Bush encroachment

Bush encroachment (also shrub encroachment, woody encroachment) is a natural phenomenon characterised by the increase in density of woody plants (bushes and shrubs) at the expense of the herbaceous layer (grasses and forbs). It is often considered an ecological regime shift and a symptom of land degradation. Bush encroachment is found to have severe negative consequences on key ecosystem services, especially biodiversity, animal habitat, land productivity and groundwater recharge. Bush encroachment can refer both to the expansion of native plants as well as the invasion and spread of invasive species. The phenomenon is observed across different ecosystems and with different characteristics and intensities globally. Among the more severely affected landscapes is the Veld in Southern Africa.

View of bush encroached land at the Waterberg Plateau Park in Otjozondjupa Region, Namibia

Causes

The earliest published notion of bush encroachment dates back to 1954.[1] Various factors have been found to contribute to the process of bush encroachment. A distinction can be made between bush encroachment due to land intensification and bush encroachment after land abandonment.

Land intensification

In the context of land intensification, a frequently cited cause of bush encroachment is overgrazing, commonly a result of overstocking and fencing of farms, as well as the lack of animal rotation and land resting periods. Studies find that overgrazing plays an especially strong role under mesic climatic conditions, where shrub encroachment is mainly limited by the than reduced competition from the herbaceous layer.[2]

A connected cause for bush encroachment is the reduction in the frequency of wildfires that would occur naturally, but are suppressed in frequency and intensity by land owners due to the associated risks.[3][4] When the lack of fire reduces tree mortality and consequently the grass fuel load for fires decreases, a negative feedback loop occurs.[5] At intermediate rainfall, fire can be the main determinant between the development of savannas and forests.[6][7]

Population pressure can be the cause for bush encroachment, when large trees are cut as building material or fuel. This stimulates coppice growth and results in shrubbiness of the vegetation.

Moreover, the reduction of browsing by herbivores, e.g. when natural habitats are transformed into agricultural land, fosters woody plant encroachment, as bushes grow undisturbed and with increasing size also become less susceptible to fire.

Land abandonment

Where land is abandoned, the rapid spread of native bush plants is often observed. This is for example the case in former forest areas in the Alps that have been converted to agricultural land and later abandoned.

Another frequently cited theory is the state-and-transition model. This model outlines how rainfall and its variability is the key driver of vegetation growth and its composition, bringing about bush encroachment under certain rainfall patterns.

Climate Change

Climate change has been found to accelerate bush encroachment. Firstly, increased atmospheric CO2 concentrations fosters the growth of woody plants. This is because woody plants with C3 photosynthetic pathway thrive under high CO2 concentrations, as opposed to grasses with C4 photosynthetic pathway.[8][9][10] Moreover, changes in precipitation can foster woody encroachment. Increased precipitation can foster the establishment, growth and density of woody plants. But also decreased precipitation can promote bush encroachment, as it fosters the shift from mesophytic grasses to xerophytic shrubs.[11] Woody encroachment correlates to warming in the tundra, while it is linked to increased rainfall in the savanna.[12]


Generally, large bushes are found to coexist with the herbaceous layer, while smaller shrubs compete with it.[13] This highlights that a mix between woody and herbaceous plants is desirable in the respective ecosystems, but an unsustainable composition can develop. Literature further suggests that the causes of woody encroachment differ significantly between wet and dry savanna.[14]

Impact

Various ecosystem services are negatively impacted by bush encroachment, such as biodiversity, groundwater recharge and land productivity.

