**2.2. Bush encroachment**

*Vachellias* are, on the whole, easy to grow and often become an aggressive invader of valuable farming land and grazing areas, a phenomenon that is usually referred to as bush encroachment (**Figure 5**). Bush encroachment has become a serious ecological and farming problem that has affected many grazing areas in grassland and savanna areas of southern Africa. It is

**Figure 5.** *Vachellia karroo* encroachment of a grassland community, Free State Province, South Africa (photo: M. Dingaan).

a transition from grassy to increasingly shrubby ecosystems [24], whereby trees and shrubs invade into open grassland or thicken up in already wooded areas [25]. *V. karroo*, in particular, has become a serious invader into the grasslands of the Eastern Cape, the Free State area, and the North-West Province [26, 27].

The most detrimental effect of *V. karroo* encroachment (and other woody species) to farming is that it depresses the production of grasses, mainly due to tree-grass competition for soil moisture [28]. Bush encroachment thus drastically reduces the carrying capacity of grazing areas because browse is generally a poor substitute for grass, especially in sheep/cattle areas [29]. For example, in some parts of the Molopo area, grass production was thought to have already decreased by over 80% due to bush encroachment, and this has subsequently affected the economic viability of many farms [30].

#### *2.2.1. Factors promoting bush encroachment*

rich in protein and are thus eaten by birds such as Grey Go-Away birds (also known as Grey Louries) and monkeys [5]. In addition, the larvae of several butterfly species feed on the pods and flowers [5, 16]. Its flowers also provide nectar for bees and are important for the production of honey [1]. *V. karroo* trees are important for bee farming as they indirectly result in the

Furthermore, parts of *V. karroo* are used as food for humans, as an example, seeds are roasted and used as coffee substitute [19]. *Vachellia* species can produce large amounts of seeds which are known to have been eaten by pastoral people when the need arose, and indirect food sources include the edible cerambycidae wood borer larvae found in the dead wood of *V. robusta* [1].

Despite its thorniness, *V. karroo* is a good fodder tree and forms an important part of the diet of a wide range of herbivore species [2]. It is palatable [20] and consumed by both domestic and wild species [21]. Its foliage is highly favoured by stock and game, so are its seeds and dehiscent pods, which are rich in protein [18]. The pods and seeds also play an important role as feed supplements during the dry season [22] as they are at times collected by farmers to feed their livestock [1]. *V. karroo* has been shown to be an important part of the giraffe diet [23], and it has also been observed that goats select *V. karroo* in preference to grass, but less so, when the amount of available browse available is limited [21]. The foliage, pods, and flowers of *V. karroo* are free of hydrocyanic poisoning, a self-protection mechanism used by many trees [6], relating to the toxic substance known as hydrocyanic acid, prussic acid, or cyanide. Some *Vachellia* species pose the danger of such poisoning to animals. These include *V. erioloba*, whose pods and young leaves contain prussic acid, as well as the wilted leaves of *V. sieberiana* [1].

*Vachellias* are, on the whole, easy to grow and often become an aggressive invader of valuable farming land and grazing areas, a phenomenon that is usually referred to as bush encroachment (**Figure 5**). Bush encroachment has become a serious ecological and farming problem that has affected many grazing areas in grassland and savanna areas of southern Africa. It is

**Figure 5.** *Vachellia karroo* encroachment of a grassland community, Free State Province, South Africa (photo: M. Dingaan).

production of a pleasantly flavoured honey [6].

116 Pure and Applied Biogeography

**2.2. Bush encroachment**

*Vachellia* species regenerate vegetatively and from seed, but regeneration from seed is most dominant [31]. The encroaching species of *Vachellia* are spread by seed, which in many of these species has impermeable seed coat resulting in a high percentage of dormancy [32]. According to O'Connor [33], the encroachment of woody species requires successful seed dispersal, germination, and seedling establishment. The two most vulnerable phases in the regeneration of *Vachellia* are during seed germination and seedling establishment; these phases are characterised by high mortality rates that influence the populations of *Vachellias* [31]. Seedling establishment can be influenced by competition from established surrounding vegetation, as well as moisture availability and irradiance [33]. Du Toit [26] has shown that *V. karroo* seedlings require high irradiance levels for optimal growth, although they may still survive under certain levels of low irradiance.

According to Trollope [25], the plausible reasons why bush encroachment was not a serious problem before the advent of commercial livestock production could have included the control of bush by fire, mechanical damage brought about by wild browsers and climatic factors. On the other hand, the factors promoting encroachment in the modern era are complex, with the most predominant being the introduction of domestic livestock and subsequent overgrazing and the elimination of veld burning [24, 33, 34]. The successful and prolific nature in which *V. karroo* has been able to encroach onto grasslands is largely due to the fact that the species is an adaptable pioneer with an ability to establish itself without shade, shelter or protection from grass fires. It is fast growing, tolerant of defoliation by herbivory and is resistant to fire and frost [18]. Its seeds do not only have a great tolerance to high temperatures produced during burning but may actually be stimulated to germinate by fire [35].
