Introduction

In many parts of the tropics and sub-tropics, savanna and grassland ecosystems have become increasingly woody over time, with an attendant decline in grass/herbage cover (van Vegten 1984; Archer et al. 1988; Da Silva et al. 2016). Temperate grasslands or prairie ecosystems, which are dominated by grasses, are also being rapidly invaded by woody perennial plants, especially shrubs (Van Auken 2000; Liu et al. 2013; Rauber et al. 2020; Ovalle et al. 2021). Even tundra ecosystems, which are dominated by herbaceous plants, particularly mosses and lichens, are becoming progressively invaded by shrubs (Myers-Smith et al. 2011; Garcia Criado et al. 2020; Aguirre et al. 2021). The rapid proliferation of woody plants, especially trees and shrubs, in savanna, grassland and tundra ecosystems is known as woody plant encroachment (Liu et al. 2013; Aguirre et al. 2021; Pillay et al. 2021; Mochi et al. 2022). Woody plant encroachment (WPE), is synonymous with the term “shrub encroachment” (Knapp et al. 2008; Maestre et al. 2009; Eldridge et al. 2011; Da Silva et al. 2016) and with “shrub invasion” (Van Auken 2000). The term, “bush encroachment” which was widely applied by researchers in Africa (Moleele et al. 2002; Wiegand et al. 2005, 2006; Wigley et al. 2009; Ngosikoma and Mogotsi 2013) also refers to WPE. The process of WPE has considerable effects on the structure and functioning of savanna and grassland ecosystems and is associated with increased carbon dioxide emission and climate change (Kgope et al. 2010; O’ Connor et al. 2014, Archer et al. 2017). Consequently, WPE has received considerable attention from scientists globally. Apart from a few studies such as those of Maestre et al. (2009) who observed that WPE reverses desertification and Moleele (1998) who opined that bush encroachment has a salutary and positive effect on browse production for cattle, an overwhelming majority of the reviews/studies on WPE considered it to be an obnoxious environmental phenomenon that causes loss of biodiversity (Da Silva et al. 2016; Soto-Shoender et al. 2018; Pinheiro et al. 2022), reduces groundwater resources (Starks and Moriasi 2017; Acharya et al. 2018); decreases grass biomass and cover (Van Vegten 1984; Van Auken 2009; Soto-Shoender et al. 2018) and also results in loss of habitat of endangered wildlife (Cuellar-Soto et al. 2020). It is considered to be patently inimical to livestock production, especially cattle (Wiegand et al. 2005; Ayalew and Mulualem 2018). An overwhelming majority of studies and reviews on WPE glossed over the benefits of the phenomenon, presumably because of the pervasive view that its impacts are negative. This review seeks to address the question whether WPE is a bad phenomenon. In addition, it will highlight the well-known and usually ignored benefits of WPE to rural people and humanity in general. As a backdrop to ascertaining whether the phenomenon is bad, an overview of its causes will be conducted.

Methods

This review is based on empirical studies published in journals and recent reviews/book chapters (e.g. Archarya et al. 2018, Fowler and Beckage 2019) that were procured from the Google Scholar database (https://scholar.google.com), using as keywords “woody plant encroachment” and its impacts on soil, vegetation, wildlife and groundwater. The keywords used for the literature search included “woody plant encroachment”, “shrub encroachment”, “shrub invasion”, “bush encroachment” and “woody plant encroachment impact on groundwater”. Apart from WPE impact of improving soil infiltrability and percolation in semiarid karst savanna (Leite et al. 2020) and carbon sequestration, the benefits of the phenomenon were inferred from the uses of the encroaching trees/shrubs. The scientific names of the encroaching trees and shrubs (e.g., Terminalia sericea, Juniperus virginiana) were used as keywords for highlighting the uses of the encroaching woody plants. This review is restricted to WPE in tropical and temperate savanna and grassland ecosystems.

