Relationship between soil-stored seed banks and degradation in eastern Nama Karoo rangelands (South Africa)

Landscape, local scale and seasonal composition of soil-stored seed banks of a southern African semi-arid rangeland (Nama Karoo) were investigated through assessment of seed germinated from field-collected soil. Variables included percentage above-ground canopy cover, numbers of seeds germinated, and selected physical and chemical soil characteristics. Organic matter content of soil sampled from under plant canopies (closed-canopy) is significantly higher than that of inter-canopy samples (open-canopy). Significantly more seeds germinated from closed-canopy soil samples are compared with those from open-canopy microsites. It is argued that reduction in rangeland canopy cover due to overgrazing (and resultant erosion through wind and water movement) leads to reduced seed retention by the leaf litter layer and thus by the soil. This degraded resource appears to have gradually lost an inherent ‘buffer capacity’, undermining attempts at ecological restoration and necessitating concerted and directed efforts to restore these fragile systems.