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Bush encroachment and control

It has been believed for many years that trees, woody shrubs and grasslands in the bushveld are in a complete dynamic equilibrium and therefore do not require active veld management. This is not true. The creation of camp systems and the absence of veld fires and large, bulk grazers such as elephants have further destroyed any equilibrium that may have existed on most wildlife ranches. In addition, many wildlife ranches are developed from old cattle ranches which have been severely overgrazed in areas with fluctuating rainfall. This has caused major bush encroachment problems which should have received attention first before any wildlife were re-established.

It is now known that an excessive density of trees in bushveld will suppress the growth of grasses because the trees compete with the grass seedlings. The soil of grasslands that suffer from bush encroachment will become impoverished of carbon and nitrogen over time on wetter areas, but on drier areas both these elements will increase in the soil. However, the balance between grasses and woody plants remains dynamic and photos of the camp of Harry Wolhuter at Shabani Hill that were taken in 1904 in the current Kruger National Park show that what is a relatively dense bushveld today was mainly an open grassland then.

In South Africa, a balance should be maintained in bushveld between the optimal density of trees and the production of excellent grazing. In the arid and mixed bushveld, the control of bush density should be done when there are more than 600 trees or woody shrubs per ha and the crown cover of these woody plants exceeds 30 per cent. Even in regions with a high annual rainfall the mere removal of grazing animals will not lead to an improvement in the grass component once it has already become overgrazed and trampled. Because of bush encroachment the growth of grasses is being impaired; rain is lost rapidly and does not penetrate into the soil; the quantity of organic material, carbon and nitrogen in the soil is being impaired and the woody plants compete with the grasses for sunlight, water and nutritional elements. This reduces the grazing capacity of an area for wild and domesticated grazers in part because of the increase of grasses that have a low grazing value, with a consequent low veld condition. In one study that was done in 1996 on 97 wildlife ranches in the North West province it was found that bush encroachment in the arid bushveld has decreased the potential stocking density with grazers almost six-fold and the annual grass production by 40 to 92 per cent over several decades. It was also found that 29 per cent of the wildlife ranches showed bush encroachment but lacked any bush control programmes.

Bush control can be done in various ways and details appear in the reference below. Mechanical methods include removing, chopping down or pushing over of woody plants following which the organic material can be mulched for charcoal or animal feed and the stumps are treated with chemicals to prevent coppicing. Heavy chains can be pulled between two bulldozers to rip trees and shrubs from the ground. Trees and shrubs can also be dug out by hand or mechanically and trees can be ring-barked to kill them. Controlled fires can be used to reduce bush encroachment or remove trees and bushes, while a series of chemicals can be used to kill woody plants. However, such chemicals should be used highly selectively and with great care not to kill large and valuable trees by applying the chemicals too close to the roots. The most important disadvantage of the use of chemicals is that they become less effective as the organic and clay content of the soil increases. In addition, some of the chemicals remain active in the soil for a long time and others are not target specific. A high density of browsing animals can be used temporarily in some cases to reduce the density of the woody plant component, while herbivorous insects with a specialized diet will also target specific types of woody plant.

The grazing component should react favourably to any bush control provided that an adequate supply of seeds of the better quality grasses is still present in the soil and the grazing is given a chance to recover first. The ideal tree density for optimal grass production in the bushveld is 300 to 400 plants per ha. It is important to have an adequate density of large trees because grasses with a high grazing quality, such as Guinea or buffalo grass Panicum maximum, germinate in shade while some wildlife require shade close to their grazing grounds. Sun-loving grasses usually have a low grazing potential and thrive in open sunlight.

In one study that was done in Namibia the density of woody plants was reduced from 4000 to 400 plants per ha through chemical control. This lead to an increase of 1500 kg per ha in grass production. In another region of Namibia where the density of woody plants was reduced by chemical aerial spraying it increased the grazing capacity by 2.1 times. In the Molopo region of the Kalahari the grass production increased by 700 per cent after bush control reduced the density of woody plants from 3000 to from 200 to 300 plants per ha by aerial chemical spraying. This has lead to a four-fold increase in the grazing capacity within three years. Near Severn in the Northern Cape Kalahari region chemical control reduced the woody plant density to 200 plants per ha and this resulted in a three-fold increase in the grazing capacity.

In the sweet bushveld regions of the Limpopo province a density of 400 or fewer woody plants per ha is regarded as ideal for grass production. In one region in Limpopo that receives an annual mean rainfall of 461 mm a decrease to such a density increased the grass production by 1000 kg per ha and the grazing capacity by 2.8 times. In the Thabazimbi district of Limpopo the grass production was increased by 1800 kg per ha after bush control in a region which receives a mean annual rainfall of 389 mm. In mopane veld in the Limpopo province there should not be more than 400 to 500 woody plants per ha for optimal grass production. In one study in the mopane veld the grass production was increased by up to 860 per cent after bush control. In the thickets of the Eastern Cape province bush control has lead to an increase of 25 per cent in the grass production.

It is a general rule that the selective reduction of the density of woody plants with a stem diameter of less than 100 mm will lead to a considerable increase in grass production in the bushveld. However, with any bush control a critical factor is the regular control of any bush that may coppice or germinate after the initial control.

 

Reference:

Van Rooyen, N 2010. Veld management in the African savannas under current climatic conditions. In J du P Bothma and J G du Toit (Eds), Game Ranch Management, fifth edition. Pretoria: Van Schaik, pp 778 - 837.

By: Prof J du P Bothma

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