Large shrubs increase soil nutrients in a semi-arid savanna

The soil under savanna woody species is often enriched in nutrients in what is termed an ‘island of fertility’. We tested for positive feedbacks between encroaching woody plants and soil fertility in two co-occurring shrub species at three sites. One of these shrub species is nitrogen-fixing, Acacia mellifera, and the other is non‑nitrogen-fixing, Tarchonanthus camphoratus; we compared these effects to the grasslands surrounding the shrub patches. We found that soil nutrient concentrations were usually related to shrub size rather than the species. Fertile patches developed underneath large shrubs as indicated by higher carbon and nitrogen concentrations, a higher CECeff (especially sodium, which is a limiting nutrient for grazing livestock) and a pH near 6. We found no difference in soil nitrogen between the N-fixing A. mellifera and T. camphoratus. Plant cover under large shrubs was less than in open grassland. There are ‘islands of fertility’ under large shrubs. However, the development of fertility islands did not facilitate understorey growth. Thus, increased soil fertility had no positive feedback on overall vegetation composition. Negative effects of the woody vegetation may override the positive effects of increased soil nutrient availability.