Biodiversity

Bush expands at the direct expense of other plant species, potentially reducing plant diversity and animal habitats.[15] These effects are context specific, a meta analysis of 43 publications of the time period 1978 to 2016 found that bush encroachment has distinct negative effects on species richness and total abundance in Africa, especially on mammals and herpetofauna, but positive effects in North America.[16] A study of 30 years of woody encroachment in Brazil found a significant decline of species richness by 27%.[17]

Evidence of biodiversity losses include the following:

Groundwater Recharge

Woody plant encroachment is frequently linked to reduced groundwater recharge, based on evidence that bushes consume significantly more rainwater than grasses. Concrete experience with changes in groundwater recharge is however largely based on anecdotal evidence or regionally and temporally limited research projects. While there is general consensus that bush encroachment has an ecohydrological impact, studies find that this impact on groundwater recharge differs between sandstone bedrocks and karst regions as well as between deep and shallow soils.[30] Although this is strongly context dependent, bush control can be an effective method for the improvement of groundwater recharge.[31]

Land Productivity

Bush encroachment directly impacts land productivity, as widely documented in the context of animal carrying capacity. In the Southern African country Namibia it is assumed that agricultural carrying capacity of rangelands has declined by two-thirds due to bush encroachment. In East Africa there is evidence that an increase of bush cover of 10 per cent reduced grazing by 7 per cent, with land becoming unusable as rangeland when the bush cover reaches 90 per cent.[32]

Also touristic potential of land is found to decline in areas with heavy bush encroachment, with visitors shifting to less encroached areas and better visibility of wildlife.[33]

Bush Control
Goats can function as a natural measure against bush encroachment or the re-establishment of seedlings after bush thinning

Types of Interventions

The term bush control, or brush management, refers to actions that are targeted at controlling the density and composition of bushes and shrubs in a given area. Such measures either serve to reduce risks associated with bush encroachment, such as wildfires, or to rehabilitate the affected ecosystems.

Literature emphasizes that a restoration of bush encroached areas to a desired previous non-encroached state is difficult to achieve and the recovery of key-ecosystem may be short-lived or not occur. Intervention methods and technologies must be context specific in order to achieve their intended outcome.[34] Current efforts of selective plant removal are found to have slowed or halted woody encroachment in respective areas, but are sometimes found to be outpaced by continuing encroachment.[35][36]

Three different categories of measures can be distinguished:[37]

  • Preventive measures (sustainable land management practices): application of proven good management practices to prevent the excessive growth of woody species, e.g. through appropriate stocking rates and rotational grazing in the case of rangeland agriculture.
  • Responsive measures (bush thinning): the reduction of bush densities through targeted bush harvesting or other forms of removal.
  • Maintenance measures (aftercare): repeated or continuous measures of maintaining the bush density and composition that has been established through bush thinning.[26]

Practices of combating bush encroachment, regardless of the exact methodology applied, are sometimes referred to as "de-bushing". Although this term is widely used and also cited in various scientific works, it is disputed. "De-bushing" can easily be misunderstood as the practice of clearing and area of all bush plants, which is not the aim in most efforts to combat bush encroachment. The terms "bush control, "bush thinning" and "selective bush harvesting" are therefore considered more adequate terms. Efforts to counter bush encroachment fall into the scientific field of restoration ecology and are primarily guided by ecological parameters, followed by economic indicators.

In grassland conservation efforts, the implementation of measures across networks of private lands remains a key challenge.[35]

Control Methods
Fire fighter administering prescribed fire as management tool to remove woody encroachment near Mt. Adams, USA

Methods of bush control encompass

  • Natural bush control: among others through the introduction of browsers, such as Boer goats,[38] aministering controlled fires,[4][39][40][41] or rewilding ecosystems with historic herbivory fauna.[42][43] Fire was found to be especially effective in reducing bush densities, when coupled with the natural event of droughts[44] or the intentional introduction of browsers.[45]
  • Mechanical bush control: harvesting of bushes and shrubs with manual or mechanised equipment
  • Chemical bush control: the application of herbicides, in particular arboricides

Encroachment Quantification and Monitoring

There is no static definition of what is considered woody encroachment, especially when encroachment of indigenous plants occurs. While it is simple to determine vegetation trends (e.g. an increase in woody plants over time), it is more complex to determine thresholds beyond which an area is to be considered as encroached. Various definitions as well as quantification and mapping methods have been developed.

