Vegetation dynamics and management models for coal-ash disposal sites at Hendrina Power Station, Mpumalanga, South Africa

Abstract

Coal-driven power stations in South Africa contribute substantially to the creation of industrial waste areas that require rehabilitation. The monitoring of rehabilitated ash disposal sites is a necessity to determine whether an ecologically sound, self-sustainable ecosystem has developed. The aim of this project is to describe the trajectory of plant community development on rehabilitated ash disposal sites and the factors governing community changes, as well as the development of condition assessment models for the ecologic al assessment of the vegetation. The study was conducted on the ash disposal sites at Hendrina Power Station, Mpumalanga Province. The study area consisted of sites on different topographic al positions, with different rehabilitation ages and treatments. Phytosociological surveys were conducted and analysed with the Braun-Blanquet method, to describe the vegetation that was used for the study. Frequency, basal cover, density and biomass surveys were conducted in 12 permanent grids over a three-year period from 1997 to 1999. Multivariate analyses techniques, analysis of variance and correlation analyses were used for analyses of quantitative data. The most abundant species on rehabilitated ash disposal sites were Eragrostis curvula, Cynodon dactylon, Hyparrhenia hirta, Cyperus escu/entus and Chamaecrista biensis . Three different plant comm unities were described on the natural grassland, on the rehabilitated ash disposal sites and unrehabilitated ash disposal sites. Newly rehabilitated disposal sites illustrated the most change in species abundances and became more similar to older rehabilitated areas during the survey period. The rate of community change of newly rehabilitated areas was significantly higher in comparison to older rehabilitated areas. Older rehabilitated areas were relatively homogenous in plant species composition and no differences in rate of change existed. Trajectories of change were towards a Hyparrheniahirta dominated grassland and followed a similar pattern as old fields on the Highveld. Although no decrease in species diversity was found over the survey period on the rehabilitated ash disposal sites, the species diversity was found to be considerably lower than that of the natural grassland. The most important factors controlling community development were fire frequency, infestations of Tatera brantsii (Highveld gerbil) and soil fertility. Important attributes which can be used for ecological assessment included the frequency of perennial grasses and annual forbs, bare ground and basal cover percentages. A state and transition model was develop to explain the dynamics of rehabilitated vegetation, and three species models as well as one model based on functional models were develop for the ecological assessment of rehabilitated vegetation. The vegetation composition on old rehabilitated ash disposal sites was found to be relatively stable due to the dominance of perennial grass species. The biomass of old rehabilitated areas were similar to the biomass of the natural grassland. Regular fires has a negative effect on litter but did not influence grass biomass as such. The densities of a number of species were correlated with biomass parameters. Results indicated that biomass is influenced by dominant species which is supportive of the mass ratio hypothesis. It is postulated that vegetation stability is a function of environmental stability especially regarding soil chemical properties. Floristic characteristics that promote plant community stability are the adaptability of the species, diversity and species persistence. Only if community stability and environmental stability have been achieved will sustainability on an autotrophic level be possible. The final conclusion is that vegetation stability has been reached from a species compositional viewpoint and will probably remain stable if the environment remains stable and viable. Diversity of rehabilitated areas is determined by the initial inputs and if the diversity of rehabilitated vegetation is to be increased, the input of adapted species must be increased to enlarge the potential species pool.