Woody plant encroachment
Woody plant encroachment 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. To the contrary, land abandonment can equally 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.
In some cases, areas affected by woody encroachment are classified as carbon sinks and form part of national greenhouse gas inventories. The carbon sequestration effects of woody plant encroachment are however highly context specific and still insufficiently researched. 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 states that woody encroachment may lead to slight increases in carbon, but at the same time mask underlying land degradation processes, especially in drylands.
The UNCCD has identified woody encroachment as a key contributor to rangeland loss globally.
Ecological definition and etymology
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. This is in line with the meaning of the term encroachment, which is "the act of slowly covering more and more of an area". 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.
The resulting process that leads to an abundance of woody plants is sometimes considered an ecological regime shift that can shift drylands from grassy dominated regimes towards woody dominated savannas. An increase in spatial variance is an early indicator of such regime shift. Depending on the ecological and climatic conditions this shift can be a type of land degradation and desertification. Progressing shrub encroachment is expected to feature a tipping point, beyond which the affected ecosystem will undergo substantial, self-perpetuating and often irreversible impact.
Research into the type of woody plants that tend to become encroaching species is limited. Comparisons of encroaching and non-encroaching vachellia species found that encroaching species have a higher acquisition and competition for resources. Their canopy architecture is different and only encroaching tree species reduce the productivity of perennial vegetation. In a comparison of Vachellia and Senegalia, Vachellia was found to be the more aggressive encroacher than Senegalia, growing faster and taller with thicker, animal-dispersed seeds, while Senegalia adapts to grass competition with denser roots and wind-dispersed seeds. There is evidence that woody plants adapt to grasslands, growing tough leaves and thick bark to protect against sun, drought, and fire, as well as efficient roots to efficiently gather nutrients.
By definition, woody plant encroachment occurs in grasslands. It is thus distinctly different from reforestation and afforestation. However, there is a strong overlap between vegetation greening, as detected through satellite-derived vegetation indices, and woody plant encroachment. Studies show that vegetation can impoverish despite a greening trend.
Grasslands and forests, as well as grasslands and shrublands, can be alternative stable states of ecosystems, but empirical evidence of such bistability is still limited.
Global extent
The UNCCD identifies woody encroachment as a key contributor to rangeland loss globally. Woody encroachment occurs on all continents, affecting an estimated total area of 500 million hectares. 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.
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 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. But 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.Susceptibility of ecosystems
There is evidence that some characteristics of ecosystem 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 coolers 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 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.