Effects of forest proximity and socio-ecological determinants on tree species composition, diversity and carbon stocks in the Vhembe Biosphere Reserve in South Africa

Abstract

This study aimed to evaluate the effects of forest proximity, and socio-ecological determinants on tree species composition, diversity, and carbon stocks in traditional agroforestry landscapes. The study was crucial considering the importance of traditional agroforestry landscapes in an effort to halt the unprecedented rise of biodiversity loss and the effects of climate change at a landscape scale. The traditional agroforestry landscape in a rural set-up was divided into three distance levels that somehow mimic the biosphere reserve zones: the immediate distances (transitional zones), intermediate distances (buffer zones) and far distances (core areas). The paramount objective of conservation has been more focused on protected areas than any other forest resource areas. In the case of the Vhembe Biosphere Reserve (VBR), the legally designated protected areas are mainly the core areas of the biosphere. Therefore, considering the traditional agroforestry landscape, specifically, the transitional zone of the biosphere as complementary to the core areas for biodiversity conservation at large is necessary. Traditional agroforestry landscapes are earmarked to play a critical role in conserving biodiversity, climate change mitigation and adaptation, and sustaining rural livelihoods through multiple ecosystem goods and services. Previously, worldwide, studies assessed tree species diversity and carbon sequestration potential of traditional agroforestry landscapes. However, there has not been a study evaluating the potential of traditional agroforestry for provisioning ecosystem services, conservation of tree species diversity and carbon sequestration in South Africa. The study was conducted in traditional agroforestry landscapes of four communities (Damani, Tshipako, Thenzheni and Tshiombo) situated in the transitional zone of Vhembe Biosphere Reserve in Thulamela Municipality, Limpopo Province. As described in biosphere zone classifications, transitional zones are characterised by multiple functions such as settlements, forestry plantations, agricultural farms and common resources use areas. To address the main objective of the study, five independent studies were conducted. An unprecedented rise in the unsustainable extractions and use of provisioning ecosystem services from the landscapes poses a dilemma as to whether traditional agroforestry landscapes help to conserve tree species diversity while sustaining rural livelihood. Understanding these dynamics was essential in order to inform the harvesting behavioural change and management interventions. The study evaluated the link between distance and socio-ecological determinants and the provisioning ecosystem services consumption behaviour. The following hypothesis was tested “rural people's preferences and extent of Provisioning ecosystem services (PESs) harvesting decrease as the distance from the village to forest patches increase, regardless of the prevalent socio-ecological conditions”. Consistent with the hypothesis, the results showed that local people harvest most of the provisioning ecosystem services at an immediate (1st) level, followed by intermediate (2nd) and far distance (3rd) levels. This study further revealed the existence of 108 useful tree species in the study areas. The fact that there is a preference to use and harvest provisioning ecosystem services from the distance regime closest to the household, shows the need for a concerted effort to conserve and enhance the abundance of multipurpose tree species in homesteads and the immediate areas. Anthropogenic disturbance has positive and negative effects on tree species richness and diversity. To ascertain and understand the effects, this study also tested the following hypotheses; first, “Tree species diversity is expected to be the highest at an intermediate distance from the villages as predicted by the immediate disturbance hypothesis”. Second, “the turnover of tree species composition between distances levels increases as the distance from the community increases as predicted by the distance decay of community similarity hypothesis.” The findings are in consonant with distance decay of community similarity hypotheses, with estimated tree species richness of 76, 93 and 95 species in an immediate distance, intermediate distance and far distance, respectively. Moreover, the highest species variation was observed at an intermediate distance, which indicates that there is greater species composition at an intermediate distance compared to immediate and far distances. The results confirm that the distance and associated factors have major detrimental effects on tree species richness and biodiversity in traditional agroforestry landscapes. Harvesting of provisioning ecosystem services is found and known to be extremely high in the study area. Effective interventions such as planting native trees near to homesteads and conserving the existing vegetation must be implemented to reduce and halt overexploitation. The defining features of traditional agroforestry landscapes (TALs) are species diversity, productivity, and complex functionality. Currently, there is a decline in endemic southern African tree species in TALs. This study, therefore, evaluated the impact of exotic species and socio-ecological determinants on endemic species diversity in these TALs. The study found that the endemic species diversity and richness were influenced by the distance level, exotic species richness, diversity, and abundance. In particular, the distance level was found to be the major driving factor of change in endemic species composition in TALs. The endemic species diversity increased as the distance increased from the epicentre of the village to the proximate forest. Conservation through endemic tree planting in traditional agroforestry landscapes would mitigate the impact of the introduction and spread of exotic species. This will ultimately enhance the traditional agroforestry biodiversity and production while benefiting people. Little has been reported regarding the effects of socio-ecological determinants on the carbon sequestration potential of traditional agroforestry in South Africa, especially in the Vhembe Biosphere Reserve (VBR). The study assessed the tree carbon stocks in the VBR traditional agroforestry landscapes along the distance levels, immediate, intermediate, and far distance from villages to the forest. The results revealed that there were no differences in total carbon stocks between the distance levels, with the same amount of carbon stocks stored in the vegetation at all distance levels. While the carbon per distance remains the same, the species richness and composition differ along the distance regimes. Thus, while carbon sequestration may be the same, the conservation value for tree species differs along the distance regimes. In general, the results showed a greater potential for TALs to sequester high carbon. When all socio-ecological determinants were considered together, elevation, density and basal areas were the most important predictors of carbon stock. Hence, this study concludes that maintaining and enhancing species diversity in traditional agroforestry landscapes can be an effective strategy to achieve the climate change mitigation goals provided this is finely balanced with the livelihoods needs of the local communities. Management practices in traditional agroforestry landscapes play a significant role in tree species conservation. The study aimed to investigate conservation strategies on tree species richness in traditional agroforestry landscapes. This study found that the local people have no conservation strategy and practices targeting the enhancement of tree species richness in the traditional agroforestry landscape. The study, therefore, advocates for the establishment of a conservation strategic framework for restoring tree species richness and composition by targeting traditional agroforestry landscapes. The findings revealed that the transitional zones are neglected in terms of conservation, as demonstrated by extreme harvesting disturbances, low tree species richness and diversity, and high dominance of exotic species at an immediate distance from households. However, this study found a great potential for intermediate and distance levels in conserving indigenous species, including endemic species despite their comparable contribution to carbon sequestration with the immediate distance. In sum, this thesis implies that human behavior and anthropogenic disturbances are linked to the decline of tree species diversity and landscape homogenization in immediate proximity to the settlement areas. If this behavioural pattern remains unchecked, the growing human population in the settlement areas may negatively affect the conservation of tree species diversity and the supply of ecosystem services that sustain rural livelihood. In addition, the possession of indigenous knowledge on conservation practices and management of native trees, may not be sufficient to tackle the fast-declining global biodiversity. Thus, there is a need to integrate scientific methods of conservation and behavioural change tools with indigenous knowledge at the local and landscape level to sustain global biodiversity while ensuring the provisioning of ecosystem service to rural livelihood.