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The study area is situated in the sour bushveld of potential bush-encroachment area. In the areas around Bela-bela (Warmbath) trees per hectare of more than 1200 are considered to be bush encroached. Bush encroachment is defined as stands of indigenous plants that tend to become abnormally abundant when the area is degraded by overgrazing or injudicious fires, examples of indigenous encroachers are Dichrostachys cinerea sickle bush and Stoebe vulgaris bankrupt bush (Van Rooyen 2002). Bush encroachment is also commonly defined as an increasing woody plant density. With the woody plant component increasing and the grass sward diminishes. When the grass layer is reduced by heavy and continuous grazing, the tree seedlings are allowed to grow beyond the control of grass competition and fire. Consequently, this leads to bush encroachment. To promote grass production in the areas of sour bushveld not more than 800 trees per hectare are recommended. All vegetation types are in South Africa are subjected to invasion of by certain grasses, forbs, woody dwarf shrubs and trees. Such invasion usually occurs when the ecosystem is disturbed either naturally or artificially (Tainton 1999). Even under high-rainfall conditions, removing domestic and/or wild herbivores from a bush-encroached area will not contribute to an improvement in the grazing, even after decades of rest. The best way of preventing bush encroachment is, therefore, to maintain a high basal cover of grasses through good management at all times (Van Rooyen 2002).
BUSH ENCROACHMENT CONTROL
Bush encroachments are of many kinds. One is the misuse of fire. Drought and human disturbance together with the absence of game of game migration and the absence of tree damage by animals, such as elephants, are more likely to control bush encroachment. The introduction of elephants is limited to larger ranches only. The most important reason for bush encroached areas in southern Africa, is the imbalance in stocking of the different feeding groups. High stocking rates of grazers, and low stocking rates of browsers, results in over-utilisation of the grass sward. Goats are well suited to controlling woody plants because the intensity and frequency with which they utilise the browse can be controlled and because they are relatively insensitive to chemical deterrents, such as the high tannin levels present in many woody species (Tainton 1999). The requirements for this approach are that the bush must be acceptable, at an available height to the browsing animal (Trollope 1974). The four basic methods for controlling bush encroachment on a wildlife farm are discussed below (Van Rooyen 2002).
Mechanical controlThere are various methods of controlling bush encroachers mechanically. The main procedures are as follows (Van Rooyen 2002): · Chopping, slashing and felling:An axe, hand-held or tractor-driven chainsaw, circular saw or bush cutter is used. The stumps are treated immediately with a chemical weed killer · Chains or cables: A heavy, 100 mm anchor chain is pulled between two caterpillars tractors in such way that the vegetation is uprooted. This method is unselective, causes considerable disturbance and is not effective for large, well-rooted trees · Digging or bulldozing: This is an effective method for uprooting plants, but is expensive and also causes considerable soil disturbance · Ring barking or girdling: The bark of the tree is removed around the trunk with an axe or a power-driven saw. The damaged area should preferably be treated with an herbicide. The trees will usually die within one to three years after being ring barked
Fire can be used to rid environment of weeds. Stem-burning of woody plants is done by packing wood around the stem of an individual tree. This is effective, but practical only when done on a small scale. A programme of periodic burning should therefore, be followed. Burning should also be followed by introducing browsing animals or by applying mechanical and means when necessary.
