The role of science in counteracting this phenomenon
Alex Rose-Innes
The Department of Environment, Forestry and Fisheries (DEFF), in its Green Heritage publication, highlighted the alarming issue of bush encroachment in forests across the African continent.
Bush encroachment, usually as a result of poor woodlot management, mostly results on overgrazed and degraded woodlands. Thickets of sicklebush (Dicrostachys cinerea) and blackthorn (Acacia melifera), effectively reduce biodiversity in any given area with resultant loss of grazing capacity for herd.
Black wattle, an invasive tree which had been introduced to southern Africa and the syringa (responsible for many human allergies) had overtaken millions of hectares as a result of a lack of local natural enemies. These trees literally sterilise the land. Despite 200 000 workers for the Working for Water Programme in SA eradicating hundreds of thousands of hectares annually, bush encroachment occurs at a faster tempo and this plan does not match these aliens. These invasive trees are in direct opposition to pioneer trees which are beneficial to soil management, nitrifying the soil and creating conditions for new tree seedlings to grow.
According to scientific data, sweet thorn and black thorn trees germinate as many as 20 000 seedlings per hectare, killing off pioneer trees. This is not altogether seen as a negative, as a greater mix of species result with, at coastal areas, red milkwood (Mimusops caffra) lead to a greater diversity and rehabilitation with pioneer trees could over many decades, be able to restore original habitat conditions.
Trees vary in water consumption and it is predicted that SA would have a water deficit, as a result of water transpiration by trees, of almost 2% within five years. Across Africa, this figure is even more alarming, with the result of not enough ecological water to maintain existing ecosystems.
Over the last five decades, scientific data showed that especially savannas, and not only in Africa but globally, were affected by this phenomenon. Bush encroachment poses a threat to domestic livestock due to its impalpability. Suppressing herbs and grasses which are preferred fodder for livestock leads to smaller patches of land and that, in turn, becomes overgrazed. Savannas and grasslands in southern and central Africa are essential to satisfy a continuously growing population and with it being encroached by bush, it in effect threatens the livelihood of Africans.
According to the De-bushing advisory service of Namibia, the primary causes of bush encroachment on savannas include a reduction in the frequency of fires and overgrazing of livestock. If grass layers on savannas loses its ability to utilise nutrients and water efficiently, higher infiltration of water and nutrients into the sub-soil results, allowing bush and tree species to dominate the land. Bush encroachment also brings with it the loss of perennial grasses which are then replaced by annual species of inferior quality and not sufficient for proper livestock grazing.
Another theory states that savanna ecosystems are event-driven with rainfall and its variability said to be playing an important role in vegetation growth more than the intensity of grazing. This model implies that bush encroachment is not a permanent phenomenon and that savanna could be changed to its grass-dominated state by favourable management or environmental conditions (Doughill et al. 1999). Woody plants establish themselves after dry periods followed by a few wet years and then maintain themselves by utilising most of the water. Rather than a gradual annual increase in numbers, the general rule is that woody plants establish in large numbers during certain years and at varying intervals (Donaldson 1969). This then shows that bush encroachment could occur rapidly and might be triggered by management practices and natural events, or a combination of both.
What can be done about it?
An article in Farmers Weekly (2013) provided pointers towards successful management of bush encroached areas, especially savanna:
- Removing trees – tree competition should be kept in balance, but workers should be properly trained which trees to remove and which alien plants
- A proper sustainable management plan is needed – a once-off action is not sufficient. The management plan should continue after burning.
- Do not plant tree seedlings too soon after the removal and chemical control of mature trees and woody plants. Use only herbicides such as Picloram, Bromacil, Glyphosate, Tebuthiuron and Triclopyr.
- Use brush cutters, axes and chainsaws as opposed to bulldozers and embark on regular burning. This would stimulate growth by increasing soil temperature. Be careful however of hard seeds of woody plants which crack open and germinate, leading to more bush encroachment.
When burning, ensure that it takes place under expert supervision and complies with a country’s relevant legislation. Browsing animals, such as goats, eland, kudu, bushbuck and black rhino, as well as mixed feeders such as elephants, impala and nyala are effective in controlling woody vegetation naturally.
Fighting bush encroachment is a long-term commitment. A proper management structure and sufficient budget are needed. But, in the long run, the land would once again become productive and grazing animals would flourish.
