture content varied in the reverse order. The low soil moisture content and the high soil bulk density reduced seedling survival. Abundant Acacia seyal seeds germinated and grew during the rainy season. During the first dry season, however, 86% of seedlings of forest died, whereas only 11% of logged-over and 14% of farm died; canopy shading did not seem to affect seedling survival. Tree seedlings and agricultural crops are not able to compete with the A. seyal trees for soil resources but appear to benefit from the ameliorated soil texture, structure and fertility following tree felling.