In Southern Africa, the BECVOL method (Biomass Estimates from Canopy Volume) finds frequent application. It determines Evapotranspiration Tree Equivalents (ETTE) per selected transect. This data is used for comparison against climatic factors, importantly annual rainfall, to determine whether the respective areas have a higher number of woody plants than considered sustainable.[15]

The comparison of panoramic photographs is found to be an effictive tool for the monitoring of vegetation change, including woody encroachment.[46]

Impact on Carbon Sequestration
Amount of carbon stored in Earth's various terrestrial ecosystems, in gigatonnes.[47]

Against the background of global efforts to mitigate climate change, the carbon sequestration and storage capacity of natural ecosystems receives increasing attention. While bush encroachment is in part caused by climate change (elevated atmospheric carbon dioxide fosters woody plant growth) and efforts to control bush densities are motivated by factors unrelated to the international climate change agenda (e.g. increasing land productivity), the impact of bush control on the carbon sequestration and storage capacity of the respective ecosystems is an important consideration. Shifts in plant species composition and ecosystem structure, especially through woody encroachment, leads to significant uncertainry in predicting carbon cycling in grasslands.[48]

Quantification

Research on the changes to carbon sequestration under bush encroachment and bush control is still insufficient.[49] Some studies find that carbon sequestration can increase for a number of years under woody encroachment, while the magnitude of this increase is highly dependent on annual rainfall. It is found that woody encroachment has little impact on sequestration potential in dry areas with less than 400 mm in precipitation.[50][51] It was further fund that encroached ecosystems are more likely to lose carbon during drought periods than open grasslands.[52]

When loosely equating bush encroachment with afforestation, considering above-ground biomass alone, encroachment could be seen as a carbon sink. However, considering the losses in the herbaceous layer as well as changes in soil organic carbon, the quantification of terrestrial carbon pools and fluxes becomes more complex and context specific. Globally, the soil organic carbon pool is twice as large as the plant carbon pool, making its quantification essential.

Changes to carbon sequestration and storage need to be determined for each respective ecosystem and holistically, i.e. considering both above-ground and below-ground carbon storage.

  • Above-ground carbon: Bush encroachment implies an increase in woody plants, in most cases at the expense of grasses. Considering that woody plants have a longer lifespan and generally also more mass, bush encroachment can imply an increase in above-ground carbon storage through biosequestration. Studies however find that this is depending on climatic conditions, with aboveground carbon pools decreasing under woody encroachment where mean annual precipitation is less than 330mm and increasing where precipitation is higher.[53][54]
  • Below-ground carbon: the impact of woody encroachment on soil organic carbon is found to be dependent on rainfall, with soil organic carbon increasing in dry ecosystems and decreasing in mesic ecosystems under encroachment. In wet environments, grasslands have more soil carbon than shrublands and woodlands. Under shrub encroachment, the losses in soil carbon can be sufficient to offset the gains of above-ground carbon gains.[50][55][56][57][58] Soil organic carbon changes need to be viewed at landscape level, as there are differences between under canopy and inter canopy processes. When a landscape becomes increasingly encroached and the remaining open grassland patches are overgrazed as a result, soil organic carbon may decrease.[59][60] In South Africa bush encroachment was found to slow decomposition rates of litter, which took twice the time to decay under bush encroachment compared to open savannas. This suggests a significant impact of woody encroachment on the soil organic carbon balance.[61]

Climate change mitigation and adaptation

In early carbon sink quantifications, woody encroachment was found to account for as much as 22% to 40% of the regional carbon sink in the USA[62], while it is considered a key uncertainty in the US carbon balance[63]. Also in Australia woody encroachment constitutes a high proportaion of the national carbon account.[64] In South Africa, woody encroachment was estimated to have added around 21.000 Gg CO2 to the national carbon sink[65], while it is has been highlighted that especially the loss of grass roots leads to losses of below-ground carbon, which is not fully compensated by gains of above-ground carbon[66].

