Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Long-term ecosystem dynamics of contrasting grasslands in South Africa

Auteur : Dabengwa, Abraham Nqabutho

Université de soutenance : University of Cape Town.

Grade : Doctor of Philosophy (PhD) 2019

Résumé partiel
Rainfall, fire, and grazing all control changes in vegetation and soil in grassland and savanna ecosystems. In these ecosystems, wetlands are key resource areas because they keep moisture and collect nutrients that support grass production. The grass production supports high grazer densities in landscapes, especially during dry climatic periods. The equilibrium idea suggests that, at high densities, herbivores reduce grass production and damage soils. In contrast, the disequilibrium idea argues that unreliable rainfall and frequent droughts lower herbivore densities to levels rendering their effects negligible. Thus, grass production and grazer densities rarely stabilise. However, nonequilibrium theories suggest the relevance of both ideas in natural systems. Spatial and temporal scales used for looking at landscapes and the resilience of persistent soil and grass states control which idea wins. In turn, stability of vegetation states is related to traits of grass biomass including palatability, flammability, and tolerance to drought. At long timescales, we remain uncertain about how grass production in landscapes are affected by indigenous herbivores, and those managed with fires by pastoralists for livestock. In this thesis, I test nonequilibrium dynamics with stability domains of grass biomass, i.e., centres of stable vegetation states (tallgrass versus shortgrass), to assess the resilience of contrasting key resource areas. Long-term sediment proxy data offer the opportunity for assessing vegetation and soil dynamics over many centuries. Grassland ecosystem dynamics were compared between two sediment cores from South African wetland grasslands. They were the productive montane grassland (Vryheid) controlled by fire and Hluhluwe-iMfolozi Park, a lowland savanna with grasses suppressed by indigenous herbivores. Vegetation change, grazing pressure, fire activity, nitrogen availability, grass biomass, soil stability, and age-depth models of sediments were studied respectively, with fossil grass phytoliths, fossil dung fungal spores, charcoal, stable isotopes, organic carbon (SOC), and x-ray fluorescence (XRF) spectrometry. Cluster and non-metric multidimensional scaling ordination methods were used to organise grass phytoliths, dung spores, and charcoal collections to uncover states of grass mosaics, grazing pressure, and fire activity, respectively. Also, grass states were evaluated by comparing changes in the relative intensity of fire and grazing with time. A long-term regional rainfall record provided a background for landscape scale changes in herbivore densities and local-scale interaction among moisture, grazing pressure, and grass biomass. Archaeological records suggested the presence of pastoralists in the region

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