Species richness, diversity, density, and spatial distribution of soil seed banks in the Moremi Game Reserve Riparian Woodlands of the Okavango Delta, northern Botswana

This study aimed to investigate the soil seed banks in the Moremi Game Reserve Riparian Woodlands (hereafter MGRWs) of the Okavango Delta, northern Botswana. We determined diversity indices, densities, spatial distribution of seeds in the soil, and compared the similarity in species composition between the standing vegetation and soil seed bank flora. The vegetation was sampled in 42 plots (20 × 50 m) and soil samples were collected from 336 subplots. Soil seed bank communities were determined using agglomerative hierarchical cluster analysis. Indicator values were calculated using indicator species analysis for species in each community of germinated seeds and across different soil layers. To infer spatial relationships of soil layers in terms of soil seed bank composition Bray-Curtis ordination was used. A total of 124 plant species were identified in the litter and top 9 cm soil layers with a mean density of 1,933 seeds m-2. Herbs, grasses, sedges, and woody plants were represented by 69, 25, 17, and 13 species, respectively, in 33 families and 92 genera. The overall diversity and evenness of the soil seed bank in the MGRWs were 3.7 and 0.77, respectively. The results revealed that Poaceae, Cyperaceae and Asteraceae are the most dominant families in all the germinated species. Four plant communities, namely Kohautia virgata-Ammania baccifera, Bidens pilosa-Urochloa mosambisensis, Setaria verticillata-Brachiaria deflexa, and Cynodon dactylon-Cyperus longus were identified from the soil seed bank. Bray-Curtis ordination showed that there was an overlap between these communities in terms of seed bank composition. However, MRPP analysis showed that there was significant (P < 0.05) separation between germinated soil seed bank communities. The overall spatial horizontal distribution of seeds varied among sampling quadrats while the vertical distribution of seeds exhibited the highest densities occurring in the upper 3cm of the soil and gradually decreasing densities with increasing depth.