Causes of woody plant encroachment

WPE has been attributed to a variety of causes including an increase in cattle population in rangeland, rainfall variability, patch dynamics of savanna vegetation in response to fluctuations in annual rainfall, biotic influences, exclusion of burning, and an increase in atmospheric carbon dioxide level. An increase in cattle population with an attendant overgrazing over time, has been reported by several authorities as a major cause of WPE in savanna vegetation in southern Africa, especially Botswana (van Vegten 1984; Moleleele et al. 2002). However, the study of Wigley et al. (2009) in KwaZulu Natal, South Africa, indicated that the rate of WPE is fastest in conserved savanna that is usually protected from grazing and burning. This presumably suggests that factors, other than cattle density, are also important determinants of WPE. The increase in cattle population hypothesis is closely related to the two—layer model of Walter (1971) who proposed that grasses and woody plants compete for soil water and nutrients and that grasses are more efficient than trees in exploiting topsoil water while trees and shrubs have virtually exclusive access to deep subsoil water. When grazing pressure is high, grasses are over utilized by cattle and so lose their competitive advantage over woody plants, in terms of exploiting topsoil water. This also opens up the grass sward and permits water to percolate downwards beyond the rooting zone of grasses to the subsoil (van Vegten 1984), thereby allowing trees to multiply rapidly at the expense of grasses. Also, an increase in the population of cattle leads to overgrazing and an attendant elimination of the grass layer, thereby making it difficult for savanna vegetation to burn (van Auken 2000; Kgosikoma and Mogotsi 2013). This will in turn facilitate the proliferation of trees. An increase in cattle population does not always result in WPE, unless it is accompanied by fire exclusion. Mochi et al. (2022) concluded from their study of the effects of grazing on the early stages of WPE in a temperate savanna in Argentina that cattle decreased seedling survival through trampling and consumption. This implies that an increase in cattle population would decrease WPE rate and not enhance it. In their simulation study of a Brazilian savanna, Pinheiro et al. (2022) pointed out that WPE reduced the regeneration of species in the ground layer of savanna ecosystems by reducing the amount of light and temperature and increased litter deposition. Their findings suggest that WPE is not triggered by competition between woody plants and grasses for water in the topsoil. The study of Silva et al. (2001) in Calabozo, Venezuela indicated that over a period of sixteen years, both plots that were protected against burning and grazing and those that were burnt and subjected to varying degrees of grazing, became increasingly invaded by woody plants. These seemingly conflicting results support the view that WPE in savanna and grassland ecosystems is complex, with several factors operating simultaneously or singly and at different spatio-temporal scales to determine the outcome of the process. WPE in savanna ecosystems depends on the interactions of factors that influence tree invasion, especially browsers, grazers, fires, rainfall and global drivers especially increasing atmospheric carbon dioxide levels, among others (Scholes and Archer 1997; O’Conor et al. 2014; Ward et al. 2014). Fowler and Beckage (2019) recognized the importance of fire elimination to WPE. They pointed that the savanna ecosystems of North America were the product of burning and when fire is eliminated, they are progressively invaded by woody plants.

Rainfall also influences WPE and the process is usually initiated and is more pronounced in wetter years (Wiegand et al. 2005). Kulmatiski and Beard (2013) observed that increasing precipitation intensity, without increasing the total amount of precipitation, would have the effect of triggering WPE by enhancing rain water percolation deeper into the soil and this would enhance the growth and multiplication of woody perennials.

Rainfall variability is central to the patch dynamics hypothesis proposed by Wiegand et al. (2005, 2006). According to this theoretical proposition, the savanna landscape is patchy, consisting of several communities of no more than a few hectares, that are in various stages of transition between grassy patches and those that are predominantly woody. WPE is a stage in the cyclical succession between open savanna and heavily wooded savanna (Wiegand et al. 2006). The transformation to densely wooded savanna usually occurs when there is sufficient rainfall to permit the germination and establishment of potential encroaching woody species. Ultimately, the woody- plant- encroached area would revert to an open savanna as a result of inter-tree competition and thinning when the trees become very numerous.