Chemical controlChemical control methods are usually expensive to apply and should be considered only under specific circumstances (Tainton 1999). Herbicides can be used effectively to control a range of problem plants. When using herbicides, however, the following considerations are important (Van Rooyen 2002):
- Toxicity of man and animals
- Volatility of the herbicide
- Length of the active period
- Precautions that are vital for proper and safe use
- Economic justification
- Proper training of the staff who use to apply the herbicide
When using an herbicide, it is vital to follow the instructions on the label strictly regarding the application, safe and proper use and storage of the product. The chemical treatment of problem plants is usually recommended in the following situations (Van Rooyen 2002): · When exceptional bush encroachment prevents the accumulation of enough fuel to sustain a hot fire · When available browse is out of reach of browsers and there is a low fuel load that prevents the use of fire · When the bush is impenetrable for browsers · When the bush is unacceptable to the browser species as food · When the chemicals have enough selectivity to control the target plant species The following basic approaches are employed when using herbicides to control problem plants (Van Rooyen 2002): · Foliar application:The chemicals are applied with a hand-operated spray apparatus, or a power driven one mounted on a trailer, tractor, truck or aircraft. The best time to spray is when the leaves are fully developed · Stem notching and application: This method is most effective on trees with a trunk diameter of less than 150 mm. Downward-sloping notches are made around the lower 300 to 500 mm of the trunk. The chemical is either sprayed or painted onto that area. The plants should die after a period varying from six months to three years · Stump treatment:Trees and shrubs are cut off approximately 200 mm above ground level. The stumps are treated immediately with herbicides · Soil treatment: The chemical is applied in the form of a water-soluble liquid or powder to the soil around the base of the trunk, or is buried in the soil in the case of pellets. The chemical is then dispersed during the rainy season and taken up by the roots of the target plant. These chemicals are most effective in sandy soils. The chemicals remain active in the soil for up to four years. Depending on the rainfall, it may take two to three years for the plants to die Different chemicals or herbicides are available commercially for controlling weeds such as broad-leaved weeds, grassy weeds and sedges. The most common plant applied herbicides are Picloram and Triclopyr. Picloram is available under the product names Tordon®, Grazon ® and Access®. Triclopyr are mixed with diesel oil or old oil at a 1 percent concentration. For safety reasons a dye should be mixed in as well. Alternatively, a water-soluble form of Picloram (Access) is available. If applied to the stamp a concentration of 2 percent Access and 2 percent Actipron Super as wetting agent is used. Leaf application as a spray affords a concentration of 0.35 percent Access and 0.5 percent Actipron. When clearing bush, it is strongly advised not to remove all woody plants. The reason is that, although an increase in the grasss development is observed in the short-term, in the long-term the quality and quantity of the grass will diminish due to eventual nutrient loss after the interruption of the nutrient cycle. By selectively clearing undesirable tree species, sufficient woody plants will remain in the cleared area.
Biological controlBiological control can be effective through the pressure exerted by various animal groups on the vegetation. The following methods can be used (Van Rooyen 2002) : · Browsers: This includes wild browser and domestic ones such as goats. The type of browse must be acceptable for the animal and occur at a suitable height. It has ecological and economic advantages because more of the natural resources are utilised and an income is provided. · Insects: Insect control of especially exotic invaders is a promising approach to plant control. Insects have been used successfully against plants such as the prickly pear, jointed cactus and the Australian species of the genus Acacia. Effective bio control agents cause a gradual thinning of dense stands of invading alien plants, which allows the natural vegetation to return as part of the natural process.
RECOMMENDATIONS FOR HIGHLANDS WILDERNIS GAME RANCH
Management units 1, 2, 3, 4, and 5 have a trees density of 764, 1183, 1237, 1195, and 995 per ha, respectively. The potential for bush encroachment in management unit 3 is high, especially in the mountainous areas. The mountain areas need to be managed carefully in order to prevent bush encroachment. The average density in management unit 3 is high with 1237 trees density per hectare. This area should be monitored by fixed-point photography in order to establish whether tree density is increasing over time. Management unit 3 is characterised by vast tree species of red bush willow Combretum apicutatum, velvet bush willow Combretum molle and common hook-thorn Acacia caffra. Even though red bush willow Combretum apicutatum and velvet bush willow Combretum molle are good fodder species for most game species, bush encroachment is detrimental to the ecosystem. A secondary effect of encroached areas could be soil erosion, especially encroached areas along drainage lines and against the slopes, since these areas soil surface is bare with no vegetation to stabilise the soil.
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