Landscape function in bush thickened and -controlled areas of the semi-arid savanna in the Molopo region, South Africa
Bush thickening (bush encroachment) and the effects thereof on the environment has been a very well debated topic over the past few decades which led to the formation of two valid but rather opposite opinions. Pasture scientists believe that bush thickening is due to overgrazing/over browsing of forage that is caused by keeping too many livestock and/or game animals, whereas other scientists believe that bush encroachment is mainly caused by changing climatic factors, including a change in CO2 levels and rainfall patterns or as other management strategies, such as the use of fire as a management tool. Consensus has been reached that bush encroachment is caused by a combination of factors that influence the ecosystem goods and services, including a loss in biodiversity and ultimately affecting the ecological services of the people using the land. This project forms part of the management and restoration sub-project (B2) of the IDESSA project (IDESSA-Integrative Decision-support System for Sustainable Rangeland Management in southern African savannas) currently carried out between the NWU and the Universities of Goettingen, Marburg and Kwazulu Natal. The project is funded by the BMBF (Federal Ministry of Education and Research or “Bundesministerium für Bildung und Forschung") in Germany. The main aim of sub-project B2 is to develop a grid-based, spatially explicit rangeland model which will be able to simulate the complex interplay of management and savanna dynamics under different environmental conditions and land use, restoration and climate change scenarios. The aim includes to determine the landscape functioning in bush thickened and controlled savannas in the Molopo region of the North-West and Northern Cape Provinces, South Africa. The land users of the Molopo region used various methods to combat the thickening of woody species of their pastures. These methods included: chemical control by aeroplane (AC), chemical control by hand (HC), double chemical control by hand (2HC), stem burning (SB) and sustainable management (SM) by using rotational grazing to prevent the bush encroachment. The sampling approached used for this study included the use of the Landscape Function Analysis (LFA) monitoring procedure to determine three main parameters (stability, nutrient cycling and infiltration) by using 11 soil surface assessment (SSA) indicators. In incorporation of the SSA indicators include biotic and abiotic factors and are measured close to the soil surface in a certain landscape. Through the landscape organizational index (LOI), the landscape id divided into patch and inter-patch zones where the SSA indicators are used to determine the three main parameters that will give an indication of the functioning of the landscape. The aim of this study also included the assessment for differences in soil chemical properties at the patch scale, contrasting bush-thickened and controlled areas. Four dominant patch types (inter patch/bare soil patch (IP), grass patch (GP), grass litter patch (GLP) and shrub litter patch (SLP)) were identified in transects used for the LFA monitoring procedure. The dominant patches types were then analysed to determine their contribution to the functionality and soil chemical properties of the landscape that are characterised by still encroached, controlled or sustainably managed. Results from the LFA’s indicated that no significant differences (p< 0.05) could be found between the functioning (stability, infiltration and nutrient cycling) of the bush thickened and bush controlled areas. The HC sites had on average the highest functionality scores as a result of a favourable tree to grass ratio, but these scores were not significantly higher (p< 0,05) than any of the other controlled or thickened areas. The bush thickened areas scored within a few points of the bush controlled areas with minimal variation over both survey years. The soil analysis indicated that the grass litter patches (GLP) from the AC sites had the highest average nutrient levels of all the different patches identified, with the SLP only scoring high in Ca % (C %) and pH (KCl). An expected result with regards to the high C and Calcium (Ca) levels at the SLP in the bush thickened sites, with high nutrient levels recorded for the GLP at the AC sites. A possible reason for the high nutrient levels can be ascribed to the tuft sizes of the dominant grass species identified at the AC sites. These large tufts were mostly characterised by one species, i.e. Stipagrostis uniplumis. Another contributing factor were the high concentration of cryptogams in biological soil crusts known for the increasing of soil nutrients and infiltration, found surrounding the base of the grass tufts, as well as around the base of the stems of Senegalia mellifera shrubs found in the bush thickened areas. The overall results from the study confirms that thickened landscapes are fully functional areas and that they will not change without human intervention. Proper management of arid ecosystems like the Molopo region, is key to prevent future woody thickening. Further research is required to determine the effect good land management can have on the functioning of Molopo rangelands. Such studies should focus on the functioning of bush control rangelands compared to thickened rangelands over a longer time period with more emphasis on the effect these actions have on the soil profile.