Given above described uncertainties, it varies widely how countries factor woody encroachment and the control thereof into their national Greenhouse Gas Inventories. Beyond difficulties to conclusively quantify the changes in carbon storage, promoting carbon storage through woody encroachment can constitute a trade-off, as it may reduce biodiversity of savanna endemics[67] and core ecosystem services, like land productivity. Some countries, such as South Africa, acknowledge inconclusive evidence on the emissions effect bush thinning, but systematically promote it as a means of climate change adaptation.[68]

Global Extent

Woody encroachment occurs on all continents in a variety of ecosystems.[69] Its causes, extent and response measures differ and are highly context specific.

In sub-Saharan Africa, woody vegetation cover has increased by 8% during the past three decades, mainly through woody encroachment. Overall, 7.5 million km2 of non-forest biomes experienced significant net gains in woody plant cover, which is more than three times the area that experienced net losses of woody vegetation.[70]

Affected Ecoregions

Central European Alps and Alpine Tundra

Areas that formerly were forests require continuous maintenance to avoid bush encroachment. When active land cultivation ends, fallow land is the result and gradual spread of shrubs and bushes can follow. Animal species once native to Central Europe effectively countered this natural process. These include herbivores such as European bison, auerochs (extinct), red deer and feral horse. Grassland and heath are considered to require protection due to their biodiversity as well as to preserve cultural landscapes. Bush encroachment is therefore frequently countered with selective removal of woody biomass or through the seasonal or year-round introduction of grazing animal species. such as sheep, goats, heck cattle or horses. Bush encroachment frequently occurs in the Alps, where structural change in agriculture often leads to the abandonment of land. Alnus viridis is the most widely distributed shrub species in the sub-alpine zone and is found to severely impair plant diversity when encroaching grassland.[71] Further, bush encroachment is common in the Alpine Tundra of Norway and Sweden[72][73]

Mediterranean Basin

The Mediterranean region is widely reported to be affected by bush encroachment. This is found to have negative effects on biodiversity and to magnify climate and related droughts. At the same time encroaching shrubs are also found to have a positive effect, reversing the desertification process.[74]

North American Grasslands

North American grasslands have been found to be affected by woody plant encroachment. Documentation of shrub encroachment caused by fire exclusion was documented as early as 1968.[75]

United States of America

In the United States, affected ecosystems include the Chihuahuan Desert, the Sonoran Desert, the Northern and Southern Rocky Mountains, the Sagebrush Steppe, as well as the Southern and Central Great Plains. Negative impacts on forage production and an interrelation with carbon sequestration are documented.[51] At the same time in the semiarid karst savanna of Texas, USA, woody plant encroachment has been found to improve soil infiltrability and therewith groundwater recharge.[76] Bird population decline as a result of woody encroachment has been identified as a critical conservation concern.[28]

Asian Temperate Savanna

China

Temperate savanna-like ecosystems in Northern China are found to be affected by shrub encroachment, linked to unsustainable grazing and climate change.[77]

India

Grassland in India

Semi-arid Banni grasslands of western India are found to be affected by bush encroachment, with affects both species composition and behaviour of nocturnal rodents.[78]

Australian Lowland Woodlands

In Australia woody encroachment is observed across all lowland grassy woodland as well as semi-arid floodplain wetlands and coastal ecosystems, with substantial implications for biodiversity conservation and ecosystem services.[79][80]

Latin American Grasslands

Brazil

Map of the Cerrado ecoregion in Brazil as delineated by the World Wide Fund for Nature

Wide ranging woody encroachment is found in the Cerrado, a savannah ecosystem in central Brazil. Studies found that 19% of its area, approximately 17 million hectares, show significant bush encroachment. Among the researched causes are fire suppression and land use abandonment.[81] This ecological change is linked to the disturbance of ecohydrological processes.[82]

Nicaragua

In Nicaragua Vachellia pennatula is known to encroach due to land intensification as well as land abandonment.[83]

Eastern African Grasslands

Ethiopia

Flowers of the Prosopis juliflora plant that is a common invasive species in Ethiopia and other countries

