VARIABILITY IN THE GROWTH OF FAIDHERBIA-ALBIDA - THE SOILS CONNECTION

Variability in the growth of Faidherbia albida (Del.) A. Chev. (syn. Acacia albida [Del.1) in the Sahelian zone of West Africa is a common observation. The ''albida effect'' has been attributed to the amelioration of soil chemical, physical, and microbiological properties by the direct action of the tree, resulting in enhanced crop production beneath the mature canopy. Crop yields within fields without F. albida can also be highly variable within short distances. A study was conducted to observe the association between the variability in soil properties and the variable growth of young (2-yr-old) F. albida trees planted at the ICRISAT Sabelian Center located near Niamey, Niger. The results of this study indicate that the variability in F. albida growth is caused, in large part, by variability in soil properties across relatively short distances. Good growth sites had higher clay contents and base saturations, and lower exchangeable acidity than poor growth sites within the field. The tallest trees in the field were associated with the proximity to abondoned termite mounds and other microhigh sites. Seedlings planted on sheet-eroded sites caused by runoff from microtopographical high sites within the field grew poorly. The variability in F. albida growth in plantations within Niger is hypothesized to be due, in part, to preexisting soil conditions. These results also suggest that the albida effect might be partially caused by these preexisting ''islands of fertility''. This site-determined variable growth of F. albida could be exploited with proper seedling placement strategies.