Bush encroachment is defined as the invasion or thickening of aggressive undesired woody species resulting in an imbalance of the grass to bush ratio. This causes a decrease in biodiversity carrying capacity of both communal (non-freehold) and commercial (free hold) farming land
The primary causes of bush encroachment on savannahs include a reduction in the frequency of fires and overgrazing of livestock. When the grass layer on savannahs loses its competitive advantage and its ability to utilise nutrients and water efficiently, higher infiltration of water and nutrients into the sub-soil results; a situation that benefits bush and tree species, allowing them to predominate (de Klerk 2004). Bush encroachment is also accompanied by a change in the dominant grasses: perennial grasses are often lost, being replaced by annual species often of inferior quality for livestock (Scholes 1997, Rothauge 2007). Annual grasses are generally less productive than perennial grasses. Thus, animal production on an annual grass sward is very precarious and less sustainable.
Another important theory is the state-and-transition model, which says that savanna ecosystems are event-driven, where rainfall and its variability play a more important role in vegetation growth (and composition) than the intensity of grazing. This model implies that bush encroachment is not a permanent phenomenon, and that a savanna can 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).
Thus bush encroachment can occur rapidly and may be triggered by management practices and natural events, or a combination of these factors.
The following trees and shrubs are recognised as the main encroacher species in Namibia
- Dichrostachys cinerea
- Acacia mellifera
- Acacia reficiens
- Colophospermum mopane
- Terminalia prunoides
- Terminalia sericea
- Acacia nebrownii
- Rhigozum trichotomum
- Catophractes alexandri
Other species of lesser importance as encroachers, include
- Combretum collinum (mainly in Zambezi Region)
- Acacia hebeclada (in areas of eastern Omaheke)
The term “encroacher bush” is used for indigenous species and “invader bush” for alien invasive species. E.g. Prosopis is an alien invasive species from the Mesquite family and was introduced into the country in 1897 from America as a fodder plant. It destroys the indigenous vegetation, uses up great amounts of groundwater and contributes to the substantial decline in land productivity caused by bush encroachment.
Bush encroachment remains a major agricultural problem in Namibia, covering about 26 t 30 million hectares of the country’s savannas, and reducing livestock productivity significantly. In 2004 De Klerk estimated that 30 % of Namibian land is affected by bush encroachment. New estimations are that 45 million hectares, equating to 60 % of the country size affected.
This presently costs Namibian farmers over N$2.7 billion annually in lost income from reduced beef production . Bush encroachment significantly reduces underground water recharge, as well as biodiversity
Original estimate of extent
Namibia’s bush encroached areas fall mainly within the semi-arid savannas, with rainfall varying from about 300 mm in the west to over 600 mm in the north-eastern parts. It is typically reported that “26 to 30 million hectares of Namibia are encroached”. This figure is based on the map compiled by Bester in 1990, showing the main areas of encroachment (Bester 1990).
Current estimate of extent
The Strategic Environmental Assessment (SEA) on bush thinning and value addition in Namibia (2016) revised the ‘Bester map’ using the field knowledge of a few recognized botanists and bush encroachment experts, to produce the maps shown in below. The individual distribution maps of the six main encroaching species were obtained from the Tree Atlas of Namibia (Curtis & Mannheimer 2005). The distributions were shown in a quarter-degree square grid, with relative abundance in each square. Squares were removed from that map where the species was identified as not encroached. These decisions were based on the team’s field experience and observations. Some areas where the species are dense, were not considered to be encroached if the level of encroachment was thought to be natural i.e. not caused by human interventions (such as overstocking or reduction of fires). This process eliminated the areas where the species was not encroached, to leave the quarter degree squares where the species was known to exist at a relatively high density that has come about in the past 60 years. This process yielded the map below which shows only the extent of encroached bush, not densities. The areas of Prosopis encroachment confined to the main ephemeral rivers in southern and central Namibia. According to this revised map, approximately 45 million hectares of Namibia are bush encroached The map below is based on the distribution of the main encroacher species, and information on where they have shown dramatic increases in density over the past +- 50 years.
Effects on the economy:
Research by the Project on the economics of land degradation in relation to bush encroachment (2016) shows a total net benefit of N$ 48 billion discounted over 25 years, or approximately N$ 2 billion annual could be gained should a national bush control programme be implemented compared to bush thinning. The same assessment has shown that such a national intervention would allow the country to gain between N$ 2.1 billion and N$ 4.2 billion worth of livestock production annually. Bush thinning would generate benefits from livestock production, groundwater recharge, production of firewood and charcoal, and generation of electricity, as well as carbon offsets for electricity. At the same time, bush control/thinning would create an estimated number of 10 000 jobs per annum in mechanical operations, which would contribute to employment and poverty reduction for semi-skilled labourers accounted over the initial round of bush control. There are also many unquantified ecosystem services which would be positively affected by bush thinning, which are not included in the dollar estimates provided