Woody plant encroachment
Woody encroachment is often interpreted as a symptom of land degradation due to its negative impacts on key ecosystem services, but is also argued to be a form of natural succession.
Depending on rainfall, temperature and soil type, among other factors, woody plant encroachment may either increase or decrease the carbon sequestration potential of a given ecosystem.
In its Sixth Assessment Report of 2022, the Intergovernmental Panel on Climate Change (IPCC) states that woody encroachment may lead to slight increases in carbon, but at the same time mask underlying land degradation processes, especially in drylands.
[16] However, this is distinctly different from woody plant encroachment that occurs due to global drivers, e.g. increased carbon dioxide in Earth's atmosphere, and unsustainable forms of land use intensification, such as overgrazing and fire suppression.
[39] Woody encroachment is assumed to have its origins at the beginning of Holocene and the start of warming, with tropical species expanding their ranges away from the equator into more temperate regions.
While moderate bush cover increases diversity, excessive encroachment leads to habitat loss and reduced niches, negatively impacting species such as insects, spiders, mammals, birds, and reptiles.
[161][162][163] Encroachment leads to plant communities developing tougher, nutrient-poor tissues, which makes the soil more acidic, causes organic matter to build up, and reduces phosphorus levels.
[164] To the contrary, in Mediterranean and very humind climates, woody encroachment often leads to enhanced soil quality by increasing concentrations of carbon, nitrogen and phosphorus, especially in the topsoil.
[165] While water loss is common in closed canopy woodlands (i.e. sub-humid conditions with increased evapotranspiration) in semiarid and arid ecosystems, recharge can also improve under encroachment, provided there is good ecohydrological connectivity of the respective landscape.
[184][185] Studies in the United States moreover find that dense encroachment with Juniperus virginiana is capable of transpiring nearly all rainfall, thus altering groundwater recharge significantly.
[194] Against the background of global efforts to mitigate climate change, the carbon sequestration and storage capacity of natural ecosystems receives increasing attention.
[196] Shifts in plant species composition and ecosystem structure, especially through woody encroachment, lead to significant uncertainty in predicting carbon cycling in grasslands.
The IPCC points out that carbon stock changes under bush encroachment have been studied in Australia, Southern Africa and North America, but no global assessment has been done to date.
[238] In the Southern African country Namibia it is assumed that agricultural carrying capacity of rangelands has declined by two-thirds due to woody plant encroachment.
[252] Targeted bush control in combination with the protection of larger trees is found to improve scavenging that regulates disease processes, alters species distributions, and influences nutrient cycling.
This data is used for comparison against climatic factors, especially annual rainfall, to determine whether the respective area has a higher number of woody plants than is considered sustainable.
[262] The probability of woody plant encroachment for the African continent has been mapped using GIS data and the variables precipitation, soil moisture and cattle density.
[263] An exclusive reliance on remote sensing data bears the risk of wrongly interpreting woody plant encroachment, e.g. as beneficial vegetation greening.
Most interventions constitute a selective thinning of bush densities, although in some contexts also repeat clear-cutting has shown to effectively restore diversity of typical savanna species.
[337] Bush that is mechanically harvested is often burnt on piles,[338] but can also serve as feedstock for value addition, including firewood, charcoal, animal feed,[339][340] energy and construction material.
[344] Some forms of mechanical woody plant removal involve uprooting, which tends to lead to better results in terms of the restoration of the grass layer, but can have disadvantages for chemical and microbiological soil properties.
[339][352] In the same vein, the World Wildlife Fund has identified invasive and encroaching plant species as a possible feed stock for Sustainable Aviation Fuel in South Africa.
[355] Literature emphasises that a restoration of woody plant encroachment 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.
[387][388][389][279] In its 2022 Sixth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) identifies woody encroachment as a contribution to land degradation, through the loss of open ecosystems and their services.
The report further stipulates that while there may be slight increases in carbon, woody encroachment at the same time masks negative impacts on biodiversity and water cycles and therewith livelihoods.
[390] Carbon focused restorations approaches remain vital and can be balanced with the need to enhance other ecosystem services through spatially mixed management strategies, leaving encroached patches and in thinned areas.
[226] Also the Intergovernmental Panel on Climate Change (IPCC) states that mitigation action, such as reforestation or afforestation, can encroach on land needed for agricultural adaptation and therewith threaten food security, livelihoods and ecosystem functions.
[92] Some countries, for example South Africa, acknowledge inconclusive evidence on the emissions effect of bush thinning, but strongly promote it as a means of climate change adaptation.
[199] The Intergovernmental Panel on Climate Change (IPCC) reflects on this trade-off: "This variable relationship between the level of encroachment, carbon stocks, biodiversity, provision of water and pastoral value can present a conundrum to policymakers, especially when considering the goals of three Rio Conventions: UNFCCC, UNCCD and UNCBD.
Clearing intense woody plant encroachment may improve species diversity, rangeland productivity, the provision of water and decrease desertification, thereby contributing to the goals of the UNCBD and UNCCD as well as the adaptation aims of the UNFCCC.
