The effects of land use and microsite availability on early seedling recruitment of Acacia tortilis (synonym: Vachelia tortilis) in a southern African savanna

Many tree recruitment studies in African savannas target individual factors that limit recruitment, yet recruitment of some species could be limited by an interplay of environmental filters. This study investigated the effects of land use and microsite availability on early seedling recruitment of Acacia tortilis in riverine woodlands in north-western Zimbabwe. The riverine woodlands were divided according to the level of human disturbance into protected and unprotected sites. Each site was further divided into riverine and off riverine microsites. In each microsite, we laid five main plots (100 x 50 m) and nine seedling subplots (5 x 5 m) nested in each main plot, making a total of 60 main plots and 180 subplots. We found higher seedling recruitment in unprotected than protected sites. Although seeder density was twice greater in protected than unprotected sites, seed bank density was lower in the former by a factor of 1.5. Tree cover and herbaceous biomass was 1.4 and 1.8 times greater in protected than unprotected sites, respectively but were negatively correlated with seedling density. Across all microsites, herbivory-related mortality was less than 40% while drought-related mortality was more than 60%. Put together, the results suggest that higher recruitment in unprotected sites was most likely facilitated by domestic ungulates through seed dispersal and grass suppression which enhanced the quality of regeneration sites. We conclude that encroachment of A. tortilis in communal grazing sites is not only important for its conservation but for a variety of goods and services derived from it.