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. 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. 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. 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. 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. 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 is assumed to have its origins at the beginning of the 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. 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. 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. Fire suppression: A connected cause for woody plant 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 and the fragmentation of landscapes. When the lack of fire reduces tree mortality and consequently the grass fuel load for fire decreases, a negative feedback loop occurs. It has been estimated that from a threshold of 40% canopy cover, surface grass fires are rare. At intermediate rainfall, fire can be the main determinant between the development of savannas and forests. In experiments in the United States it was determined that annual fires lead to the maintenance of grasslands, 4-year burn intervals lead to the establishment of shrubby habitats and 20-year burn intervals lead to severe woody plant encroachment. 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. Already one decade of land management change, such as the exclusion of fires and overgrazing, can lead to severe woody plant encroachment. The global increase in atmospheric CO2 contributes to the reduction of wildfires, as it decreases flammability of grass. Competition for water: a positive feedback loop occurs when encroaching woody species reduce the plant available water, providing a disadvantage for grasses, promoting further woody encroachment. According to the two-layer theory, grasses use topsoil moisture, while woody plants predominantly use subsoil moisture. If grasses are reduced by overgrazing, this reduces their water intake and allows more water to penetrate into the subsoil for the use by woody plants. Moreover, research suggests that bush roots are less vulnerable to water stress than grass roots during droughts. Population pressure: population pressure can be the cause for woody plant encroachment, when large trees are cut as building material or fuel. This stimulates coppice growth and results in an increase of the shrub vegetation. Ecosystem restoration: active interventions and changes of ecosystem, such as the creation of wetlands, can trigger woody encroachment.
Climate change
While changes in land management are often seen as the main driver of woody encroachment, some studies suggest that global drivers increase woody vegetation regardless of land management practices. For example, in a representative sampling of South African grasslands, woody plant encroachment was found to be the same under different land uses and different rainfall amounts, suggesting that climate change may be the primary driver of the encroachment. Once established, shrubs suppress grass growth, perpetuating woody plant encroachment. Suitable habitat for key encroacher species is expected to increase under climate change. Predominant global drivers include the following:
Atmospheric CO2: climate change has been found to be a cause or accelerating factor for woody plant encroachment. This is because increased atmospheric CO2 concentrations foster the growth of woody plants. Woody plants with C3 photosynthetic pathway thrive under high CO2 concentrations, as opposed to grasses with C4 photosynthetic pathway. The ability to increase nitrogen availability through N₂ fixation helps encroaching trees thrive as CO₂ levels rise. Moreover, tolerance to herbivory is found to be enhanced during the plants' recruitment stage under increased CO2 concentrations. Physiological acclimation to elevated CO2 is found to vary considerably between species: some savanna trees show clear photosynthetic upregulation under elevated CO2, particularly under well-watered conditions, while others downregulate, indicating that CO2-driven encroachment potential differs markedly among species. Rainfall patterns: a frequently cited theory is the state-and-transition model. The approach has for example been applied to describe transitions between grassy and bushy vegetation states in the Highland savanna of Namibia, driven by rainfall variability, competition and browsing pressure on Acacia mellifera seedlings. This model outlines how rainfall and its variability is the key driver of vegetation growth and its composition, bringing about woody plant encroachment under certain rainfall patterns. For example, if rainfall intensity increases, deep soil water typically increases, which in turn benefits bushes more than grasses. Both the amount of rainfall and its timing are important and distinct factors. Changes in precipitation can foster woody encroachment. Increased precipitation can foster the establishment, growth and density of woody plants. Also decreased precipitation can promote woody plant encroachment, as it fosters the shift from mesophytic grasses to xerophytic shrubs. Global warming: woody encroachment correlates to warming in the tundra, while it is linked to increased rainfall in the savanna. Species such as Vachellia sieberiana thrive under warming irrespective of the competition with grasses. The Intergovernmental Panel on Climate Change (IPCC) in its report "Global warming of 1.5°C" states that high-latitude tundra and boreal forests are at particular risk of climate change-induced degradation, with a high likelihood of shrub encroachment under continued warming. In other ecosystems, such as sub-Sahara grasslands, rising aridity may cause woody plants to be more prone to hydraulic failure. Droughts: droughts contribute to woody plant encroachment, if they reduce the perennial grass cover and the latter recovers slowly, providing shrubs with a competitive advantage with regard to the acquisition of deep-soil water. Drought, in combination with high levels of grazing pressure, can function as the tipping point for an ecosystem, causing woody encroachment.
