Bush Encroachment

The Negative Impact of Bush Encroachment in Savannah Ecosystems

Bush encroachment is defined as the increase of woody plant densities so that the natural equilibrium of woody plant layer (trees and shrubs) and herbaceous (grass and forb) layer densities is shifted unfavourably. It results in the degradation of the grass and forb layer, whereby changes in the vegetation composition can be observed.

Bush encroachment is a separate phenomenon from “alien invasion” as many species of indigenous woody species in South Africa are bush encroachers and can become problem plants.

Bush encroachment is not something you want occurring on your reserve. It is indicative of a “sick” savannah ecosystem, where there should be a balanced co-existence of tree and grass species.

Bush encroachment is bad for the following reasons:

• It results in a weakened grass and forb (herbaceous) layer, meaning that good grasses and forbs become scarcer and may even disappear.
• When good grass disappears, it negatively affects many animal grazer species as their staple food source is significantly depleted, and results in the decline in grazing carrying capacity, which means the land cannot sustain as many grazers as before.
• The tree species that encroach are often thorny trees that make feeding grounds inaccessible to game, including browsers.
• Bush encroachment negatively affects browsers as well. This is because the high density of woody plant roots in the soil depletes the soil moisture which causes trees to drop their leaves earlier in winter. This means that browsers have limited browse available for the dry winter months.
• As you may have experienced in many reserves, thick bush makes it very difficult to view game. This poor game viewing can lead to the loss of tourism potential in certain areas.

Causes of Bush Encroachment:

• Poorly Placed Waterholes causes bush encroachment. For example, the placing of artificial waterholes to close together in a uniform pattern, has been known to result in the herbaceous community around these water holes becoming sparse. The sparser herbaceous layer allows for woody plant seedlings to successfully compete and dominate, so the open savannah areas change to enclosed woodlands.
  
  This then leads to a loss of biodiversity as different antelope species prefer different habitats. For instance, the less common Waterbuck prefers long grass to hide in, where-as Wildebeest and Zebra like an open area in which they can scan for predators. When a large area becomes enclosed by woody plants as a result of the weakened herbaceous layer, the grass is too short and sparse for Waterbuck, and the area becomes too enclosed by woody plants for Zebra (Bothma, 2010).
  
  
• Veld Mismanagement where the exclusion of fire in a savannah system results in bush encroachment. Fire is needed to maintain the grass vigour as it kills off the dead sward material, and allows for new growth to emerge. There is a concept known as “moribund”, where grasses “grow themselves to death”. When grasses become moribund, it means that they have grown to their maximum, but all the above ground material dies off. This results in dormancy of the grasses, because no new material can come through as a result of all the dead material still remaining.
  
  Fire is very important in removing this dead material, and allowing the new green material to come through after the first rains. Without fire, grasses become unhealthy due to moribund, and the woody community remains unchallenged, as it is now easy to outcompete the grass which is no longer active. Fire is important in killing and restraining the woody plant population in such a way that grass species have an opportunity to grow (Jordaan, 1992).
  
  Good grass cover creates the necessary fuel load to produce hot fires that retard woody growth. 

• Incorrect Stocking Rates of animal species within a reserve, especially in a closed system, can lead to bush encroachment. This is a function of either over or under utilisation of grasses, influencing basal cover, growth vigour and inter species competition.
  
  
• Climate Change or particular climates favour the growth of trees, especially in places where there is a lack of frost. Trees, unlike grasses, have their growth cell sites (primary meristems) at the tips of their branches. These primary meristems produce the cells which are important for plant regeneration and growth. Due to the location of their meristems, trees are more vulnerable to unfavourable weather conditions such as black frost and severe wind. Grasses are better protected as their growth reserves are kept underground where they are insulated and protected from damage. It is also understood by ecologists, that climates that are moister with higher levels of rain, result in more trees flourishing. A tree is a large plant that requires a lot of water to survive, this means that where there are a lot of trees, the water supply from the climate is adequate for their survival and more plentiful (Brewer, 2010).
  
  
• Lack of Competition from a Healthy Herbaceous Layer: One of the greatest reasons for bush encroachment is the absence of competition from a healthy, resilient herbaceous (grass) layer that is kept healthy by fire and the correct levels of grazing. As mentioned before, when veld is weakened through moribund by the absence of fire, or is weakened due to over or under grazing, it means that the grass is not active and is not growing well, and their roots (which are better than trees for preventing soil erosion) are not taking up space in the soil. This allows woody seedlings to out compete the grasses. Trees can cause a negative shading effect, blocking out the sunlight for grasses. 

• Low Browser Impact: Studies have shown that where vegetation exclusion blocks are established and monitored, the absence of large herbivores in the land scape such as elephants and black rhinos influences grasslands becoming woodlands.
  
  For research purposes, an exclusion block is a fenced off area within a natural functioning system that prohibits any plant utilisation by animals. It can also be a fire exclusion plot, where fire breaks are maintained around the plot to maintain a fire free zone.

 

JL: Thanks Paige, for your time and effort in contributing to the Reserve Report.

References
Bothma, D. T. (2010). Game Ranch Management . Pretoria: Struik Punlishers.
Brewer, R (2002). The Science of Ecology. USA. Sedage Learning Publishers.
Jordaan, J. J & Roux, A. L. (1992) The Short-Term Effect of Fire, Boer Goats and Cattle on The Woody Component of the Sourish Mixed Bushveld in the Northern Province of South Africa. Warmbaths.
Towoomba Agricultural Development Centre