Soil seed bank dynamics in relation to land management and soil types in the semi-arid savannas of Swaziland

In the semi-arid savannas of Swaziland, communal grazing, commercial ranches and game reserves are the main land management systems for animal production. These land uses can have different ecological effects on the rangelands. This study was conducted to investigate the differences in soil seed bank characteristics between three land management systems having high (communal land), low (government ranch) and medium (game reserve) stocking rates, and soil types (lithosol and vertisol or raw mineral). Two grazing areas (Bigbend and Simunye) each with the three land management systems and soil types were selected for this study. A total of 40 plant species were identified in the seed bank of both grazing areas. Panicum maximum dominated both grazing areas. Cencrhus ciliaris was dominant in Bigbend area, while Digitaria eriantha was dominant in Simunye area. Seedling density varied (P<0.05) among the land management systems and soil types. The difference in past and recent grazing pressure may be the primary cause of the observed differences. In all study sites, the soil seed bank was a poor reflection of the aboveground vegetation as revealed by weak similarity. In Bigbend, P. maximum and Urochloa mosambicensis were well represented in the seed bank. In Simunye, however, these two species were comparatively under-represented. Whereas the regeneration of the aforementioned two species from the seed bank may be high and have a profound effect in restoration of rangelands after disturbance, this may be affected by spatial differences in terms of rainfall and soil. Valuable species under-represented in the seed bank can be prone to extinction under heavy utilization and therefore, in situ conservation within certain localities should receive high priority.