The effect of fire on savanna vegetation dynamics in the semi-arid Molopo Bushveld region of the North-West Province, South Africa
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Land degradation in semi-arid areas is a worldwide phenomenon. Both land users and researchers have become increasingly aware of the environmental changes that occur over time due to land degradation. Among such changes, bush encroachment results from land degradation, especially in savanna rangeland areas. Fire is known as a major driver of the dynamics of woody and herbaceous vegetation dynamics in the savanna biome. A workshop held in Potchefstroom, North-West Province, South Africa, in 2015 for the IDESSA project (IDESSA: An integrative decision-support system for sustainable rangeland management in southern African savannas) within the BMBF (German Federal Ministry of Education and Research) SPACES framework (SPACES: Science Partnerships for the Assessment of Complex Earth System Processes) indicated that more in-depth scientific information was needed on the potential role of fires in the dynamics and shaping of the savanna vegetation of the Molopo Bushveld region in the North-West Province, which is a semi-arid area. Several studies have indicated the potential use of fire to maintain the balance between woody and grass species and to prevent bush encroachment. However, information is limited with regards to this specific area. The study area comprised three locations, namely the Molopo Nature Reserve (Molopo), Khamab Reserve (Khamab) and a commercial cattle farm (Farm) in the Molopo Bushveld region. Each area was divided into reference and burnt sites. The reference sites included unburnt sites within the Molopo and Khamab area and burnt sites within the Farm area as there were no non-burnt sites within the Farm area. The first objective of this study was therefore to assess the effects of fire on the vegetation composition and structure in the Molopo Bushveld region. This objective was achieved by conducting vegetation surveys that included the use of belt transects for the woody component and a step-point method for the herbaceous component. The second objective was to evaluate the use of fire as a management tool to make long-term predictions and management decisions in semi-arid savanna areas. This objective was achieved by using semi-structured interviews with land users and questionnaires which included relevant questions regarding management, vegetation, rainfall, causes of fires and damage suffered due to the fire; as well as by analysing and comparing the gathered quantitative and qualitative data. Considering the various studies done on the influence of fire on savanna vegetation, the hypothesis for this study was that fire events influence the structure and composition of the vegetation in semi-arid savanna regions and can be used as a management tool, especially in terms of land degradation caused by bush encroachment. The results indicated vegetation differences within reference and burnt sites. However, the results were inconsistent, except for Grewia flava, which had higher densities in burnt sites than in reference sites. The inconsistency within the data regarding species composition for all three areas (Molopo, Khamab and Farm) may be due to different management practices and the type of animal being kept (i.e. game or livestock), as some areas were grazed and others browsed. With regards to the overall woody density, the burnt sites had lower woody densities compared to that of the reference sites with some sites showing significant differences. The sites with the lowest overall woody densities were chemically controlled reference and burnt sites. Differences in canopy volume were also observed between the reference and burnt sites, but no consistent differences were observed between species and sites. This may be due to the differences in when fires occurred, meaning that some species could either have recovered or were browsed over a longer period by the time that the surveys were done. Regarding herbaceous species composition, a clear distinction occurred between the sampling areas (Molopo Nature Reserve, Khamab Reserve and commercial cattle farm) according to canonical correlation analyses. This was presumably mainly due to the different management strategies within each site rather than the influence of fire however differences in soil types and rainfall across the sites should not be excluded from having influenced the vegetation. Due to these differences, the results for the study sites were discussed separately. The commercial cattle farm had the highest woody species richness, which may be due to the dispersal of seeds by cattle, whereas the two nature reserves had lower woody species richness, which may be due to the presence of both browsers and grazers rather than just grazers. The results indicated that most land users had no management strategy prior to or after the fires. Some land users did, however, reduce their stock after the occurrence of a fire or started rotating/resting their camps. The land users’ observations of vegetation were varied, which may be due to how they perceived their land and whether the land is overgrazed/browsed. However, most land users agreed that G. flava increased after a fire, corresponding to the vegetation surveys conducted. Most land users also stated that the amount of rainfall was not higher prior to the fire occurring, but most land users did not keep records and the land-users’ memories are not necessarily accurate after a time has passed since the fire event. Most land users were opposed to the use of fire as a management strategy for woody vegetation, as they did not want to lose valuable grazing and, with rainfall being unpredictable in the area, they excluded fire management completely. The first part of the hypothesis, namely that fire events influence the structure and composition of vegetation in semi-arid savanna regions, was accepted based on this study’s results as there were differences in the vegetation within the reference and burnt sites, however not significant differences. The second part of the hypothesis, namely that fire can be used as a management tool, especially in terms of land degradation caused by bush encroachment, was accepted to some extent, as it was dependent on the specific area. More information with regards to fire frequency, the exclusion of fire by land-users, management practices in the area and the effects of frequent fires should be gathered with regards to the semi-arid Molopo Bushveld region.