The evaluation of the establishment and growth of indigenous trees to restore deforested riparian areas in the Mapungubwe National Park, South Africa
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The deforestation of riparian areas is a major concern in southern Africa. These areas are characterized as fragile ecosystems which contribute largely to the regional and global biodiversity of the world. It is therefore important to restore these degraded areas along the natural rivers of South Africa to ensure the sustainability and biodiversity of riparian corridors. Riparian areas inside the National Parks of South Africa, and especially in Mapungubwe National Park, have a high esthetical value and should be preserved for future generations. The study was conducted in the Mapungubwe National Park, which is listed as a cultural world heritage site. Plans are in place to convert it into one of Africa's biggest Transfrontier Parks, called the Limpopo/Shashe Transfrontier Conservation Area (TFCA), which will be situated between neighbouring countries Zimbabwe, Botswana and South Africa. The main purpose of this project was to establish a demonstration site for the restoration of degraded, previously cultivated lands in the deforested riparian areas in the Mapungubwe National Park, Limpopo Province. Another aim of the project was to evaluate the theoretical assumption that the growth of trees on so called "activity lines" in the environment due to geological and soil characteristics is enhanced. "Activity lines" were identified by Mr. Lynn van Rooyen of South African National Parks (SANParks) and trees of which the growth was tested, were planted both on and off "activity lines". The selection of the right type of trees for the restoration of the deforested riparian areas during active restoration applications is very important and depends on a multitude of factors. These factors include aspects such as the location with its specific vegetation, soil type and climatic conditions, the historical background of the management practices such as previous land uses, as well as other environmental impacts that previously occurred in the area to be restored. The latter can be gained through interviews with previous and present managers of the area, as well as maps, reports and aerial photographs. Ten different indigenous tree species that previously occurred in the area were planted in an experimental demonstration site of approximately 70ha, which was enclosed by an electrical game fence. The ten tree species that were evaluated included: Faidherbia albida (Ana tree), Acacia nigrescens (Knob thorn), Acacia tortilis (Umbrella thorn), Schotia brachypetala (Weeping boer-bean), Acacia xanthophloea (Fever tree), Lonchocarpus capassa, recently renamed Philenoptera violacea (Apple-leaf), Salvadora australis (Narrow-leaved mustard tree), Adansonia digitata (Baobab), Combretum imberbe (Leadwood) and Xanthocercis zambesiaca (Nyala tree). With the aid of aerial photographs, phytosociological studies, interviews with previous and present land users and managers, as well as existing surrounding vegetation, four different zones within the enclosure were identified according to ecotones. The establishment, growth and survival rate of the different tree species were monitored using morphological and physiological vegetation sampling techniques, as well as leaf component analyses on individuals of selected species. Soil physical and chemical analyses were carried out in the four different blocks identified within the experimental site. Data analysis was carried out on both the soil and leaf component analyses using the CANOCO-package. The establishment of the experimental site was successful, and important information was collected on various aspects of restoration activities. Positive growth effects were also observed in certain indigenous tree species concerning the "activity line" effect, especially with regard to Acacia tortilis and Combretum imberbe. However, the preliminary results obtained through this pilot study showed no conclusive evidence to what exactly stimulated the enhanced growth phenomena observed in certain individual tree species planted on "activity lines". Additional watering was identified as the most important factor contributing to successful establishment and growth of indigenous tree species in this semi-arid area. Various results showed a multiplying effect when a combination of additional watering and "activity lines" was applied. It was concluded that, should any further restoration work be conducted in the degraded areas of the Mapungubwe National Park, the planting of trees should be done on "activity lines" and with the addition of water. This will result in higher establishment rates of transplanted trees and speed up the succession processes involved in the natural "healing process" of degraded areas. Parameters that should be used for monitoring tree growth include the trunk thickness at the base, trunk thickness at 30cm from the base, and the length of the tree in its natural growth form. Recommendations were also made as reference for future restoration practices to ensure better and more successful and sustainable outcomes in the planting of trees. These include the use of nurse plants such as Acacia tortilis and Salvadora australis to establish a more favourable microclimate for climax species, as well as the establishment of a preferred herbaceous layer. Care should be taken in the period required for the cultivation of indigenous trees before they are transplanted into the field, as a prolonged cultivation period could lead to a circular growth form of the root system, preventing sufficient penetration ability of the roots into deeper, more nutrient rich soils. Before trees can be planted into the field, a hardening period must be applied to all seedlings for at least a three week period. This entails the exposure to more direct sunlight for longer periods as well as a reduction in the water applied weekly. Special attention should be paid to the stresses caused by herbivory, especially that of termites and porcupines. The maintenance of the exclosure is a critical factor contributing to the successful outcomes of this particular restoration project. Problem animals, especially elephants, should be kept out of the exclosure at all costs. The results of this project can be used in this ongoing restoration program, as well as in other related projects in semi-arid, degraded savannah areas over the long-term.