Soil and substrate conditions are also important determinants of WPE into savanna and grassland ecosystems. In the Kalahari of Botswana, the proportion of woody plants is negatively correlated with soil clay content (Kgosikoma and Mogotsi 2013). If the substrate consists of un-weathered rocks, it is difficult for the seeds of trees and shrubs to germinate and become established as adult plants. After more than 10 years of fire protection of savanna in Calabozo in the Llanos of Venezuela, there was no significant invasion of woody plants in areas characterized by un-weathered rocks (San Jose and Farinas 1983).

Humans are key players in the process of WPE. In North American savannas, fire suppression by humans is of pivotal importance to WPE (Fowler and Beckage 2019). Also, the role of humans in raising cattle in large numbers, especially near watering points such as boreholes, thereby triggering WPE in southern Africa, has already been referred to.

Finally, WPE has been attributed to the build-up of carbon dioxide in the atmosphere (Myers-Smith et al. 2011; Archer et al. 2017). Unlike grasses which are mainly C4 plants, woody encroachers are predominantly C3 plants that are able to take advantage of increased atmospheric carbon dioxide levels to grow and proliferate (Archer et al. 2017). The experimental study of Kgope et al. (2010) showed that two tree species (Vachellia (syn.Acacia) karroo and V. nilotica) responded vigorously to increasing atmospheric carbon dioxide levels and grew rapidly while a C4 grass (Themeda triandra) showed very little response.

Is woody plant encroachment bad?

The notion that WPE is bad probably originated from the temperate grasslands (especially the prairie ecosystems of North America) which are dominated by grasses and are used mainly for livestock and commercial crop production. As with the European Union, browse from wild trees and shrubs are hardly used to feed livestock in Canada and USA. In Africa, in contrast, large-scale commercial livestock farming and ranching are restricted to a few countries such as Botswana and South Africa. The vast majority of African livestock farmers are traditional pastoralists who depend on grasses in savanna and grasslands, and also on browse from wild plants to feed their livestock. Hence, WPE which impacts commercial farmers adversely as a result of reduced availability of grasses, would usually have a salutary and more positive effect on small-scale farmers and herders in Africa. As Moleele (1998) has pointed out, WPE in savanna in Botswana resulted in an increase in browse for cattle. In fact, in savannas of South and East Africa, invading trees such as Vachellia (syn. Acacia) tortilis do not only provide additional feed for livestock but also food for the farmer and his household (Winrock International 1991). In Ethiopia, the tree provides a safety net for rural dwellers in times of drought and crop failure as the wood is either sold as fuelwood or charcoal to earn income (Milkias and Toru 2018). WPE has more severe negative impacts on commercial farmers and ranchers than small-scale farmers and pastoralists. Wigley et al. (2009) interviewed people in KwaZulu Natal, South Africa, regarding their perception of WPE and compared the responses of communal and commercial land users. The commercial land users observed that the phenomenon had a negative impact on them due to a decrease in grass cover and the invasion of pasture by alien species. In contrast, the communal land users did not consider WPE to be a problem, citing its benefits, such as increased supplies of fuelwood, poles for building and increased availability of browse for feeding livestock.

