Edaphic and Topographic Gradients Have Differential Influence on Woody Species Assemblages on Ultramafic and Non-Ultramafic Soils in an African Savanna

Background and aims Understanding the determinants of plant species associations on unique ultramafic substrates is crucial for the study of restoration ecology. We investigated the influence of local edaphic and topographic gradients on woody species associations on ultramafic and non-ultramafic substrates along the Great Dyke of Zimbabwe. Methods Vegetation attributes were assessed in 62 plots on ultramafic and adjacent non-ultramafic substrates at varying slope magnitude and orientation. Plant community comparisons and relationships with soil and topographic variables were analyzed using ANOVA and ordinations. Results Aspect had more influence on woody composition, species associations and densities on ultramafic compared to non-ultramafic substrates. Lower species richness and tree/shrub densities were observed on ultramafic substrates. Soil Mg, Mg/Ca ratio, total Ni, Cr and Mn, and available Ni were significantly higher on ultramafic substrates. Most parameters (pH, Ca, Mg, Mg/Ca ratio; available Ni, Cr, Mn and total Mn) were similar between ultramafic east- and west-facing slopes, but only total Cr and Ni were higher on east-facing slopes. Only available Ni and Mn were higher on ultramafic piedmont than on slopes. Tree/shrub density and species richness were positively correlated with available Mn and Cr while negatively correlated with total and available Ni, pH, Mg/Ca ratio and herbaceous plant cover. Conclusion Vegetation patterns on ultramafic substrates are partly driven by intra-site edaphic (metals and Mg/Ca ratios) and topographic gradients. Aspect has differential influence on woody vegetation assemblages on ultramafic and non-ultramafic substrates. Species associations and environmental determinants observed can be used in mine site rehabilitation planning.