CHAPTER 2 LITERATURE REVIEW
2.4 The management and control of bush encroachment
As BE became a serious problem in semi-arid rangelands, land users, land managers and governments officials implemented better sustainable land-use management (SLM) strategies to prevent and/or reduce BE. Turpie et al. (2019) also indicated that pro-active measures are important for reducing existing and preventing further BE and proposed several ideas that should be implemented to reduce or prevent further BE. These include biological management (fire and herbivory), manual and mechanical thinning or clearing, chemical spraying, and income generation strategies through wood harvesting. Some of these strategies will be discussed below.
2.4.1 Biological clearing
The biological clearing of woody vegetation includes methods regarding the management of fire and herbivory, whereas biological control involves the deliberate introduction of invertebrates or diseases (such as wood fungi) and is aimed at reducing the effects (invasion and reproduction) of specific woody vegetation (Van Wilgen et al., 2004; De-bushing Advisory Service Namibia, 2017).
In certain instances, fire and/or herbivory are described as “biological clearing” methods to decrease the woody vegetation causing BE. Rangelands where BE occur are mainly burnt for (1) the removal of accumulated organic material (particularly in areas of high precipitation) and (2) to combat or prevent BE (Naudé, 2018). High-intensity fires are applied at the right time of year to
prevent woody plant seedlings from surviving (Smit et al., 2010). When burning too early (early winter), the burnt surface is exposed to wind, frost, and sunlight, which leads to fertile surface material being lost (Naudé, 2018). It is also important for rangeland managers to burn areas consisting of dense stands of perennial grasses, as when rangelands consisting of primarily pioneer and/or subclimax species are burnt, the rangeland might further deteriorate. Intensive and frequent fires also often kill woody plants, preventing the establishment of a continuous canopy cover (Smit et al., 2010, in Hare et al., 2020).
Another biological control method to combat BE includes the use of browsing animals and management of grazing animals (Naudé, 2018). The common strategy of grazing management is to allow adequate rest for the veld through carefully-managed rotational grazing, rather than continuous grazing (Turpie et al., 2019). Continuous grazing refers to a system by which animals graze in one area without any barriers to their movement. Rotational grazing is usually applied using a system of fenced camps (paddocks) in which the livestock is “rotated” between pastures to meet the forage needed for livestock. Tainton (1999) and Smit (2004) have established two important approaches to either adapt the livestock system to the existing vegetation or to modify the vegetation to suit specific livestock systems. Where browsers form an important component of the livestock system, woody plant densities should be high and palatable, as browsing animals, such as goats, are well suited to control woody plants. This is because of the intensity and frequency with which they utilise the browse (Hare, 2020). Goats also have been used together with mechanical methods in the initial thinning of woody vegetation in a bush control programme (Smit et al., 1999, in Hare et al., 2020). Earlier studies have reported that after mechanical methods were used, plots were stocked with 1.2 to 2.2 Boer goats per hectare (ha-1) for commercial farmers who wished to increase the grass production by reducing bush (Hare et al., 2020).
2.4.2 Manual clearing
The manual clearing of woody species causing BE relies on “man-power” to remove unwanted bush (Naudé, 2018). Manual removal is carried out by using tools, such as hand saws, pruning loppers, bush axes, spades, and in advance cases chainsaws. This is done to remove woody vegetation causing BE (De-bushing Advisory Service Namibia, 2017). As manual control is labour intensive, it creates job opportunities, especially in communal areas (Nghikembua et al., 2020).
Other advantages of manual control include that specific encroaching species and size-classes of plants can be targeted, limiting soil disturbances. A disadvantage of manual clearing of woody vegetation is that it may be time-consuming (De-bushing Advisory Service Namibia, 2017). This is a very selective method and has to be applied correctly, i.e. cutting off all the stems of a multi- stemmed shrub (Naudé, 2018). Manual clearing/control is often followed up by chemical methods.
2.4.3 Mechanical clearing
The clearing of woody vegetation causing BE by mechanical technologies involves the use of heavy machines, such as bulldozers, bush rollers, or bush cutters (De-bushing Advisory Service Namibia, 2017; Naudé, 2018). Wheel- and track-mounted heavy machines can be outfitted with saws, clippers, and leveraged scoops to help remove unwanted woody vegetation. This method is effective in clearing large areas of land in a relatively short amount of time, with bulldozers able to clear up to 8 ha/day (De-bushing Advisory Service Namibia, 2017). Mechanical clearing/control is faster than manual clearing, but it causes substantial soil disturbance, which may favour the establishment of woody seedlings (De Klerk, 2004). Smaller machines or “mini-heavies” have been designed to reach difficult areas and can manoeuver easier through dense thickets (De- bushing Advisory Service Namibia, 2017). These smaller machines are also more selective and can treat 3-5 ha of unwanted woody vegetation/day and with smaller wheels, cause less soil disturbance compared to the larger bulldozers. Unfortunately, mechanical clearing is very expensive (cost, maintenance, and fuel) and has to be operated by skilled operators, which is not always possible for landowners, especially in communal areas (Barac, 2003; De-bushing Advisory Service Namibia, 2017).
2.4.4 Chemical clearing
Chemical clearing/control involves the application of arboricides (herbicides specific for eliminating woody plant species) that are applied selectively to kill targeted plants only (De- bushing Advisory Service Namibia, 2017; Naudé, 2018). The chemical control of woody vegetation causing BE is a common practice applied in southern African rangelands, to decrease the density of the woody layer and stimulate the growth of the herbaceous layer by restoring the soil moisture content of the upper soil layers (see mentioned models) (Harmse et al., 2016). This control method is generally based on the size of the target area, funds available, and availability of a labour force that is trained to apply the arboricide (De Klerk, 2004). Arboricides are applied to the foliage, stems of woody plants, or to the soil around plants (Turpie et al., 2019). Chemical application is usually used for a very large area, where mechanical and manual clearing is not possible, as well as the tree density is such that animal access is severely restricted. Tebuthiuron or bromacil (chemical formula: C9H16N4OS) is the active ingredient present in many arboricides commonly used by rangeland owners and bush clearing contractors (Harmse et al., 2016; Naudé, 2018). Other chemicals used include Picloram and Triclopyr or arboricides coming by trade names, such as Hyvar, Bushwhacker, Brush-free, Spike, Graslan, Grazer, Limpopo, Molopo, or Reclaim (De-bushing Advisory Service Namibia, 2017; Naudé, 2018). The drawback of chemical control is the decrease of soil health and the treated woody plants are “virtually un-harvestable”, after which it cannot be used to generate income (De Wet, 2015, in Turpie et al., 2019). Other
disadvantages of chemical clearing/control include: (1) the arboricides can cause harm if it comes into contact with skin, eyes, mouth, and nostrils and (2) if spilled, arboricides can cause unwanted pollution in water sources (De-bushing Advisory Service Namibia, 2017). Compared to manual methods of bush clearing, chemical control is also very costly and less applied by local, poor communities (De-bushing Advisory Service Namibia, 2017).