Phytosociology of the Mpumalanga high altitude grasslands

Abstract

A phytosociological study covering approximately 12 000 km2 was completed within Southeastern Mpumalanga high altitude grasslands. The towns of Belfast, Barberton, Piet Retief and Wakkerstroom represented the four corners of the area. The study formed part of the Grassland Biome Project sponsored by the Department of Environmental Affairs and Tourism. The Braun-Blanquet approach was applied. Grassland research results from the western side of the country, determined that soil patterns and vegetation distribution are significantly correlated. Based on these results, it was hypothesised that the same correlation would exist in the east. The eight soil patterns used during the survey as homogenous units, were Ab, Ac, Ad, Ae, Ea, Fa, Fb and lb. A pro rata, randomly stratified sample size of 405 plots were used. An in-depth literature study of the environmental and other factors related to vegetation distribution, indicated on local scale that a significant correlation exists between landform and plant distribution. Two geographic information systems, Idrisi and Arc-Info/Niew, aided in the modelling and extraction of environmental attributes from existing databases. A TWINSPAN classification of the complete floristic data set falsified the null hypothesis based on the soil patterns but verified the null hypothesis based on landforms. The TWINSPAN dendrogram revealed clusters associated with three landforms (mountains, hills and lowlands, and plains) in two ecosystems, the terrestrial and the wetlands or aquatic. The three landforms represented three mapping units: Southeastern Mpumalanga Mountain Vegetation Type, Southeastern Mpumalanga Hills and Lowland Vegetation Type and Southeastern Mpumalanga Plain Vegetation Type. A second TWINSPAN classification was executed on each of the vegetation types. The resulting clusters were tested for uniqueness and informational value using a set of criteria. Those clusters which qualified were arranged in a Braun-Blanquet table to determine communities, sub-communities and variations using constancy and fidelity values. The plant communities within the vegetation types were described in terms of floristic composition and environmental attributes. The indirect gradient analysis ordination program DECORANA was used to determine environmental trends and was confirmed by using multiple regression. Soil water availability was the most significant environmental trend between the two ecosystems and within the communities of the ecosystems in each vegetation type. Soil water availability is influenced by numerous factors, whose significance differs from one vegetation type to another. lt was concluded from this study that, in high rainfall areas, soil patterns and vegetation distribution are not significantly correlated but that landforms and vegetation distribution are significantly correlated. Landscape slope configuration or landform influences soil water availability through soil characteristics (rockiness, texture and depth) and local climatic conditions (aspect, perpendicular insolation and precipitation). lt is suggested that the current Grassland - Savanna Biome boundary be changed towards the west using the boundary between the covered and exposed shields. This suggestion is supported by the presence of Hyparrhenia hirta/Hyperthelia dissoluta - Acacia sieberiana communities to the east of the study area in the different vegetation types which correlate significantly with the thorntree-tall grass savanna formation on a global scale. AFRIKAANS : 'n Fitososiologiese studie is gedoen van die Suid-oostelike Mpumalanga hoe liggende grasveld tussen die dorpe Belfast, Barberton, Piet Retief en Wakkerstroom. 'n Area van ongeveer 12 000 km2 is bestudeer. Die studie is deel van die Grasveld Bioom Projek gefinansier deur die Departement van Omgewingsake en Toerisme. Die Braun-Blanquet benadering is gebruik. Resultate van grasveldnavorsing in die westelike deel van die grasveld bioom, het aangedui dat 'n betekenisvolle verband tussen grondpatrone en plantegroei verspreiding bestaan. Na aanleiding van hierdie resultate, is die hipotese gemaak dat dieselfde verband in die ooste sal voorkom. Agt grondpatrone, Ab, Ac, Ad, Ae, Fa, Fb, en lb is as homogene eenhede beskou. 'n Eweredige ewekansige steekproefneming van 405 punte is gebruik. 'n Deeglike literatuurstudie van die omgewing en faktore wat verband hou met plantegroei verspreiding, het aangedui op lokale skale, dat 'n verband tussen landvorm en plantegroei verspreiding bestaan. Twee geografiese inligtingstelsels, ldrisi en Arc-lnfo/View, is gebruik in die modelering en verkryging van omgewings data vanuit bestaande databasisse. 'n TWINSPAN klassifikasie gebaseer op die volledige spesiesamestelling, het die nul hipotese oor die grondpatrone ongeldig verklaar maar die nul hipotese oor die landvorme bevestig. Die TWINSPAN dendrogram het groeperings bevat wat verband hou met drie landvorme (berge, heuwels en laaglande en vlaktes) in twee ekosisteme, terrestrieel en vleiland van akwatiese omgewings. Die drie landvorme is beskou as drie karteerbare eenhede: Suid-oostelike Mpumalanga Berg Plantegroeitipe, Suidoostelike Mpumalanga Heuwels en Laagland Plantegroeitipe en Suid-oostelike Mpumalanga Vlakte Plantegroeitipe. 'n Tweede TWINSPAN klassifikasie is op elk van die plantegroeitipes toegepas. Elke plantegroeitipe se groeperings is met behulp van bepaalde voorwaardes vir uniekheid en inligtingswaarde getoets. Die groeperings wat gekwalifiseer het, is met behulp van konstantheids en getrouheids waardes in 'n Braun-Blanquet tabel gerangskik in gemeenskappe, sub-gemeenskappe en variasies. Elke plantegroei tipe se gemeenskappe is beskryf in terme van floristies spesiesamestelling en omgewingsfaktore. 'n indirekte gradientanalise ordeningsprogram DECORANA is gebruik vir die bepaling van omgewings tendense en is bevestig met behulp van veelvuldige regressies. Grondwaterbeskikbaarheid was die mees betekenisvolle omgewingstendens tussen die ekosisteme en die gemeenskappe van die plantegroei tipes. Grondwaterbeskikbaarheid word deur 'n aantal faktore beïnvloed, waarvan die belangrikheid van plantegroei tipe tot plantegroeitipe wissel. 'n Gevolgtrekking van die studie is, dat in hoë reënvalomgewings daar nie 'n betekenisvolle verband is tussen grondpatrone en plantegroei verspreiding nie maar wel tussen landvorme en plantegroeiverspreiding. Landskap hellingsamestelling of landvorme beïnvloed grondwaterbeskikbaarheid deur middel van grondeienskappe (klipperigheid, tekstuur en diepte) en lokale klimaatstoestande (aspek, loodregte bestraling en presipitasie). Daar word voorgestel dat die huidige Grasveld- Savanna Bioom grens weswaarts geskuif word om ooreen te stem met die grens tussen die bedekte en blootgestelde plate. Die voorstel word ondersteun deur die teenwoordigheid van Hyparrhenia hirta/Hyperthelia dissoluta - Acacia sieberana plantgemeenskappe in die ooste van die plantegroeitipes wat betekenisvol ooreenstem met die langgras-doring boom savanna formasie op globale skaal.