Grasslands in the Borana Zone in southern Ethiopia are found to be effected by bush encroachment, specifically by Senegalia mellifera, Vachellia reficiens and Vachellia oerfota.[84] The invasive species Prosopis juliflora has expanded rapidly since it was introduced in the 1970s, with direct negative consequences on pasture availability and therewith agriculture. The Afar Region is most severely affected. The wood of the invasive species is commonly used as household fuel in the form of firewood and charcoal.[85][86][87][88]

Southern African Savanna

Namibia

Dichrostachys cinerea, a common encroacher species in Namibia

Bush encroachment is estimated to affect up to 45 million ha. Agricultural productivity in Namibia has declined by two thirds throughout the past decades, mainly due to the negative impact of bush encroachment. The phenomenon affects both commercial and communal farming in Namibia, mostly the central, eastern and north-eastern regions.[89] Common encroacher species include Dichrostachys cinerea, which is most dominant in areas with higher precipiation.[90]

The Government of Namibia has recognised bush encroachment as a key challenge for the national economy and food safety. In its current National Development Plan 5, it stipulates that a total of 82.200 ha of land shall be treated against bush encroachment annually, i.e. through the thinning of the bush density by means of selective bush harvesting[91] The Government of Namibia, through the Support to De-bushing Project, pursues a value addition strategy, promoting the sustainable utilisation of bush biomass, which in turn is expected to finance bush harvesting operations.

In 2019, the three Namibian farmers’ unions (NNFU, NAU/NLU, NECFU) together with the Ministry of Agriculture, Water and Forestry published a best strategy document called “Reviving Namibia’s Livestock Industry”.[92] The document states that the Namibian livestock industry is in decline due to the loss of palatable perennial grasses and the increase in bush encroachment. Namibia’s rangelands show higher levels of bare ground, lower levels of herbaceous cover, lower perennial grass cover, and higher bush densities over large areas. Bush thickening leads to direct competition for moisture with desirable forage species and detrimentally influences the health of the soil. The best practice document identifies tried and tested practices of both emerging and established farmers from communal and title deed farms. These practices include the Split Ranch Approach, several Holistic Management approaches and the Mara Fodder Bank Approach. Other best practices include bush thinning, landscape rehydration and fodder production. The unions state that there is major scope for enhancing livelihoods through the sustainable use of bush products. In addition, increased profitability and productivity of the sector will have a major impact on the 70% of the Namibian population that depends directly or indirectly on the rangeland resource for their economic well-being and food security.

Increasingly, the encroaching bush is seen as a resource for a biomass industry. Economic assessments were conducted to quantify and value various key ecosystem services and land use options that are threatened by bush encroachment. The evaluation was part of the Economics of Land Degradation (ELD) Initiative,[93] a global initiative established in 2011 by the United Nations Convention to Combat Desertification, the German Federal Ministry for Economic Cooperation and Development, and the European Commission. Based on a national study, cost-benefit analysis suggests a programme of bush control to generate an estimated and aggregated potential net benefit of around N$48.0 billion (USD 3.8 billion) (2015 prices, discounted) over 25 years when compared with a scenario of no bush thinning. This implies a net benefit of around N$2 billion (USD 0.2 billion) (2015 prices, discounted) per annum in the initial round of 25 years.

The following products are currently produced from the encroacher bush biomass:[94]

Namibia has a well-established charcoal sector, which currently comprises approximately 1.200 producers, which employ a total of 8.000 workers. Most producers are farmers, who venture into charcoal production as a means to combat bush encroachment on their land. However, increasingly small enterprises also venture into charcoal making. As per national forestry regulations, charcoal can only be produced from encroaching species. In practice, it however proves difficult to ensure full compliance with these regulations, as the charcoal production is highly decentralised and the inspection capacities of the Directorate of Forestry are low. Voluntary FSC certification has sharply increased in recent years, due to respective demand in many off-take countries, such as the United Kingdom, France and Germany. Due to exclusive use of encroacher bush for charcoal production, rendering the value chain free from deforestation, Namibian charcoal has been dubbed the "greenest charcoal" in an international comparison.[95] In 2016 the Namibia Charcoal Association (NCA) emerged as a legal entity through a restructuring process of the Namibia Charcoal Producers Association, previously attached to Namibia Agricultural Union. It is a non-profit entity and the official industry representation, currently representing an estimated two-thirds of all charcoal producers in the country.

