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

[摘要] ENGLISH ABSTRACT: The composition and state of soil-stored seed banks on- and off the mesa Tafelberg, in theNama Karoo rangelands of the Eastem Cape, South Africa, were investigated within thecontext of a broader restoration ecology project Restoration of degraded Nama Karoorangelands: the role of conservation islands'.The premise for this seed bank study was that restoration of degraded semi-aridrangelands is possible through applied management programs based on the methodologyand practice of ecological restoration. Broadly acknowledged properties of non-equilibriumenvironments (e.g. unpredictable climates and varying degrees of disturbance) and soilstoredseed banks (e.g. spatial and temporal distributions) formed the basis forinvestigating. the general environment and the properties.. of existing seed, banks, in theTafelberg locality. The potential role of hills as refugia for palatable plant species was anunder-lying element of the investigation.Following a brief investigation, of historical and contemporary research and policy onrangeland degradation in semi-arid regions of the world, the fundamental need forcomprehensive and applied seed bank research in the Nama Karoo is emphasised.Within the framework of the umbrella project, the seed bank study examined localenvironmental criteria commencing with an investigation into seed bank- and vegetationhabitats. Chemical and physical properties of soils from twenty two sites on- and off Tafelbergwere described. Substantial soil habitat variation, between the top, the north west slopes andplains and the south east slopes and plains of Tafelberg, was identified. Micro-site variationbetween open-canopy (interplant spaces) and closed-canopy (under plant cover) microhabitatswas found to be significant Primary soil habitat differences were linked to soil organicmatter content which was found to be low on the plains relative to the top and slopes; and, lowin open-canopy micro-sites relative to closed-canopy sites. Soil texture and nutrient compositionon- and off Tafelberg was found to be highly variable with significant differences between thetop and the plains as well as between the north west and south east plains. The slopes werefound 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 propertiesof soils. Anthropogenic factors for accelerated erosion, deposition, leaching and salt-crustformation were also considered. It was concluded that while properties of soils on- and offTafelberg are inherently related to parent soils, changes to textural and nutrient propertiesmay be occurring and these changes may have been exacerbated by high levels of grazing.An investigation (focusing on small shrubs) of plant phenological response, in relation torainfall and grazing gradients, identified trends of peak budding- and flowering seasonsfollowing rainfall during summer and autumn respectively. A continuum of seeding activity,with peaks in late autumn and early winter, was construed from quarterly data. Flushvegetative growth was noted for most small shrubs during spring, autumn and summersurveys. 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 highintensity disturbance (including grazing) might impact negatively on plant survival, leadingto reduced reproductive input (i.e. seeds) into future generations.Plant communities on the top and plains are described noting significant differences betweenvegetation on top of Tafelberg (comprising primarily high production, palatable grass andshrub species) and that on the plains (comprising mainly spinescent, ephemeral, toxic andlow production species). Given the high grazing pressure on the plains (relative to the lessutilised slopes and top of Tafelberg), differences in vegetation composition are discussed inrelation to studies elsewhere that describe degraded rangelands. It is concluded firstly thatthe plains surrounding Tafelberg are degraded, secondty that long term over-utilisation hasaltered vegetation composition and finally that inter-grazing rest periods of three- or sixmonths alone may not restore vegetation diversity nor 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, slopesand top of Tafelberg followed distribution patterns observed in above-ground vegetation.Species-specific data was not finalised for this thesis since not all seedlings matured andflowered within given time constraints. In order to compare seedling emergence data, plantcategories were developed that distinguished ephemerals versus persistent (perennial)species and dicotyledonous species versus grasses and other monocotyledonous plants.Samples from the top and the middle to upper slopes showed a high percentage of palatableand persistent grass and shrub species present in soil-stored seed banks while over twothirdsof plants germinating from plains' soil samples were ephemeral species and most ofthese were both tiny «Scm) and short-lived «3 months). Most of the perennial speciesgerminating from plains' samples were seedlings of Pentzia incana, Chrysocoma ciliata andsucculents (mostly Mesembryanthemaceae) but few perennial grasses were present. Onthe other hand, roughly 94% of seedlings germinating from samples from the top and 63%of seedlings germinating from two slopes of Tafelberg respectively were persistent- grassor shrubby species. Roughly 89% of ephemeral species recorded from slopes' samplesgerminated from the lowest footslope sites.Seed densities were closely linked to micro-habitats with roughly three-fold differences betweenopen- (lower seed density) and closed-canopy (higher seed density) micro-sites. Multivariateanalysis of variance (MANOVA) indicated that most of the variation in seed density data wasexplained at the micro-habitat level. Seasonality played a secondary role with significant twowayinteraction between the two effects. It is concluded that both factors must be consideredwhen developing restoration programmes that aim to improve both overall plant cover andimproved plant species diversity. While the original or pristine state of vegetation composition and seed banks on the plainsis unknown, concems were raised regarding the apparently degraded state of total above- andbelow-ground plant diversity on these plains. A likely consequence of habitat degradation isthat species with specific soil-, nutrient ratio-, aspect- and altitudinal requirements from the topand slopes of Tafelberg (as well as from nearby plains' refugia) may not be able to establish indegraded habitats on the plains. The identification of pioneer plant species (tolerant of habitatdegradation) that allow increased vegetation cover and safe-sites for seedling germination ofdesirable plant species is recommended. Restoration programmes will need to be coupledwith strict grazing management principles that allow seedling germination. establishmentand successful reproductive output of desirable plants for future rangeland regeneration.Common and salient features of the soil habitat, plant pbenological response and germinationtrial studies are brought together in an examination of habitats and related seed bank diversityon- and off Tafelberg. Acknowledging the brevity of this research study, but utilising casestudies from elsewhere and integrating both lines of questioning, the conclusion is againreached that the plains surrounding Tafelberg are degraded through decades and probablycenturies of grazing by domestic livestock. It is considered crucial that restoration throughimprovement (or rehabilitation) of habitats and increased seedling safe-sites be considered.Both climate and grazing management appear to play an irrevocably linked role in shapingvegetation composition in rangelands. While rangelands are intrinsiCalfy adapted to survivingextremes of climatic variability found in non-equilibrium regions it is argued that the impactsof grazing, particularly during times of drought and climate change, are slowly reducing theintrinsic buffer-capacity of rangelands to withstand these changes and extremes.It seems apparent from research elsewhere that restoration through passive management isslow and probably not economically achievable within a viable time frame. The instifution andpromotion of integrated and strategic programmes that identify and address issues of landdegradation and land use change in semi-arid rangelands is recommended. The input andendeavours of different authorities, ministries and a broad public participation incentive areencouraged in these proposed programmes in order to ensure broadly-based input into longterm sustainability and conservation of the considerable biological diversity of these regions.Seed bank assessment is considered to be a valuable means of indicating restoration potentialand ranqeland condition with potential for the identification of both degraded and conservationworthyareas.Finally, some limitations and challenges of this study are examined through a process offirstly identifying alternative approaches to research methodologies and secondly throughproposing recommendations for future research projects. While alternative methods couldhave been applied for the purposes of accomplishing this study it is concluded that, withinthe given time- and other constraints, the appropriate methods were applied.

[发布机构] Stellenbosch University