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University of Stellenbosch (2000)

An assessment of the potential for utilisation of soil-stored seed, from on- and off ’conservation islands’ (isolated mountains), as an indicator of restoration potential of degraded sites in semi-arid Karoo areas

Jones, F. Elizabeth

Titre : An assessment of the potential for utilisation of soil-stored seed, from on- and off ’conservation islands’ (isolated mountains), as an indicator of restoration potential of degraded sites in semi-arid Karoo areas

Auteur : Jones, F. Elizabeth (Freda Elizabeth)

Université de soutenance : University of Stellenbosch

Grade : Master of Science (MSc) 2000

Résumé
The composition and state of soil-stored seed banks on and off the mesa Tafelberg, in the Nama Karoo rangelands of the Eastern Cape, South Africa, were investigated within the context of a broader restoration ecology project Restoration of degraded Nama Karoo rangelands : the role of conservation islands. The premise for this seed bank study was that restoration of degraded semi-arid rangelands is possible through applied management programs based on the methodology and practice of ecological restoration. Broadly acknowledged properties of non-equilibrium environments (e.g. unpredictable climates and varying degrees of disturbance) and soil-stored seed banks (e.g. spatial and temporal distributions) formed the basis for investigating the general environment and the properties of existing seed banks in the Tafelberg locality. The potential role of hills as refugia for palatable plant species was an under-lying element of the investigation. Following a brief investigation, of historical and contemporary research and policy on rangeland degradation in semi-arid regions of the world, the fundamental need for comprehensive and applied seed bank research in the Nama Karoo is emphasized. Within the framework of the umbrella project, the seed bank study examined local environmental criteria commencing with an investigation into seed bank and vegetation habitats. Chemical and physical properties of soils from twenty two sites on and off Tafelberg were described. Substantial soil habitat variation, between the top, the north west slopes and plains and the south east slopes and plains of Tafelberg, was identified. Micro-site variation between open-canopy (interplant spaces) and closed-canopy (under plant cover) micro-habitats was found to be significant. Primary soil habitat differences were linked to soil organic matter content which was found to be low on the plains relative to the top and slopes ; and, low in open-canopy micro-sites relative to closed-canopy sites. Soil texture and nutrient composition on and off Tafelberg was found to be highly variable with significant differences between the top and the plains as well as between the north west and south east plains. The slopes were found to be intermediate (showing some level of gradient) between the top and the plains. Soil nutrient variation was interpreted as a function of textural and parent-material properties of soils. Anthropogenic factors for accelerated erosion, deposition, leaching and salt-crust formation were also considered. It was concluded that while properties of soils on and off Tafelberg are inherently related to parent soils, changes to textural and nutrient properties may be occurring and these changes may have been exacerbated by high levels of grazing. An investigation (focusing on small shrubs) of plant phonological response, in relation to rainfall and grazing gradients, identified trends of peak budding and flowering seasons following rainfall during summer and autumn respectively. A continuum of seeding activity, with peaks in late autumn and early winter, was construed from quarterly data. Flush vegetative growth was noted for most small shrubs during spring, autumn and summer surveys. Since some form of activity related to reproductive output (flush growth, budding, flowering or seeding) was apparent at almost all times of the year, it is argued that high intensity disturbance (including grazing) might impact negatively on plant survival, leading to reduced reproductive input (i.e. seeds) into future generations. Plant communities on the top and plains are described noting significant differences between vegetation on top of Tafelberg (comprising primarily high production, palatable grass and shrub species) and that on the plains (comprising mainly spinescent, ephemeral, toxic and low production species). Given the high grazing pressure on the plains (relative to the less utilized slopes and top of Tafelberg), differences in vegetation composition are discussed in relation to studies elsewhere that describe degraded rangelands. It is concluded firstly that the plains surrounding Tafelberg are degraded, secondly that long term over utilization has altered vegetation composition and finally that inter-grazing rest periods of three of six months alone may not restore vegetation diversity or desirable plant species to the plains. Results of germination trials (investigating soil-stored seed banks) from two sampling episodes (Spring and Autumn 1998) revealed that seed banks in soils removed from the plains, slopes and top of Tafelberg followed distribution patterns observed in above-ground vegetation. Species-specific data was finalized for this thesis since not all seedlings matured and flowered within given time constraints. In order to compare seedling emergence data, plant categories were developed that distinguished ephemerals versus persistent (perennial) species and dicotyledonous species versus grasses and other monocotyledonous plants. Samples from the top and middle to upper slopes showed a high percentage of palatable and persistent grass and shrub species present in soil-stored seed banks while over two-thirds of plants germinating from plains’ soil samples were ephemeral species and most of these were both tiny (smaller than 5cm) and short-lived (less than 3 months). Most of the perennial species germinating from plains’ samples were seedlings of Pentzia incana, Chrysocoma cliata and succulents (mostly Mesembryanthemaceae) but few perennial grasses were present. On the other hand, roughly 94 percent of seedlings germinating from samples from the top and 63 percent of seedlings germinating from the slopes of Tafelberg respectively were persistent grass or shrubby species. Roughly 89 percent of ephemeral species recorded from slopes’ samples germinated from the lowest footslope sites. Seed densities were closely linked to micro-habitats with roughly three-fold differences between open (lower seed density) and closed canopy (higher seed density) micro-sites. Multivariate analysis of variance (MANOVA) indicated that most of the variation in seed density data was explained at the micro-habitat level. Seasonality played a secondary role with significant two-way interaction between the two effects.

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