An investigation of the root distribution of Parkia biglobosa in Burkina Faso, West Africa, using a logarithmic spiral trench

The root systems of nine mature Parkia biglobasa trees were excavated using a logarithmic spiral trench. The trees were well isolated from other trees and shrubs, and were situated in fallow fields at three sites representative of different vegetation types and climatic conditions in Burkina Faso. The tree roots extended to at least 10 m from the trunk, thereby exploiting an area twice that of the crown. The vertical distribution of roots was shallow, with approximately 60% of the roots present in the upper 20 cm of soil, with the highest density of roots occurring at a depth of 10-20 cm. Rooting intensities of tree roots reached a maximum of 350 m(-2) at 1 m from the tree trunk in the 0-50 cm soil horizon. Roots larger than 10 cm in diameter were only present in the first 6 m from the trunk. Soil nitrogen and potassium concentrations were higher close to the tree trunk, in the superficial soil layers, suggesting amelioration by the tree. The results for phosphorus were not conclusive, possibly because of inputs from rock phosphate fertilisers and defecation by cattle beyond the crowns of the trees. pH showed no change with increasing proximity to the tree. The pattern of root distribution of the tree and the amelioration of soil fertility beneath the tree crown revealed by this study indicate that there is likely to be limited competition for nutrients between the tree and crops planted beneath the tree crown. In sub-Sahelian West Africa where soil fertility is a major factor limiting food production, these areas of soil fertility beneath P. biglobosa could be maximised by cultivating crops whose root systems have complementary distribution to that of the tree, such as sweet potatoes, yams and cassava, peppers and other vegetables. (C) 1998 Elsevier Science B.V.