Woody plant encroachment (also called woody encroachment, bush encroachment, shrub encroachment, shrubification, woody plant proliferation, or bush thickening) is a natural phenomenon characterised by the area expansion and density increase of woody plants, bushes and shrubs, at the expense of the herbaceous layer, grasses and forbs. It refers to the expansion of native plants and not the spread of alien invasive species. Woody encroachment is observed across different ecosystems and with different characteristics and intensities globally. It predominantly occurs in grasslands, savannas and woodlands and can cause regime shifts from open grasslands and savannas to closed woodlands.
Causes include land-use intensification, such as overgrazing, as well as the suppression of wildfires and the reduction in numbers of wild herbivores. Elevated atmospheric CO2 and global warming are found to be accelerating factors. Conversely, land abandonment can likewise lead to woody encroachment.
The impact of woody plant encroachment is highly context specific. It can have severe negative impact on key ecosystem services, especially biodiversity, animal habitat, land productivity and groundwater recharge. Across rangelands, woody encroachment has led to significant declines in productivity, threatening the livelihoods of affected land users. 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.
Various countries actively counter woody encroachment, through adapted grassland management practices, controlled fire and mechanical bush thinning. Such control measures can lead to trade-offs between climate change mitigation, biodiversity, combatting desertification and strengthening rural incomes. The carbon sequestration effects of woody encroachment are highly context specific; the IPCC states that it may lead to slight increases in carbon, but can simultaneously mask underlying land degradation processes.
Definition
Woody plant encroachment is the increase in abundance of indigenous woody plants, such as shrubs and bushes, at the expense of herbaceous plants, grasses and forbs, in grasslands and shrublands. The term encroachment is thus used to describe how woody plants outcompete grasses during a given time, typically years or decades. Although such differentiation is not always applied, encroachment refers to the expansion of woody plants into open areas and thickening refers to the increasing density in a given area, including sub-canopy cover plants. Among earliest published notions of woody plant encroachment are publications of R. Staples in 1945, O. West in 1947 and Heinrich Walter in 1954.
While the terms are used interchangeably in some literature, woody plant encroachment is different from the spread of invasive species. As opposed to invasive species, which are deliberately or accidentally introduced species, encroacher species are indigenous to the respective ecosystem and their classification as encroachers depends on whether they outcompete other indigenous species in the same ecosystem over time. As opposed to alien plant invasion, woody plant encroachment is thus not defined by the mere presence of specific plant species, but by their ecological dynamics and changing dominance.
In some instances, woody plant encroachment is a form of secondary succession or forest regrowth. This applies to cases of land abandonment, for example when previous agricultural land is abandoned and woody plants re-establish. 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. Such drivers disrupt the ecological succession in a given grassland, specifically the balance between woody and herbaceous plants, and provide a competitive advantage to woody plants.
By definition, woody plant encroachment occurs in grasslands. It is thus distinctly different from reforestation and afforestation.
Global extent
The UNCCD identifies woody encroachment as a key contributor to rangeland loss globally.UNCCD. 2024. Global Land Outlook Thematic Report on Rangelands and Pastoralism. United Nations Convention to Combat Desertification, Bonn.UNCCD & CMS. 2025. Global Land Outlook Thematic Report on Ecological Connectivity and Land Restoration. Bonn: UNCCD & CMS. ISBN 978-92-95128-22-4. Woody encroachment occurs on all continents, affecting an estimated total area of 500 million hectares (5 million square kilometres). Its causes, extent and response measures differ and are highly context specific. Ecosystems affected by woody encroachment include closed shrublands, open shrublands, woody savannas, savannas, and grasslands. It can occur not only in tropical and subtropical climates, but also in temperate areas. Woody encroachment occurs at 1 percent per decade in the Eurasian steppes, 10–20 percent in North America, 8 percent in South America, 2.4 percent in Africa and 1 percent in Australia. In the European Alps, recorded expansion rates range from 0.6 percent to 16 percent per year.