Those who claim that WPE is bad (e.g. van Vegten 1984; Wiegand et al. 2005; Gil-Romera et al. 2010; Ayalew and Mulualem 2018) argued that the phenomenon leads to a substantial reduction in grass biomass and cover and hence, a considerable decline in herbage for livestock. This would in turn lead to loss of means of livelihood for ranchers and pastoralists. Grass cover does not always decline as a result of WPE. The data of Eldridge et al. (2011), who summarized the results of 53 studies from Spain and Australia, indicated that in 69.8% of the studies, grass cover declined, while in 20.8% and 9.4% of the studies, grass cover remained unchanged and increased respectively. Also, the concentrations of soil organic carbon, total nitrogen, exchangeable calcium and available nitrogen significantly increased in the soil, following the transformation of grasslands into shrubland as a result of WPE (Eldridge et al. 2011). Several empirical studies indicated that the encroachment of trees and shrubs into savannas and grasslands increased soil organic matter and herbaceous productivity underneath tree canopies more than in the open savanna/grassland. The study of Belsky et al. (1989) in the Tsavo National Park, Kenya, indicated that the herbaceous layer above-ground net primary productivity under the canopies of the trees Vachellia (syn. Acacia) tortilis (an encroaching tree species) and the baobab (Adansonia digitata) were twice the amounts recorded in the open savanna. Grasses and other herbaceous plants grow better underneath the crowns of isolated trees and shrubs because the woody perennials accumulate organic matter and nutrients in the soil underneath their crowns (Belsky 1994). Weltzin and Coughenour (1990) observed that herbage production under the canopies of Vachellia (syn. Acacia) tortilis trees in savanna vegetation in Turkana district in western Kenya was more than twice the level in the interspaces between trees. They pointed out that the reduced temperatures and water stress and the enhanced nutrient levels under the tree canopies were the main factors accounting for the much higher herbage production underneath the trees. Also, in the central Rift valley of Ethiopia, herb biomass and cover as well soil nutrients were higher under the canopies of Vachellia (syn. Acacia) tortilis trees than in the open savanna (Yadeta et al. 2018). Soil organic matter and nutrients were also higher in the topsoil under the encroaching tree, Peltophorum africanum, than in the open grassland in south-eastern Botswana (Aweto and Dikinya 2003). In the Rio Grande Plains of Texas, USA, the study of Liao et al. (2006) showed that soil carbon and nitrogen stocks in the soil increased following the inception of WPE. The concentrations of carbon and total nitrogen in the 0–15 cm soil layer under shrubs were 100–500% greater than in the open grassland. Also in north central Texas, following 68 years of the development of the invading tree, Prosopis glandulosa, aboveground biomass increased substantially, but there was no corresponding increase in the concentrations of carbon and nitrogen in the 0–10 cm soil layer (Hughes et al. 2006). The study of Cable et al. (2009) in a mesquite-encroached semi-arid grassland in Arizona, southwestern USA also showed that the 0–50 cm layer of soil under large mesquite (Prosopis. velutina) shrubs had carbon and microbial biomass that were 1.9–2.6 times greater than in the open grassland.

Another negative impact attributed to WPE is that it reduces the quantity of water available for human and plant use, as a result of increased transpiration and interception of rainwater by tree and shrub canopies. Starks and Moriasi (2017) conducted a modelling study of the impact of eastern redcedar (Juniperus virginiana) encroachment into grasslands of the central reach of the North Canadian River Basin in Central Oklahoma, USA, and concluded that redcedar encroachment will greatly reduce river discharge and water supply downstream. The effects of woody plants on soil moisture and groundwater vary with species and their functional traits and topo-edaphic conditions. Acharya et al. (2018) conducted a review of the impacts of WPE/woody plant removal on groundwater in different countries including USA, Brazil, Argentina and South Africa. In 80% of the fifteen studies they reviewed, WPE had negative impact on groundwater while in the remaining 20%, it had no effects or beneficial effects on groundwater resources. It is, therefore, not in all circumstances that WPE reduces river discharge and groundwater recharge. Leite et al. (2020) reported that WPE significantly increased soil infiltrability and percolation in a semiarid karst savanna in Texas, USA. This implies that the phenomenon enhanced groundwater recharge and availability in a semiarid karst environment. Aldworth et al. (2023) examined the effects of encroaching mopane trees (Colophospermum mopane) on evapotranspiration in a semi-arid savanna in Mthimkulu Game Reserve, which is a part of the Kruger National Park in South Africa. Their experiment involved comparing the rates of evapotranspiration in a savanna plot from which mopane trees have been removed with a control plot with the trees intact, for the period 2019 to 2022. Their results indicated that the removal of mopane trees had little effect on evapotranspiration for two years which were relatively dry. However, for the wettest year, evapotranspiration decreased by 12% in the plot without mopane trees and this suggests that the mopane trees have the potential of increasing evapotranspiration.

WPE has also been reported to cause a decline in biodiversity (Soto-Shoender et al. 2018; Cuellar-Soto et al. 2020; Pinheiro et al. 2022). The phenomenon causes the spatial heterogeneity of grasslands to increase considerably as the density of woody plants increases and this may lead to an increase in species diversity, especially in tropical and subtropical areas that are rich in species (Archer et al. 2017). Andersen and Steidl (2019) observed that the species richness of breeding birds in grasslands in southeastern Arizona, USA, increased dramatically, before attaining a peak, as the cover of woody plants increased.