Namibia Biomass Industry Group is a non-profit association under Section 21 of the Companies Act (Act 28 of 2004) of Namibia, founded in 2016. It functions as the umbrella representative body of the emerging bush based biomass sector in the country with voluntary paid membership. The core objectives, as enlisted in the Articles of Association, include to develop market opportunities for biomass from harvested encroacher bush as well as to address industry bottlenecks, such as skills shortages and research and development needs. The De-bushing Advisory Service is a division of the association, mandated with the dissemination of knowledge on the topics of bush encroachment, bush control and biomass utilisation. Services are provided upon inquiry and are considered a public service and therefore not charged. According to its websites, services include technical advice on bush control and biomass utilisation, environmental advice, the strengthening of existing agricultural outreach services and linkage with service providers.[96] [97]

Both the Forestry Act and the Environmental Act of Namibia govern bush control. Special harvesting permits as well as Environmental Clearance Certificates are applicable to all bush harvesting activities. Responsible Authority is the Ministry of Environment, Forestry and Tourism.Effective April 2020 the Forest Stewardship Council introduced a national Namibian FSC standard (National Forest Stewardship Standard) that is closely aligned to the global FSC certification standard, but takes into consideration context specific parameters, such as bush encroachment.[98] In early 2020, the total land area certified under the FSC standard for the purpose of bush thinning and biomass processing was reported to amount to 1.6 million hectares.[99]

Botswana

Healthy savanna landscape in Botswana

Ecological surveys in Botswana have found bush encroachment affecting both communal grazing areas and private farmland, with particular prevalence in semi-arid ecosystems.[100][101] In selected areas, charcoal production has been introduced as a measure to reduce bush densities.[102][103][94] Bush encroachment in Botswana has been documented from as far back as 1971.[94]

South Africa

In South Africa bush encroachment entails the abundance of indigenous woody vegetation in grassland and savanna biomes.[46] These biomes make up 27.9% and 32.5% of the land surface area. About 7.3 million hectares are directly affected by bush encroachment, impacting rural communities socio-economically.[104][105]

Common encroaching species include Vachellia karoo, Senegalia mellifera, Dichrostachys cinera, Rhus undulata and Rhigozum trichotomum.[106]

Through the public works and conservation programme Working for Water, the governmetn of South Africa allocated approximately 100 million USD per annum for the management of native encroaching species.[107] Land users in South Africa commonly combat woody encroachment through clear felling, burning, intensive browsing or chemical control in the form of herbicide application.[106]

Eswatini

Studies in the Lowveld savannas of Eswatini confirm different heavy woody plant encroachment, especially by Dichrostachys cinerea, among other factors related to grazing pressure. In selected study areas the shrub encroachment increased from 2% in 1947 to 31% in 1990. In some affected areas, frequent fires, coupled with drought, reduced bush densities over time.[44][108][109]

Zambia

Woody encroachment has been recorded in Southern Zambia. Between 1986 and 2010 woody cover increased from 26% to 45% in Kafue Flats and Lochinvar National Park. A common encroacher species is Dichrostachys Cinerea.[110]

Zimbabwe

There is evidence of bush encroachment in Zimbabwe, among others by Vachellia Karroo.[111]

Reference Map

The following map displays the countries that are addressed in this article, i.e. countries that feature ecosystems with woody encroachment.

See also


Notes
1.^ Dune shrub encroachment differs significantly from bush and shrub encroachment of grasslands. It is often intermixed with grass encroachment and both the underlying ecosystem as well as the causes differ from other contexts.

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