The implementation of selected technologies to enhance the restoration of indigenous tree species in the deforested riparian areas in the Mapungubwe National Park, South Africa : a case study
Stretches of forest along the Limpopo and Shashe Rivers have been classified as a unique forest type in the vegetation of South Africa and are considered as being "critically endangered" by the South African Biodiversity Institute. Roughly 400 hectares of this riverine forest area inside the western section of the Mapungubwe National Park (MNP), a UNESCO World Heritage site, were deforested and therefore degraded due to previous agricultural cultivation practices. Given the extent of forest degradation that has occurred, the restoration of this area by means of the re–vegetation of indigenous trees to its former composition is one of the objectives of the MNP's management plan. The successful establishment of tree seedlings, especially in semi–arid systems, is however presented with a wide range of constraints and limiting conditions, which often result in very high mortality rates during restoration projects. An experimental exclosure, as identified by South African National Parks (SANParks), was therefore fenced off inside the degraded old lands to act as a demonstration site for the restoration of indigenous trees. A pilot study conducted in 2006, involved the transplantation of selected indigenous tree species with the aim of evaluating suitable re–vegetation technologies. The research contained in this dissertation was also conducted inside the experimental exclosure, where recommendations derived from the pilot study were evaluated, including the assessment of new re–vegetation technologies to enhance the establishment of the indigenous trees. This study was therefore a follow–up project which involved both field– and greenhouse trials. Seedlings of the following species were either transplanted into the experimental exclosure (field trial) or cultivated inside a controlled environment in the greenhouse at the North–West University: Acacia xanthophloea Benth. (fever tree), Berchemia discolor (Klotzsch) Hemsl. (brown–ivory), Combretum imberbe Wawra (leadwood), Faidherbia albida (Delile) A. Chev. (ana tree), Philenoptera violacea (Klotzsch) Schrire (apple–leaf), Salvadora australis Schweick. (narrow–leaved mustard tree) and Xanthocercis zambesiaca (Baker) Dumaz–le–Grand (nyala tree). During the follow–up study the effects of various enhancement treatments were tested regarding the survival, growth and physiological performance of seedlings in both the field– and greenhouse trials. The enhancement treatments consisted of the addition of compost and indigenous arbuscular mycorrhizal fungi (AMF). In addition, seedlings transplanted during the pilot study, which did not include enhancement treatments, were also monitored for establishment and growth. The potential use of established Acacia tortilis Hayne trees to facilitate growth and establishment and to act as "nursing plants", was also assessed. In addition, various pre–sowing treatments were also applied to seeds of selected tree species in the greenhouse to assess the germination rate. The survivorship and growth of seedlings in both the field– and greenhouse trial were determined by using three growth parameters, namely "stem diameter at the base", "stem diameter 30 cm from the base", and "height of the tree in its natural growth form". Chlorophyll fluorescence induction (JIP test) was measured on seedlings in both trials, using the multi–parametric expression, namely performance index (PIABS), as a measure of the overall vitality of the plants of each species–treatment combination. Physical and chemical analyses were carried out on the soil inside the experimental exclosure. Basic descriptive statistics were used to analyse seedling survival and germination rates, and a two–way analysis of variance (ANOVA) was used to determine the statistical significant effects of the various enhancement treatments on diameter growth in each species (p < 0.05). Fluorescence data were processed using the Biolyzer software and significant effects in each species were determined using the Student's t–test (p < 0.05). Multivariate data ordinations using the CANOCO package were used to determine the differences in soil types inside the experimental exclosure. Moisture stress due to transplantation shock, competition with dense grass cover and herbivory, resulted in an overall 55.8% seedling survival rate and negative stem diameter growth for transplanted seedlings in the field. In comparison, seedlings cultivated in the greenhouse had much higher survival rates and showed positive stem diameter growth. Most species in the greenhouse showed higher growth rates and significantly higher vitality values when planted with enhancement treatments. The responses of transplanted seedlings to the enhancement treatments were very species–specific in the field trials. Based on these results, it was concluded that the enhancement treatments were beneficial with regard to the establishment and growth of most of the species. The beneficial effect was however cancelled out by the various abiotic and biotic factors encountered in the natural environment. Seedlings transplanted in the understory of established pioneer A. tortilis trees had much lower survival rates as the extensive root system of A. tortilis most likely out–competed the transplanted seedlings for moisture and nutrients. Many seedlings were also predated by insects or small mammals which reduced the growing potential. The germination trials recorded the highest germination rates for most species when germinated in the compost–containing treatments. These trials also indicated that all of the investigated species showed higher survival rates when pre–sowing treatments, such as soaking, mechanical scarification and removing the seed from fruit, were applied. Various recommendations emphasising long–term monitoring, proper maintenance and after–care of future restoration efforts are made. These include experimental layout of exclosure plots and pretransplantation treatments of seedlings while cultivated in the nursery. During this study, the experimental exclosure was also used as a demonstration site for training and capacity building for SANParks personnel and students from academic institutions.