Determining the effect of soil on bush encroachment between 1993 and 2018 in the North West Province

Abstract

Bush encroachment (BE) is a serious form of land degradation and South Africa alone has lost an estimated 8 million hectares (ha) of grazing or cultivation land due to BE. This consequently leads to decreased food security. To prevent BE, one needs to understand the drivers and mechanisms that control the process and to advise when and where certain management actions should be implemented. Unfortunately, the proposed drivers for BE in African savannas are still widely debated given that the causes for this process is still poorly understood. The focus of this study was to understand the effect of soil type and certain soil properties on BE in the North West Province (NWP) between 1993 and 2018. For this study, the main driving factors of BE extent and spread were identified in the study area for the specified period by taking a GIS approach on provincial (NWP) and regional scales (four significant areas). Maps indicating the percentage (%) of woody cover for the years 1993, 1998 and 2018 were sourced from Symeonakis et al. (2020). The layers indicating the % woody cover in the NWP were used for calculating the spread of bush and bush spread maps were created for time frames, 1993-1998, 1998-2018 and 1993-2018. Potential driving factors of BE were sourced from various sources and used to analyse the bush spread and determine the driving factor/s of the specific bush spread from 1993 to 2018 on a provincial scale and regional scale. On a provincial scale, mean annual precipitation (MAP) was the main driving factor of BE, while in land-managed areas, land-use and MAP together with soil, were important driving factors of bush encroachment from 1993 to 2018. Therefore, soil can be regarded as a minor driving factor of BE in the NWP from 1993 to 2018. Vegetation surveys were also carried out at the study sites, characterising different soil types, soil properties and degrees of BE. The belt-transect method was used for the vegetation survey to determine the composition, density, and structure (height classes) of the woody component (tree- and shrub species). Soil profiles were described per soil horizon, soil samples were taken within each transect, which were analysed at the laboratory to determine the soil particle distribution (soil texture), pH, electrical conductivity (EC), and water retention of the soil. From the vegetation surveys, Dichrostachys cinerea and Diospyros lycioides were found to be the main woody encroacher species as D. cinerea occurred at all study sites with D. lycioides mainly occurring at the Kgomo-Kgomo study site. The other recognized woody encroacher species included Grewia flava, Grewia flavescens, Senegalia mellifera, Vachellia karroo, Vachellia tortillis, and Ziziphus mucronata.

Soil types and properties did not have a significant influence on all the woody species identified at each study site, but rather on specific encroacher species causing BE in the NWP. The results indicated that D. cinerea mostly occurred on soils with low clay content, while G. flava favoured soils with higher clay content. The highest extent of BE occurred at the Legkraal and Kgomo-Kgomo study sites, where the soil was characterised as deep soils with sandy loam texture. Species such as Combretum apiculatum, Combretum inberbe, and Combretum hereroense, occurred on shallow soils, while Vachellia tortillis preferred deep soil types. The encroacher species, D. lycioides, occurred on the subsoil with an alkaline pH, while both D. lycioides and D. cinerea preferred soils with EC higher than 25 mS/m. D. lycioides and G. flava both occurred in soils with high dry bulk densities (Pbs), especially at the Kgomo-Kgomo study site. The highest Pbs was also recorded at the Kgomo-Kgomo study site. It therefore seems that soil types with specific soil properties, influence the occurrence of specific woody species causing BE in the NWP. It is recommended that land-managed areas that experience BE should in general be considered as important future restoration and/or research study sites. Areas where deep soils occur, with predominantly sandy or sandy loam textures, should be regarded as priority areas. Restoration actions that could be considered in the priority areas include the application of manual, mechanical, chemical, biological or a combination of these methods in BE areas to stimulate the growth of grasses. To improve soil condition for grass growth, soil organic matter in the form of livestock manure could be added to the topsoil instead of fertilizer, as fertilizers are usually too expensive for land managers. A knowledge, training and skills development program should also be impleemted for land managers. It is recommended that future research be conducted on determining the main driving factors of BE of other Provinces, such as Limpopo and Northern Cape and using different GIS methods for determining the main driving factors of BE on provincial and regional scale. Future research should also be done on the effect of other soil properties, such as soil temperature and soil organic matter, on woody species causing BE.