Ecological aspects of vegetation establishment on landfills.

Abstract

A high level of plant mortality on the Bisasar Road landfill, Durban, South Africa initiated an investigation into the primary causes of the mortalities and a search for potentially tolerant plant species. Field studies revealed that volunteer grass growth on cover soils was primarily limited by elevated soil C02, with high soil conductivity and low soil moisture possibly compounding the effect. Cynodon dactylon, the most abundant coloniser of the site appeared to be relatively sensitive to high soil CO2, whilst less common species such as Sporobolis cifricanus and Paspalum Paspoloides appeared to be less sensitive. Further research focused on the high mortality of trees planted on the landfill providing insight into the important variables limiting survival· and the relative differences in performance of 20 tree species. A more rigorous 14-month field experiment was designed and constructed, to assess the performance of 10 of the more promising tree species, the environmental conditions limiting tree growth and the benefit of a deeper layer of better quality topsoil. Some species, such as Barringtonia racemosa, performed relatively well in the field experiment, whilst other species such as Syzygium cordatum, and Harpephyllum caffrom experienced high mortalities and poor growth. The better quality topsoil layer provided little improvement in the performance of the stronger or the weaker species, however significant improvements were recorded for species with relatively intermediate performance. The composition of the soil atmosphere was shown to determine rooting depth. Species that performed better had deeper roots, possibly assisting them in utilising deeper soil moisture reserves. It was concluded that high soil CO2 and low soil O2 levels were the key variables responsible for poor tree survival and growth in this field experiment. A soil fumigation system was designed to provide more control of soil gas concentrations and to experimentally investigate differential species responses and the relative effects of soil CO2 and O2 on tree survival and growth. The apparatus fumigated, for a period of 140 days, the rhizosphere of 80 potted 'tolerant' (Barringtonia racemosa) and 'non tolerant' (Harpephyllum caffrum) trees with 4 treatments consisting of varying combinations of CO2 and 02. The difference in performance of Barringtonia racemosa and Harpephyllum caffrum in the experiment on the landfill was similar to that of the elevated CO2 low O2 fumigation treatment, supporting the premise that landfill gas was the key cause for poor performance of plants. Reduced stomatal conductance and resultant limitations on photosynthesis were found to be indicative of species sensitivity. Low O2 had an additive effect on the impact of elevated CO2 in Harpephyllum caffrum however, even with normal soil O2 levels, 25% soil CO2 had negative growth effects on this sensitive species. Maintenance of plant health and better performance of Barringtonia was attributed to a high inherent level of tissue porosity and aerenchyma. The research provided a greater understanding of the causes of poor vegetation growth and the possible mechanisms of species tolerance to landfill conditions.