Undoubtedly, WPE has negative impacts on crop or animal farmers whose farms and ranches are turned into grasslands/savanna dotted with islands of woody plants or shrublands or even close-canopy woodland, as has occurred in several countries including Brazil (da Silva et al. 2016; Passos et al.2018), Argentina (Rauber et al. 2020; Vogel et al. 2022), southern USA (Archer et al. 1988), Australia (Eldridge and Ding 2021) and South Africa (Mogashoa et al. 2021). These adverse effects, however, pale into insignificance when compared with the surpassing benefit of carbon sequestration in soil and vegetation, as a result of WPE, that helps to mitigate global warming and climate change in the biosphere.

Benefits of woody plant encroachment

WPE is beneficial to rural communities and the entire human race. However, the benefits are usually overlooked as a result of the pervading perception that the phenomenon is bad and is a major threat to livelihood of ranchers and farmers (Van Vegten 1984; Ayalew and Mulualem 2018). The shrubs and trees that encroach into commercial ranches and farms are regarded by the farmers as weeds and a nuisance because they reduce grass cover and are not suitable for feeding livestock (Van Auken 2000; Auken 2009). This belies the fact that many encroaching trees and shrubs are useful and WPE provides several benefits for rural communities and humankind in general. The benefits of WPE largely derive from the usefulness of the encroaching shrubs and trees and the ecosystem services they provide.

It is important to say, right from the outset, that what is considered a benefit of WPE in one part of the world may not be considered beneficial in another part of the world. Trees and shrubs are a major resource used for heating and cooking in developing countries, unlike in the economically developed countries. Hence, WPE is beneficial to people in developing countries (as it ensures a more abundant supply of fuelwood), unlike in the developed countries. Similarly, the leaves, roots and barks of trees in the wild are commonly utilized as herbal remedies for different diseases in developing countries and are considered more useful and cheaper than orthodox medicine by the people. This is, however, not the case in countries in Europe and North America where orthodox medicine has gained wide acceptance. The benefits of WPE include provision of browse for livestock, provision of medicine and food for humans, reversing the process of desertification, environmental conservation and stabilization of sand dunes in arid and semi-arid regions, and carbon sequestration in soil and vegetation.

Provision of browse

Many trees that invade grassland and savanna ecosystems provide additional browse for livestock but this advantage appears to be limited to Africa and parts of South America and Asia, where the leaves and sometimes the fruits of encroaching trees are used for feeding livestock. The fruits of encroaching shrubs such as Vachellia (syn. Acacia) tortilis provide valuable food for livestock in parts of West, East and southern Africa, especially toward the end of the dry season, when fresh grazing becomes scarce (Winrock International 1991). Other trees and shrubs that are sources of browse for livestock in southern Africa include Vachellia (syn. Acacia) erubescens, V. mellifera, V. erioloba, V. karroo, Dichrostachys cinerea and Grewia flava (Moleele 1998). In the Olifants Drift in south-eastern Botswana, Moleele (1998) reported that browse from encroaching woody plants, especially from D. cinerea and G. flava accounted for more 48% of cattle diet in March and July when browse was plentiful. However, in September, the proportion of browse from encroaching woody plants in the diet of cattle declined to 21.4%. Wilson (1977) fed the dried leaves of four shrubs and four trees, that grew in New South Wales, Australia, to sheep and goats in order to determine their digestibility. The ruminants voluntarily ate the leaves of the trees and shrubs and this presumably indicates that the leaves of wild plants can provide suitable browse for livestock, subject to the proviso that they are not poisonous. It may well be that the leaves and fruits of encroaching woody plants can provide browse for livestock as they do for wild animals.

