| dc.description.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. | en_US |
