Bush Encroachment Theories

• Semi-arid rangelands globally have transitioned from grasslands to bush-encroached states over the last half-century
• Bush encroachment involves the spread of local woody species or the invasion of woody species from other ecosystems into semi-arid savannas and grasslands
• Bush encroachment-induced rangeland degradation presents challenges, impacting livestock production and pastoral livelihoods in Africa
• Scientists have proposed theories to explain bush encroachment involving well-regarded theories

Theories explaining bush encroachment:

• Walter's two-layer model
• Moir's one-layer model
• State-and-transition theory
• Equilibrium theory
• Disequilibrium theory
• Non-equilibrium theory
• Overgrazing, availability of soil nutrients and moisture, elevated CO2 levels, fire frequency/intensity, and seed dispersal by animals are driving factors explaining bush encroachment based on these theories
• Interactions among driving factors are important in determining bush encroachment
• Managing semi-arid ecosystems requires a critical understanding of the cause-effect relationships of underlying factors through an integrated approach
• Research should be multi-disciplinary and involve multi-partnership approaches to address woody plant encroachment

Introduction to dryland ecosystems in Africa:

• Most dryland ecosystems in Africa feature unique vegetation types like grassland, savanna, woodland, scrubland, and desert
• Vegetation structure and composition in these ecosystems have changed dynamically, resulting in ecosystem degradation and bush encroachment
• Bush encroachment is the "aggression of destructive and unwanted woody plants," causing an increase in bushes, decline in biodiversity, and decreased grazing capacity for herbivores in semi-arid rangelands
• Encroaching woody species include both local and foreign species and is marked by the proliferation of indigenous woody plants and the invasion of foreign/alien woody species

Factors contributing to bush encroachment

• Continuous heavy grazing
• Frequent fires
• Declines in soil moisture and nutrients
• Climate change-related global warming
• Most factors are spatially correlated and scale-dependent, making it hard to pinpoint a single cause for bush encroachment
• Encroaching woody species were tied to the degradation of semi-arid rangelands but can have positive effects on ecosystem structure and function
• Walter's two-layer and Moir's one-layer models, as well as state-and-transition system explains bush encroachment
• Longer-term data led to the emergence of more comprehensive theoretical schemes of equilibrium, disequilibrium and non-equilibrium rangelands systems
• Bush encroachment, the spread of indigenous woody species and invasion of woody species from other ecosystems into semi-arid savannas and grasslands, is widely reported
• The term "bush encroachment" is similar to others like thickening of woody plants, woody re-growth, woody weed invasion, xerification, and shrub invasion
• Bush encroachment can decrease grazing and browsing values ​​and productivity, and is thus a destruction of pleasant grass species indigestible to herbivores
• Bush encroachment is widespread in arid and semi-arid savannas in North/South America, Africa, Southeast Asia, and Australia, causing herbaceous vegetation loss

Bush Encroachment and Land Degradation

• Bush encroachment is linked to rangeland degradation and perceived negatively.
• Bush encroachment is described "the aggression of destructive and unwanted woody plants" which decreases biodiversity and grazing capacity
• Woody species can be a source of food for herbivores, firewood etc. and can enhance resource stocks like carbon and nitrogen in the soil
• Bush encroachment can be an issue when trees, shrubs and bush covers exceeds 30%
• Land degradation is a constraint in most rangelands in dry lands of Africa
• Processes leading to degraded conditions for primary productivity are called “land degradation”
• Desertification is decrease in the ability of a dry land environment to provide various goods and services

Bush Encroachment Debate.

• Bush encroachment happens globally in dry lands and is a problem in Africa
• Bush encroachment is tied to rangeland degradation and is a difficulty faced by the socio-economic development of drylands
• Bush encroachment changes soil characteristics and local biodiversity
• Bush encroachment provides livestock fodder and improves soil quality through nitrogen fixation
• Ecological and economic effects depend on trees or shrubs that are indigenous or invasive
• Rangeland quality is improved by removing encroached land, and land degradation can be addressed with its removal

Woody Species

• Both Indigenous and exotic woody species are known encroachers in semi-arid rangelands
• Acacia, Dichrostachys, Prosopis, Balanites, and Grewia are common across the globe
• Maytenus, Euclea,and Acalypha frequently encroach African rangelands and savannas
• Acacia, Prosopis, Opuntia, Lanthana and Larrea are exotic plants infringing on African rangelands
• Native bush poses a great threat to the ecological structure and function while exotic bushes associated with greater risk

Bush Encroachment Mechanisms

• Walter and Moir's models explain the mechanism through two-layer and one-layer models, respectively
• Grasses are faster growing and absorb moisture from the surface, inhibiting water intake for Trees with deep roots as described by the two Layer Model
• Nitrogen loss from grass fires limits power of acacia bushes that are nitrogen fixing species
• State-and-transition model explains the change due to biotic and abiotic factors involving one state to another

Concepts rangeland ecosystem.

• Equilibrium suggests that ranges respond to grazing influence. High grazing can initiate growth of woody plants
• Disequilibrium is balance of tree/grass is burning and human dominance can result in range deterioration leading to plant growth
• Non-Equilibrium are determined by abiotic factors; rainfall effects plant growth, drought limit the species

Driving Factors of Bush encroachment.

• Precipitation variablity determines growth and abundance of species, an intermittent wet season can promote vegetation and dry stunts growth
• Grazing level influences the species, high herbivores lowers grasses but alter above and below ground biomass. Soil or groundwater can increase establishment of bushes when there is decreased biomass
• Wildfires control habitat by reducing the damage of grassland fodders but also increase the sprouting of seeds
• Soil nutrients contribute to success in dry location. Where Clay content is high is associated with higher number of plants
• Increasing CO2 can aid growth of woody plants for fuel. Species may substitute depending on available resources

Strategies for Controlling Bush Encroachment

• Woody plant removal should be viewed nationally and not simply in relation to the immediate local community
• Government, Stakeholders and pastoralists are key in coming up with a solution
• Establish plans for burning and removal of fuel mass
• Community involvement is most successful

Conclusions on Bush Encroachment

• Extensive research shows relationships between the synergies and trade-offs between woody growth encroachment
• Rangeland degradation affects native plants and environments in many different ways