Provision of food and medicine

Humans have depended on the fruits of wild plants from time immemorial, although, with the development of agriculture, they are now less dependent on food from wild animals and plants. This notwithstanding, humans still depend on food from wild plants to some extent, especially in developing countries. The small tree, Geoffroea decorticans, which is widely distributed in South America, is one of the encroaching woody plants in Argentina (Rauber et al. 2020). The fruits of the wild plant are harvested by local communities for food and for medicinal uses (Fern 2014a). Honey mesquite (Prosopis glandulosa) which is one of the trees encroaching into grasslands in the Central Great Plains of USA (Liu et al. 2013) is a tree with several uses, including the provision of food and medicines and it was particularly important for the sustenance of native desert people (Fern 2014b). The redcedar (Juniperus virginiana) is another important encroaching tree in USA, especially in the Central Great Plains (Starks and Moriasi 2017). It is a multipurpose tree. The young leaves are boiled in water to provide tea for human consumption while the tea made from the fruits and leaves is used as herbal therapy for colds and coughs (Lady Bird Johnson Wildflower Center 2015).

The use of encroaching woody plants for food and medicine assumes considerable importance in Africa. Important encroaching trees and shrubs in Africa that are sources of food and medicine for humans include Vachellia (Acacia) tortilis. The pods and flowers of the shrub, not only provide browse for livestock, but also food for humans, especially in Kenya where they are consumed by the Masai people and people in Turkana area whereas the leaves, barks and seeds are important sources of medicine for local people (Winrock International 1991). Mopane worms, the larvae of the emperor moth (Imbrasia belina) that feed mainly on the leaves of encroaching mopane trees, are an important dietary supplement in southern Africa (Makhado et al. 2014). They are collected by villagers, dried and eaten or sold in markets in Botswana, Zimbabwe and South Africa. Other shrubs and trees that encroach into grasslands and savanna in Africa that are notable sources of medicine include Terminalia sericea, Ziziphus mucronata, Vachellia melifera (syn. Acacia mellifera ), and Peltophorum africanum. T. sericea has gained wide acceptance in traditional medical practice in Africa, especially in southern Africa (Fern 2014c, Mongalo et al.2016, 2020). A number of pharmacological compounds have been extracted from the tree, especially the roots and leaves and they have been shown to exhibit wound-healing, anti-inflammatory, antifungal and antibacterial properties (Fern 2014c) The plant is used for the treatment of various diseases including sexually-transmitted diseases, tuberculosis, diarrhea and skin infections (Mongalo et al. 2016). Ziziphus mucronata is another important tree that is widely used in African traditional medicine (Mokgolodi et al. 2011). Extracts from the plant exhibit strong antioxidant, antiviral and anti-inflammatory activities and herbal products from the plant are used for treating human and animal infections including skin infections, diarrhea and respiratory diseases (Mongalo et al. 2020). The fruits and leaves of the tree are edible and are consumed by both wildlife and humans (Mokgolodi et al. 2011).

Source of lumber and fuelwood

Honey mesquite (Prosopis glandulosa) is an important source of wood for constructional purposes, fencing, fuelwood and production of charcoal (Fern 2014b). The redcedar, referred to in the preceding sub-section, also provides wood for making tables, benches and fences, especially by the early American settlers (Lady Bird Johnson Wildflower Center 2015). In Africa, particularly in East and southern Africa, the wood of Vachellia tortilis is used for making high quality charcoal and plantations of the shrub have been established in India to provide fuelwood (Winrock International 1991). Terminalia. sericea is a source of timber and is used for making furniture. It is also used as poles for building houses and huts. Charcoal and fuelwood are other products obtained from the bole and its branches (Fern 2014c).

