Abstract.The relationship between plant diversity and productivity has been hotly debated over the last two decades. What makes the relationship complex is, in part, the interplay of several biotic and abiotic effects, which include rainfall variability, soil characteristics and grazing. We investigated the influence of grazing intensity on the diversity-productivity relationship in a wide range of soil pH along a rainfall gradient in semi-arid grassland. Vegetation and soil surveys were conducted in high grazing (HG) and low grazing (LG) grasslands around Bethlehem (716 mm mean annual rainfall, moderately acidic soil), Bloemfontein (543 mm, slightly acidic/neutral soil) and Kimberley (467 mm, neutral/slightly alkaline soil) in South Africa. Plant species occurring in the study area were recorded, and aboveground biomass was estimated by clippings. Soils sampled were analysed for chemical properties. Species richness increased with increasing biomass and decreasing pH in Kimberley and Bloemfontein and subsequently declined with increasing biomass and decreasing pH in Bethlehem. The relationship between species richness and biomass was hump-shaped across the study area but when we differentiated between the degrees of grazing, we found linear positive relationships at both LG and HG sites. This suggests that the diversity-productivity relationship needs to be carefully examined in grazing land.
Vachellia karroo is a useful and widespread tree in Africa. It belongs to the family Fabaceae, which is the third largest woody plant family in southern Africa. This is an ecologically and economically important species as almost all of its parts, including bark, pods, seeds, leaves and thorns, are extremely useful to both humans and animals. Various commercial products are also obtained from the tree, and gum is one of the most important products. V. karroo in South Africa has an extensive distribution range that includes several biomes. It is very adaptable and has wide habitat tolerance, growing under many differing conditions of soil, climate, and altitude. Although it is often associated with heavy, clayey soils on the banks of rivers and streams, it also grows in bushveld, dry thornveld, grassland and woodland. V. karroo is easy to grow and as a result can become an aggressive invader of valuable farming land and grazing areas, a phenomenon usually referred to as bush encroachment. An analysis of historic data comprising 1553 relevés and 2006 species, compiled from all areas of South Africa where V. karroo is known to occur was conducted, and TWINSPAN classification produced five main vegetation types.
Natural vegetation in urban environments is greatly impacted by human activities and it is in constant threat of degradation and destruction as a result of urbanisation. This vegetation, although fragmented, serves an important ecological function and needs to be properly managed and conserved. Studies on urban vegetation are lacking in South Africa, with only a handful having been carried out since the end of the last century. This study was initiated to identify, classify and describe the grassland communities of the urban open spaces in Bloemfontein. Relevés were compiled in 61 sample plots, where species present and habitat information were recorded. Care was taken to restrict sample plots to vegetation in pristine condition, wherever possible, and severely degraded stands were avoided. A two-way indicator species analysis (TWINSPAN) classification, refined by Braun-Blanquet procedures, revealed two distinct major communities, seven communities and four sub-communities. Both detrended and canonical correspondence analyses indicated the vegetation units to be associated with soil texture and pH, although biotic factors such as overgrazing, burning and mowing also influence the composition of the vegetation. The proper management and conservation of urban open spaces requires in-depth knowledge of the spatial distribution, floristic, structural and functional compositions within the major vegetation types in this environment. The present study further contributed towards formulating ways for the proper management, utilisation and functioning of the open spaces within the Bloemfontein area.Conservation implications: The Grassland Biome of South Africa is poorly conserved, mainly because of its status as an agricultural hub of the country. The preservation of natural and semi-natural forms of urban vegetation is important because such vegetation, although often disturbed and degraded, could form dispersal corridors between peri-urban and rural vegetation.
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