Ecological and socio-economic impacts Woody encroachment constitutes a major global shift in plant composition, structure and function, with far-reaching impact on the affected ecosystems. The accelerating rate of woody encroachment across grasslands globally may lead to an abrupt decline of this biome type, owing to human impact. For example, the Great Plains biome is found to be at the brink of collapse due to woody encroachment, with 62% of North American grassland lost to date. Encroachment is commonly identified as a form of land degradation, with severe negative consequences for various ecosystem services, such as biodiversity, groundwater recharge, carbon storage capacity and herbivore carrying capacity. Nevertheless, negative impact is not universal. Impacts are dependent on species, scale and environmental context factors. Woody encroachment can have significant positive impacts on ecosystem services as well. Research suggests that ecosystem multifunctionality increases under woody encroachment. Affected ecosystem services fall into the category of provisioning (e.g. forage value), regulating (e.g. hydrological regulation, soil stability) and supporting (nutrient cycling, carbon sequestration, biodiversity, primary production). A systematic review of woody encroachment's impacts on nature's contributions to people in Africa and North America found that reductions in herbaceous forage availability were the most consistently reported material impact on both continents, while negative effects on non-material contributions such as recreation and tourism were reported mainly for Africa. There is a need for ecosystem-specific assessments and responses to woody encroachment. Generally, the following context factors determine the ecological impact of woody encroachment:
Prevailing land use: While positive ecological effects can occur in unmanaged landscapes or certain land-uses, negative ecological effects are observed especially in landscapes used for livestock grazing. Density of woody plants: Plant diversity and ecosystem multifunctionality typically peak at intermediate levels of woody cover and high woody covers generally have negative impacts. The magnitude of these structural changes scales with the degree of encroachment, with impacts on ecosystem structure intensifying as shrub cover increases relative to grassland baselines. Environmental conditions: Arid ecosystems show more negative responses to woody encroachment than non-arid ecosystems. In arid ecosystems woody encroachment is sometimes regarded as a form of land degradation and an expression of desertification Due to its ambiguous role in these dry ecosystems, it has been termed "green desertification". To the contrary, in ecosystems of the Mediterranean region and in Alpine grasslands, encroachment can enhance ecosystem functionality and reverse desertification trends. A key difference is that during woody encroachment the herbaceous cover in the inter-canopy zones can remain intact, while during desertification these zones degrade and turn into bare soil devoid of organic matter.
Biodiversity
Woody encroachment causes widespread declines in the diversity of herbaceous vegetation through competition for water, light, and nutrients. Bush expands at the direct expense of other plant species, potentially reducing plant diversity and animal habitats. Woody encroachment impacts animal diversity by altering the structural diversity of vegetation, which affects habitat quality and species interactions. 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. These changes can cascade through ecosystems, affecting herbivores and top predators, altering behaviors like hunting efficiency and foraging strategies. For example, maintaining open savannas can benefit diurnal avian scavengers (e.g. vultures), while the same ecosystem in an encroached state may favour nocturnal mammalian scavengers. These effects are context specific, a meta-analysis of 43 publications of the time period 1978 to 2016 found that woody plant encroachment has distinct negative effects on species richness and total abundance in Africa, especially on mammals and herpetofauna, but positive effects in North America. However, in context specific analyses also in North America negative effects are observed. For example, piñon-juniper encroachment threatens up to 350 sagebrush-associated plant and animal species in the US. A study of 30 years of woody encroachment in Brazil found a significant decline of species richness by 27%. Shrub encroachment may result in increased vertebrate species abundance and richness. However, these encroached habitats and their species assemblages may become more sensitive to droughts. As encroachment is not a stable state, but characterised by changing bush densities, it is important to identify how different density thresholds affect plant and animal species.