In Sub-Saharan Africa, woody vegetation cover has increased by 8% during the past three decades, mainly through woody plant encroachment. Overall, 750 million hectares 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. In around 249 million hectares of African rangelands, long-term climate change was found to be the key driver of vegetation change. Across Africa, 29 percent of all trees are found outside classified forests. In some countries, such as Namibia and Botswana, this percentage is above 80 percent and likely linked to woody encroachment. In Southern Africa, woody encroachment has been identified as the main factor of greening, i.e. of the increase in vegetation cover detected through remote sensing. The future trend of biome change through woody encroachment in Africa bears great uncertainty. A global counterfactual assessment of natural vegetation cover suggests that, absent human influence, 43% of land could support trees, 39% shrubs and grasses, and 18% bare ground, and finds that adjustments in fire frequency and wildlife herbivory could have a greater impact on natural vegetation cover than expected climate change by 2050.
In Southern Europe an estimated 8 percent of land area has transitioned from grazing land to woody vegetation between 1950 and 2010.
In the Eurasian Steppe, the largest grassland globally, climate change linked woody plant encroachment has been found to occur at around 1% per decade.
In the Arctic Tundra, shrub plant cover has increased by 20 percent during the past 50 years. During the same time period, shrub and tree cover increased by 30 percent in the savannas of Latin America, Africa and Australia.
Causes Woody encroachment has a long history: with the warming at the onset of the Holocene, tropical woody species expanded their ranges away from the equator into more temperate regions. However, it has occurred at unparalleled rates since the mid-19th century. As such, it is classified as a type of grassland degradation, which occurs through direct and indirect human impact during the Anthropocene.
There is evidence that some characteristics of ecosystems render them more susceptible to woody encroachment than others. For example, coarse-textured soils promote woody plant growth, while fine-textured soils limit it. Moreover, the likelihood of woody encroachment is influenced by soil moisture and soil nutrient availability, which is why it often occurs on downslope locations and cooler slopes. The causes of woody encroachment differ significantly under different climatic conditions, e.g. between wet and dry savanna.
Various factors have been found to contribute to the process of woody plant encroachment. Both local drivers (i.e. related to land use practices) as well as global drivers can cause woody plant encroachment. Due to its strong link to human induced causes, woody plant encroachment has been termed a social-ecological regime shift. Research shows that both legacy effects of specific events, as well as plant traits can contribute to encroachment. There is still insufficient research on the interplay between the various positive and negative feedback loops in encroaching ecosystems.
Land use
Where land is abandoned, and respective anthropogenic pressures cease, a rapid spread of native bush plants is often observed. This is for example the case in former forest areas in the Alps that had been converted to agricultural land and later abandoned. In Southern Europe encroachment is thus linked to rural exodus. In such instances, land use intensification, e.g. increased grazing pressure, is found to be effective against woody encroachment. More recently, it is observed that land use cessation is not the only driver of woody encroachment in aforementioned regions, since the phenomenon occurs also where land continued to be used for agricultural purposes.
In other regions land use intensification is the main cause of woody plant encroachment. This is due to the interrelated fragmentation of landscapes and the loss of historical disturbance regimes, mainly in the following forms: Overgrazing: In the context of land intensification, a frequently cited cause of woody plant encroachment is overgrazing, commonly a result of overstocking and fencing of farms, as well as the lack of animal rotation and land resting periods. Overgrazing plays an especially strong role in mesic grasslands, where bushes can expand easily when gaining a competitive advantage over grasses, while woody encroachment is less predictable in xeric shrublands. Seed dispersal through animals is found to be a contributing factor to woody encroachment. While overgrazing has in the past frequently been found to be a main driver of woody encroachment, it is observed that woody encroachment continues in the respective areas even after grazing is reduced or ceases. Absence of large mammals: linked to the introduction of rangeland agriculture as well as unsustainable hunting practices, the reduction of large mammals such as elephant and rhino (in Africa) or elk (in North America) is a contributing factor to woody encroachment. A continent-wide energetics analysis found that total food energy consumption by birds and mammals across sub-Saharan Africa has declined by over one third since around 1700, including an approximately 75% decrease in the ecosystem functions performed by megafauna, reducing a key historical check on woody plant expansion. A continent-wide synthesis of historical tree-toppling rates found that elephants knock down on average 5.4% of the trees in an ecosystem per year, but that their impacts are now spatially constrained, so that most African landscapes lack this agent of tree mortality; historically, elephant tree toppling would have removed on average about one third of woody biomass production, and locally more than was produced. Landscape-scale research in Hluhluwe-iMfolozi Park, South Africa, similarly found that the poaching-driven loss of white rhinoceros (Ceratotherium simum), the world's largest grass-eating megaherbivore, was associated with changes in vegetation structure and fire regimes, with indirect implications for soil carbon dynamics.