Improving soil fertility and sand dune stabilization

Terminalia.sericea helps to stabilize soils, especially sandy soils and to replenish the fertility of impoverished soils (Fern 2014c). Trees and shrubs have the effects of accumulating organic matter and nutrients under their canopies, resulting in the formation of “islands of fertility” around the trunks of isolated trees and shrubs in grasslands and savanna (Vetaas 1992). Encroaching woody plants are no exception. The study of Aweto and Dikinya (2003) in southeastern Botswana has shown that the levels of soil organic matter, total nitrogen, exchangeable calcium, magnesium, potassium and cation exchange capacity in the 0-10 cm layer of soil under the canopy of the encroaching tree, Peltophorum africanum, were 47–106% significantly higher than in the open grassland. The data of Mogashoa et al. (2021) in an encroached savanna in northern South Africa indicated that soil organic carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil increased substantially as the degree of WPE increased. In a semi-arid savanna in central Argentina, Gonzalez-Roglich et al. (2014) observed that soil organic matter increased threefold with increasing tree density along a gradient of grassland to closed forest. Vogel et al. (2022) reported that soil under shrubs, which invaded a subhumid grassland in Patagonia, Argentina, had higher fertility level than soil under grassland. Similarly, Jessup et al. (2003) who compared the soil under woody patches of oak (Quercus virginiana) and juniper (Juniperus ashei) with open grassland in Texas Plateau of Texas, USA (an area that is undergoing WPE) observed that the levels of soil organic matter and total nitrogen were higher under the two tree species than in the open grassland.

Some encroaching woody plants reclaim and stabilize sand and sand dunes. Acacia tortilis (now known as Vachellia tortilis) has been successfully used for stabilizing sand dunes in Somalia, United Arab Emirate and India (Winrock International 1991). In northern China, a leguminous encroaching shrub, Caragana microphylla, has been successfully deployed to stabilize shifting sand and the shrub improved soil fertility through the accretion of organic carbon, total nitrogen, available potassium in the soil (He et al. 2008). Also, in Mongolia, China, there was a significant build-up of soil organic matter and total nitrogen in the soil under shrub patches in a woody-plant- encroached grassland, relative to the levels in the open grasslands (Li et al. 2019).

Potential of reversing desertification

Some woody plants that encroach into grasslands and savannas such as Vachellia tortilis, Vachellia. (syn. Acacia) erioloba and Faidherbia albida in Africa and Caragana microphylla in China can be established on sand and have been successfully deployed to stabilize mobile sand dunes (Centre Technique Forestier Tropical 1989; Winrock International 1991; He et al. 2008; Fern 2014d). Such woody species not only stabilize sand dunes but also have the potential of combating and reversing the process of desertification. This is particularly so, if they establish themselves aggressively and enrich nutrient-deficient sands, thus making it possible for other trees and shrubs to be subsequently established. The ensuing process of succession and the proliferation of woody plants would ultimately counter the process of desertification.

Maestre et al. (2009) observed that shrub encroachment can reverse the process of desertification in semi-arid Mediterranean grasslands. They studied 13 experimental sites in Stipa tenacissima-dominated semi-arid grassland in Spain, which is impoverished and had undergone some degree of desertification. In each site, they delimited two sample plots, one with adult sprouting shrubs and the other without sprouting shrubs and compared them. Their results indicated that the plots with sprouting shrubs had more vascular plants, higher levels of soil organic carbon, nitrogen and potential nitrogen mineralization and greater biomass of fungi and bacteria. They interpreted their results, especially the higher vascular plants richness and the higher level of fertility in the plots with sprouting shrubs to be a step in the reversal of desertification in the semi-arid Mediterranean grasslands. It is important to observe that their study did not prove conclusively that a reversal of desertification has occurred. This will require long-term monitoring of the plots for ten years or more in order to ascertain whether the “desertified” grasslands are becoming noticeably woody.