Evidence of biodiversity losses includes the following:
Grasses: encroachment results in substantial loss of herbaceous diversity, with a loss of richness that is not replaced. Research on Mediterranean grassland-shrubland mosaics similarly found that woody plant encroachment drives biotic homogenisation and species turnover in herbaceous plant communities, with the strength of these effects depending on local environmental context. Studies in South Africa have found that grass richness reduces by more than 50% under intense woody plant encroachment. In North America, a meta-analysis of 29 studies from 13 different grassland communities found that species richness declined by an average of 45% under woody plant encroachment. Rare species and those with lower stature are at risk of going extinct. Among the severely affected flora is the small white lady's slipper. Generally, large bushes are found to coexist with the herbaceous layer, while smaller shrubs compete with it. Increased shade is a contributing factor to the reduction of grass abundance and diversity. Mammals: woody plant encroachment has a significant impact on herbivore assemblage structure and can lead to the displacement of herbivores and other mammal types that prefer open areas. Among other factors, predation success of various mammals is negatively impacted by bush encroachment. Among the species found to lose habitat in areas affected by woody plant encroachment are cats such as cheetah, white-footed fox, as well as ungulates such as the Common tsessebe, Hirola and plains zebra. In Latin America the habitat of the almost extinct Guanaco is threatened by woody encroachment. In some rangelands, woody plant encroachment is associated with a decline in wildlife grazing capacity of up to 80%. Among rodent species, those specialists on grasslands typically decline in abundance under woody encroachment, while those specialised on forests might increase in abundance. Also burrowing mammals can lose habitat when woody encroachment occurs. Birds: the impact of woody encroachment on bird species must be differentiated between shrub-associated species and grassland specialists. Studies find that shrub-associated species benefit from woody encroachment up to a certain threshold of woody cover (e.g. 22 percent in a study conducted in North America), while grassland specialist populations decline. Experiments in Namibia have shown that foraging birds, such as the endangered Cape vulture, avoid encroachment levels above 2,600 woody plants per hectare. In Southern Africa, woody encroachment drives population decline of 20% of the common open ecosystem bird species, on average at a rate of 50% population decline over fifty years. In North American grasslands, bird population decline as a result of woody encroachment has been identified as a critical conservation concern. Among the birds negatively affected by woody plant encroachment are the Secretarybird, Grey go-away-bird, Marico sunbird, lesser prairie chicken, Greater sage-grouse, Archer's lark, Northern bobwhite, Kori bustard, and Yellow cardinal. Insects: woody plant encroachment is linked to species loss or reduction in species richness of insects with preference for open habitats. A multi-year study of arthropod communities in Namibia similarly found that shrub encroachment altered invertebrate diversity and abundance, with responses to encroachment-control interventions such as shrub thinning varying by taxon. Affected species include butterfly, ant and beetle. Encroachment often creates connected bare plant interspaces where water and wind erosion can occur.
Soil and vegetation structure Soil quality under woody encroachment in dryland ecosystems is determined by a combination of plant cover, precipitation, soil physiochemical characteristics, and topographic variables. Encroachment has a significant impact on soil bacterial communities. Soil quality can decline significantly in arid and semi-arid regions under woody encroachment, manifesting through reduced soil moisture levels, nutrient availability and microbial activity. This drives soil drought conditions and decreases perennial herbaceous plants, while increasing bare ground. A global meta-analysis of 117 studies encompassing 635 paired invaded-uninvaded comparisons found that, in contrast to these arid-region moisture losses, woody plant invasions were on average associated with increases in soil nitrogen (~50%), phosphorus (~34%) and organic carbon (~29%), with effects most pronounced in tropical, Mediterranean and semi-arid regions and shaped by traits such as nitrogen fixation, rooting depth and mycorrhizal association. 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. To the contrary, in Mediterranean and very humid climates, woody encroachment often leads to enhanced soil quality by increasing concentrations of carbon, nitrogen and phosphorus, especially in the topsoil.
Groundwater recharge and soil moisture
Woody plant encroachment is frequently linked to reduced groundwater recharge, based on evidence that bushes consume significantly more rainwater than grasses and encroachment alters water streamflow. Woody encroachment generally leads to root elongation in the soil and the downward movement of water is hindered by increased root density and depth. The impact on groundwater recharge differs between sandstone bedrocks and karst regions as well as between deep and shallow soils. Woody plant encroachment can accelerate soil drought through enhanced soil porosity. While water loss is common in closed canopy woodlands (i.e. sub-humid conditions with increased evapotranspiration), in semi-arid and arid ecosystems recharge can improve under encroachment, provided there is good ecohydrological connectivity of the respective landscape. Ecohydrological connectivity is suggested as a unifying framework for the understanding of different groundwater impacts under encroachment. Besides groundwater recharge, woody encroachment increases tree transpiration and evaporation of soil moisture, due to increased canopy cover. Woody encroachment leads to the drying up of stream flows. Further, woody plant control can effectively improve the connectivity of water resources. Although this is strongly context dependent, bush control can be an effective method for the improvement of groundwater recharge. Studies in South Africa have shown that approximately 44% of rainfall is captured by woody canopies and evaporated back into the atmosphere under woody encroachment. This effect is strongest with fine-leaved species and in events of lower rainfall sizes and intensities. It was found that up to 10% less rain enters the soil overall under woody encroachment. A meta-analysis of studies in South Africa further finds that woody encroachment has low water loss effect in areas with limited rainfall. Streamflow can increase after targeted removal of invasive and encroaching species, as showcased in South Africa.
