**3.1. Domestic uses**

conditions [30]. Concerns about tebuthiuron on the other hand pertain to the accumulation and persistence of the chemical in the soil thus posing potential threats to non-target species [36, 37]. Biological methods sometimes employed include the controlled use of herbivores (especially goats) and fire. Du Toit [29] observed in a study in the Eastern Cape that in comparison to continuous/rotational sheep grazing of a *V. karroo* stand, there was a higher mortality of trees and more efficient control of seedling regrowth under continuous goat grazing than rotational grazing. Goat grazing resulted in a marked improvement in the cover, compo-

Fire has also been extensively used in combating bush encroachment in savanna because it is known to maintain a balance of grass to trees and shrubs in the savanna areas [25]. Trollope [25] has observed that fire generally has different roles in controlling bush encroachment in the moist and arid savannas. In the moist savanna regions (>600 mm p.a.), bush encroachment may be controlled with fire alone because there is adequate grass material under grazing conditions to support frequent enough fires to burn down and control the bush. This is unlikely in the arid savanna regions (<600 mm p.a), which constitute the major portion of the South African savannas, because the rainfall is too low and erratic to support frequent enough fires under grazing conditions to prevent the regeneration of bush [25]. In grassland, Du Toit [34] made observations that the application of fire to combat *V. karroo* intrusion in the Eastern Cape sweetveld was not a practical approach. While fire was found to retard *V. karroo* seedling

All in all, eradication of *V. karroo* is difficult once the thorn has invaded an area where it was previously absent, since a seed bank which did not previously exist is established. *V. karroo* trees can produce large amounts of seeds annually, and these have a high longevity. As a result, destruction of a stand of *V. karroo* is often times still followed by seedling establishment

The effect of *V. karroo,* and other tree species, on herbaceous species (and grasses) may not always be negative, and there is evidence that trees may actually have a beneficial effect on neighbouring plants. For example, increased herbaceous layer productivity has been reported under tree canopies, due to favourable conditions such as improved soil water status and soil fertility [38, 39]. Likewise, *V. karroo* has various favourable influences on herbaceous production. First, *V. karroo* is a leguminous tree known to form root nodules [40], which are swellings on the root that contain nitrogen-fixing microorganisms (bacteria) known as *Rhizobium. Rhizobium* possesses the enzyme systems (including nitrogenase enzyme complex) that convert atmospheric nitrogen to nitrogen compounds useful to plants [41, 42]. Legumes like *V. karroo* then use the compounds to construct amino acids and protein [41, 43]. This ability of *V. karroo* to fix nitrogen is beneficial to other plants as well, mainly because the nitrogen content in the soil increases, and soil fertility is thus enhanced under these trees. In addition, *V. karroo* is able to use water and nutrients from deep underground because it has a long taproot, and this again leads to grasses and other plants thriving in its shade [6]. The ability of *V. karroo* to use water from deep underground means that it can grow in arid and otherwise inhospitable

development, it could however not prevent the seedling establishment.

sition and vigour of the grass sward [29].

118 Pure and Applied Biogeography

and considerable regeneration [34].

**2.3. Role in soil fertility**

*Vachellia karroo* is one of the most preferred species for fuelwood [48] because the wood has excellent fuel properties. It burns clean with little smoke and is valued for its sustained high temperature [18] and thus produces high-quality fuelwood for many rural communities which still rely on wood for cooking and heating. The wood is also used as rough construction material for building traditional huts and fences in many rural communities [1, 18]. The thorns are used as sewing needles, pegs or pins, while its branches are used in farms to make fencing kraals for livestock, to protect them from predators [1, 2]. The bark, leaves, gum and other parts are used medicinally in many ways. An infusion of the bark is used to cure diarrhoea and dysentery, while the dried and powdered form of its gum is used for eye treatments [2]. A boiled liquid from the bark is sometimes used to treat cattle which have tulp poisoning caused by *Moraea* (*Homeria*) species, which are bulbous plants poisonous to cattle [6]. Other *Vachellia* species are known to have medicinal properties as well. For example, the bark of *V. erioloba* is used to treat headaches and that of *V. xanthophloea* is used for fevers and eye complaints [1].
