Composition and structure of woody vegetation in thickened and controlled bushveld savanna in the Molopo, South Africa

Abstract

A large portion of South Africa’s semi-arid Molopo Bushveld savanna is thickened (encroached) by indigenous increaser shrubs and trees (also known as bush encroachment), mainly as a result of poor grazing management practices and suppression of fires. A common approach to restore a balanced and productive woody:grass ratio in this savanna is the use of systemic, soil-applied arboricides, containing the active chemical ingredient Tebuthiuron, which can either be applied selectively by hand or non-selectively by aeroplane. Land owners also use non-chemical bush control (thinning) methods, such as stem burning. All bush-thinning practices create a highly dynamic and competitive postcontrolled environment for different vegetation components. In this dissertation, the impact of selective and non-selective bush-control technologies on the composition and structure of the woody vegetation is compared to non-controlled sites in four commercially managed areas in the Molopo Bushveld savanna, with emphasis on the response patterns of selected key woody species, namely Grewia flava, Senegalia mellifera subsp. detinens, Vachellia luederitzii var. luederitzii, V. erioloba and Boscia albitrunca. The study design included four bush control treatments, namely (1) selective chemical control by hand, (2) selective chemical control by hand with re-application, (3) non-selective chemical control by aircraft and (4) selective control by stem burning with re-application, which were located on three commercial cattle farms and one game farm. Three benchmark sites were also identified, forming part of the study design, including bush-thickened sites (BT), partially thickened sites (PT) and sustainably managed open rangelands (SM). The results showed relatively low species richness per study area, with selected species dominating the Molopo Bushveld study area, as they outcompete other vegetation layers and woody individuals for available soil moisture and nutrients. These species include G. flava, which revealed the highest mean density, V. erioloba, B. albitrunca and the encroacher species S. mellifera subsp. detinens. The threat of savanna re-thickening following non-selective bush control, which is attributed to initial low grass densities and extensive loss of larger, mature woodies such as V. erioloba, accompanied by a change in inter- and intra-lifeform competition is predicted. The non-selective bush control sites revealed a poorly structured woody vegetation layer dominated by woody recruits, with scattered mature B. albitrunca individuals. This counteracts progressive development of vegetation towards a stable and productive grassy state, as is the case with a well-balanced woody:grass ratio. Thus, to avoid vegetation retrogression, a selective follow-up and maintenance programme of controlling woody recruits, in particular those of S. mellifera subsp. detinens and V. luederitzii var. luederitzii, is mandatory within the first years after arboricide application. The best practice towards a sustainable woody:grass ratio is the selective chemical application approach. The resulting post-control environment is complemented by a betterstructured, species-rich woody layer, with higher tree equivalents/ha and mean height of favourable woody species compared to that in non-selective chemical control sites. Mature woody individuals, in combination with a well-established graminoid layer, prevent the rethickening of a regenerative recruitment layer of thickening woody species such as S. mellifera subsp. detinens and V. luederitzii var. luederitzii. As a result, a lower relative abundance of these thickening woody species was recorded in selective chemical control sites compared to non-selective control sites.