Carbon sequestration The impact of bush control on the carbon sequestration and storage capacity of the respective ecosystems is an important management consideration. Against the background of global efforts to mitigate climate change, the carbon sequestration and storage capacity of natural ecosystems receives increasing attention. Grasslands constitute 40% of Earth's natural vegetation and hold a considerable amount of the global Soil Organic Carbon. Shifts in plant species composition and ecosystem structure, especially through woody encroachment, lead to significant uncertainty in predicting carbon cycling in grasslands. Research on the changes to carbon sequestration under woody plant encroachment and its control is still insufficient. The Intergovernmental Panel on Climate Change (IPCC) states that woody plant encroachment generally leads to increased aboveground woody carbon, while below-ground carbon changes depend on annual rainfall and soil type. 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. Considering above-ground biomass alone, woody plant encroachment may appear to function as a carbon sink. However, when accounting for losses in the herbaceous layer and changes in soil organic carbon (SOC), the quantification of terrestrial carbon pools and associated fluxes becomes significantly more complex and highly context-dependent. Accurate assessments of changes in carbon sequestration and storage therefore require a holistic approach, considering both above-ground and below-ground carbon stocks, and must be tailored to each specific ecosystem. Elevated atmospheric CO2 levels generally promote woody plant growth, which increases nutrient uptake from the soil and can in turn reduce the soil's capacity to store carbon. In contrast, grasses contribute less biomass above ground but play a significant role in below-ground carbon sequestration. Several studies have found that gains in above-ground carbon during encroachment can be entirely offset by losses in below-ground carbon. Empirical evidence shows that while woody encroachment may lead to overall carbon increases in wetter ecosystems, it can result in carbon losses in arid environments. Some studies indicate that carbon sequestration may increase for a number of years following encroachment, but the extent of this increase is strongly influenced by annual rainfall. In dry regions with less than 400 mm of precipitation, woody encroachment appears to have minimal impact on carbon sequestration potential. This suggests that the carbon benefits of woody plant encroachment may decline with future climate change, particularly in ecosystems projected to experience reduced rainfall and increased temperatures. Moreover, woody encroachment has been linked to increased fluvial erosion, which can lead to the release of previously stabilised organic carbon from legacy grasslands. Encroached systems are also more vulnerable than open grasslands to carbon loss during periods of drought. Among the ecosystems anticipated to lose carbon storage under advancing woody encroachment is the tundra. Factors relevant for comparisons of carbon sequestration potentials between encroached and non-encroached grasslands include the following: above-ground net primary production (ANPP), below-ground net primary production (BNPP), photosynthesis rates, plant respiration rates, plant litter decomposition rates, soil microbacterial activity. Also plant biodiversity is an important indicator, as plant diversity contributes more to soil organic carbon than the quantity of organic matter. Woody plant 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, woody plant encroachment typically implies an increase in above-ground carbon storage through biosequestration. Studies however find that this is dependent on climatic conditions, with above-ground carbon pools decreasing under woody encroachment where mean annual precipitation is less than 330 mm and increasing where precipitation is higher. A contributing factor is that woody encroachment decreases above-ground plant primary production in mesic ecosystems. Globally, the soil organic carbon pool is twice as large as the plant carbon pool, making its quantification essential. Soil organic carbon makes up two-thirds of total soil carbon. Comparisons of grasslands, shrublands and forests show that forest and shrubland hold more above-ground carbon, while grasslands boast more soil carbon. Generally, herbaceous plants allocate more biomass below-ground than woody plants. 