A geobotanical investigation of mountain ecosystems in Griqualand West, South Africa

Abstract

The Griqualand West Centre (GWC) of plant endemism harbours a unique flora of which 24 species are endemic. Heterogeneous geology, climate and topography are considered drivers of the unique flora and local endemism. However, these drivers have not yet been investigated and our understanding of the effects thereof on vegetation dynamics remains poor. Four mountain ecosystems, each underlain by different rock types and with distinct climatic patterns, provided a setting to investigate the effects of ecological drivers shaping vegetation dynamics of this semi-arid area. Therefore, the primary aim of this study was to disentangle the effects of rainfall and geology, through soil properties related to the underlying geological parent material, as drivers of floristic patterns, plant diversity and structure, biomass production, and the relationship between diversity and biomass production. The objectives of this study were to (i) redefine the borders of GWC to establish which mountain ranges fall within the centre by using a MaxEnt spatial model based on geology, climate and topography in combination with distribution data of GWC endemics, (ii) describe the flora within the newly redefined borders of GWC based on dominant plant families and -species, indicator plant species, endemic species and species composition, (iii) compare soil properties, rainfall, plant diversity and structure between mountain ecosystems to test whether mountains, within the newly defined borders of GWC, differ significantly from each other, (iv) determine whether soil properties, rainfall or a combination thereof act as drivers of plant diversity and structural differences between mountains, (v) test for differences in total biomass production (above ground green plant material and debris), live biomass production (only live green above ground plant material) and respective plant functional group (PFG) biomass production between the four mountain rangelands, (vi) relate differences to specific soil properties and rainfall to identify the strongest drivers of biomass production, (vii) investigate diversity-biomass relationships for total plant species and for species representing different PFGs, and (viii) present an optimal range of biomass production at which herbaceous species diversity can be maintained at regional scale. Results obtained from this study revealed that each mountain plant community was characterised by unique herbaceous plant communities with specific indicator plant species, driven by soil properties and rainfall. Herbaceous plant composition, density, height, cover and shrub frequencies were related to a combination of soil properties and mean annual rainfall. However, plant diversity, and grass, lignified forb and tree frequencies, as well as woody plant height and canopy area, could only be related to soil properties. Grasses, lignified forbs and herbaceous forbs contributed to biomass production in descending order. At regional and local scales, diversity-productivity relationships followed non-linear trends. However, optimum biomass production was reached at highest diversity. Semi-arid mountain landscapes in GWC provide important ecosystem services through their unique plant diversity. It is necessary to follow a holistic, multi-disciplinary conservation and management approach to not only manage for species diversity, but to conserve the underlying environmental drivers in semi-arid mountain plant communities.