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Titre : Structural and functional attributes of Heuweltjies in the fynbos and succulent Karoo biomes : the interaction of termites, vegetation and geochemistry

Auteur : Vermonti, Nicola

Université de soutenance : Stellenbosch University

Grade : Master of Science ( MScConsEcol) 2022

Résumé partiel
The structural and functional attributes of heuweltjies in the Fynbos and Succulent Karoo biomes were studied with the aim of investigating the interaction amongst vegetation, termites, and geochemistry on heuweltjies found across the West Coast of South Africa. This study takes a multi-disciplinary approach to determine possible sources of salinity in the biotic community, that can be considered inputs into heuweltjie mound soils. Heuweltjie vegetation communities were found to host different plant species that were structured differently and consisted of different plant functional types. These differences occurred between heuweltjies found in the Fynbos and Succulent Karoo, and among the centre, slope, and off areas of the heuweltjie mound. Anthropogenic disturbance such as grazing was also found to bring about changes in the structure of the vegetation communities. A subsample of the most common plant species growing in these communities were found to host variable concentrations of plant tissue ions, with sodium, magnesium, and potassium as the major ions found in higher concentrations, in the species growing on the mounds. These plants are suspected to have an important role in maintaining, but not causing the higher ion concentrations which are seen in central heuweltjie soils, as these plants could take up ions through their roots into their tissues and return them when abscission or senescence occurs. This creates a feedback loop with the soil environment. Where soils are saline, halophytic plants would be expected to form part of this cycle. Two external sources of salt were identified as viable contributors to the accumulation of salts that are associated with heuweltjie mounds. Aerosols deposited on the outer surfaces of the plants were identified to have a marine source of sodium, magnesium, and sulphate. Calcium deposits were enriched compared with seawater ion ratios. The foraging activities of Microhodotermes viator and foraging rodents were the second external source investigated to assess their impact on heuweltjie soils, using stable δ13C and δ15N isotope analysis. An evaluation was made of the plants that most likely were targeted by termites and rodents, using observations and δ13C. It appeared that termites collect live plant material nearby to their foraging holes, with the exception of any succulent plant species in the Fynbos, or stem and leaf succulents in the Succulent Karoo. This study shows that calcium oxalate crystals (whewellite CaC2O4·H2O) are present in plant tissues on centre mound soils as well as in the plant material collected by termites and rodents while weddellite (Ca(C2O4)·2H2O) was present but only in the centre mound plants of the Fynbos.

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