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. Degradation of grasslands has in some areas led to the loss of up to 40% of the ecosystem's soil organic carbon. An important factor is that under woody plant encroachment the increased photosynthetic potential is largely offset by increased plant respiration and respective carbon losses. Moreover, woody encroachment can decrease the water holding capacity of soils, which in turn also implies that CO2 runs out more quickly and isn't replaced. In tropical savanna soils, most soil organic carbon is derived from grass, not woody plants. For example, research in South Africa found that soil organic carbon from tree input matched grass-derived soil organic carbon only after 70 years of fire exclusion, challenging the view that increased tree density leads to SOC improvements. Contributing factors vary broadly in different settings, as is also evident in the role of litter. Generally, organic carbon in the topsoil can benefit from increased litter under encroachment. However, in South Africa woody plant encroachment was found to slow decomposition rates of litter, which took twice the time to decay under woody plant encroachment compared to open savannas. Soil organic carbon changes need to be viewed at the 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. In pastoral lands of Ethiopia, woody plant encroachment was found to have little to no positive effect on soil organic carbon and woody encroachment restriction was the most effective way to maintain soil organic carbon. In the United States, substantial soil organic carbon sequestration was observed in deeper portions of the soil, following woody encroachment. An important factor is that rooting depth increases with woody encroachment, on average by 38 cm and up to 65 cm. Deeper rooting may promote the accumulation of organic carbon in the deep soil layers, but at the same time also lead to a positive priming effect, i.e. the stimulation of microbial activity and decomposition of organic matter. The trajectory of deep soil carbon under woody encroachment will depend on the balance of increased SOC accumulation and priming losses. A meta-analysis of 142 studies found that shrub encroachment alters soil organic carbon (0–50 cm), with changes ranging between -50 and 300 percent. Soil organic carbon increased under the following conditions: semi-arid and humid regions, encroachment by leguminous shrubs as opposed to non-legumes, sandy soils as opposed to clay soils. The study further concludes that shrub encroachment has a mainly positive effect on top-soil organic carbon content, with significant variations among climate, soil and shrub types. There is a lack of standardised methodologies to assess the effect of woody encroachment on soil organic carbon.
Land productivity and rural livelihoods Woody plant encroachment directly impacts land productivity, as widely documented in the context of animal carrying capacity. In the western United States, 25% of rangelands experience sustained tree cover expansion, with estimated losses for agricultural producers of $5 billion since 1990. The forage lost annually is estimated to be equal to the consumption of 1.5 million bison or 1.9 million cattle. In North America, each 1 percent increase in woody cover implies a reduction of 0.6 to 1.6 cattle per 100 hectares. 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. In East Africa there is evidence that an increase of bush cover of 10 percent reduced grazing by 7 percent, with land becoming unusable as rangeland when the bush cover reaches 90 percent. The relationship between shrub cover and productivity is not necessarily linear. A study across 102 sites along a shrub-cover gradient found that grassland productivity responded non-linearly to encroachment, declining sharply between 0% and 3% shrub cover (grass-dominant stage), stabilising between roughly 3% and 25% cover (grass–shrub transitional stage), and declining substantially again beyond 25% cover (shrub-dominant stage). Woody encroachment is often considered to have a negative impact on rural livelihoods. In Africa, 21% of the population depend on rangeland resources. Woody encroachment typically leads to an increase in less palatable woody species at the expense of palatable grasses. This reduces the resources available to pastoral communities and rangeland based agriculture at large. Woody encroachment has negative consequences on livelihoods, especially in arid areas, which support a third of the world population's livelihoods. Woody plant encroachment is expected to lead to large scale biome changes in Africa and experts argue that climate change adaptation strategies need to be flexible to adjust to this process. In South Africa, the shrub Seriphium plumosum is commonly referred to as "bankrupt bush" due to its association with farm productivity reductions of up to 80%. Studies suggest a link between woody plant encroachment and outmigration from rural areas. Touristic potential of land is found to decline in areas with heavy woody plant encroachment, with visitors shifting to less encroached areas and better visibility of wildlife.
Human health and climate feedbacks In the United States, woody encroachment has been linked to the spread of tick-borne pathogens and respective disease risk for humans and animals. In the Arctic tundra, shrub encroachment can reduce cloudiness and contribute to a raise in temperature. In North America, significant increases in temperature and rainfall were linked to woody encroachment, amounting to values up to 214 mm and 0.68 °C respectively. This is caused by a decrea