Carbon sequestration in soil and vegetation

One very important and not-readily appreciated function that WPE performs for humanity and all organisms in the biosphere, is that it stores carbon both in the soil and in the vegetation. Open grasslands and savanna ecosystems also store carbon in the soil and vegetation ( Li et al. 2019; Vogel et al. 2022). Their invasion by trees and shrubs substantially increases the quantity of carbon stored in the soil-vegetation system (Gonzalez-Roglich et al. 2014). The total carbon stocks in the soil-vegetation system for grassland, shrubland, open forest and closed forest in woody-plant-encroached savannas in central Argentina were 4.5, 8.3, 12.4 and 16.4 kg Cm−2 (Gonzalez-Roglich et al. 2014). The amount of carbon stored in the shrubland is nearly twice, while that of the closed forest is more than thrice the total carbon stock of the grassland. Thus, there is a progressive build-up of carbon in the soil-vegetation system as WPE becomes more pronounced. Through the accumulation of carbon in the soil and plant biomass, WPE (which increases tree biomass) reduces atmospheric carbon dioxide and decelerates its build-up, thereby helping to combat the threat of global warming and climate change (Alemu 2014; Fino et al. 2020). Trees and shrubs, being much larger than herbs, substantially enhance the capacity of grasslands to accumulate carbon in the soil and vegetation. The study of Halefom et al. (2020) in Tigray, Ethiopia, has shown that following 15 years of using enclosures to prevent grazing in a semi-arid savanna, carbon stored in the vegetation rose to 29.3 Mg ha−1, compared to 7.2 Mg ha−1 in open grazing plots, in which tree regeneration is largely inhibited, as a result of grazing. Several studies in different continents, which indicated accumulation of organic carbon in soil underneath encroaching trees (relative to levels in soil in open savanna/grassland), have been referred to (e.g. Jessup et al.2003; Liao et al. 2006; He et al. 2008; Li et al. 2019; Mogashoa et al. 2021) and will not be discussed further here.

Conclusion

WPE has some disadvantages, prominent among which, is that it usually reduces grass cover and biomass (Van Auken 2009; Soto-Shoender et al. 2018). WPE may ultimately result in the transformation of grassland and savanna ecosystems into closed-canopy woodlands, shrublands or even forests (Passos et al. 2018; Vogel et al. 2022). This would result in loss of revenue for farmers, especially commercial farmers and ranchers, who may be forced out of business. In spite of this disadvantage, WPE should not be regarded as patently bad because of the overwhelmingly surpassing advantage of carbon sequestration in the soil and vegetation, that slows down the pace of climate change. When farmers, ranchers and other land users become financially incapacitated. as a result of WPE, they should be compensated as they are bearing the brunt of WPE on behalf of the entire humankind. They should be entitled to payments, under the Clean Development Mechanism of the Kyoto Protocol, for carbon sequestered in the vegetation and soil in their farms which are transformed into shrubland, close-canopy woodland or forests (Miller 2000; World Bank 2012). The Food and Agriculture Organization of the United Nations could assist such farmers with grants to start other business enterprises or compensate them, after it has been ascertained that their land has been heavily encroached by woody plants. Small- scale farmers and herders of livestock in the developing countries, especially in sub-Saharan Africa, appear more resilient to the problems of WPE. Many encroaching woody plants provide people with food, medicine, fuelwood, wood/poles for building houses or huts and also browse for their livestock. The challenge to livestock scientists in the developed countries is to screen encroaching trees and shrubs for suitability as livestock feed. This is with a view to finding nutritious invading woody plants, with digestible and palatable leaves and fruits, that can be used as browse and supplementary feed for livestock. This will help to mitigate the effects of WPE and turn some of the invading wild plants into sources of browse/feed for livestock.

Also, shade-tolerant grasses which provide good grazing for livestock, such as guinea grass (Panicum maximum), can be grown in pastures and rangelands at the inception of WPE or even after its commencement. The grass provides good grazing for livestock, even when the density of the invading trees and shrubs becomes high. The studies of Mogashoa et al. (2021) and Ward et al. (2022) in South Africa showed that P. maximum grew well in the shade under the canopy of trees in woody plant-encroached grassland and savanna. In Minas Gerais, Brazil, the growth parameters of P. maximum, especially leaf and stem elongation rates and leaf blade length were significantly greater under 58% shade than under 0 and 37% shade (Paciullo et al. 2017). The above findings clearly show that P. maximum is not only shade-tolerant but tends to grow better when the shade or canopy cover of trees is high. Hence, the grass and other shade-tolerant grasses that are palatable to livestock, can be judiciously used for feeding livestock, even in advanced stages of WPE.

With the possible exception of the Republic of South Africa, the pharmaceutical industry is weakly developed in Africa, where the use of traditional herbal medicine is widespread and strongly entrenched. Many of the woody plants that invade grasslands and savannas in Africa such as Ziziphus mucronata, and Terminalia sericea are important sources of medicine (Mokgolodi et al. 2011; Fern 2014c). Mokgolodi et al. (2011) lamented the underutilization of Z. mucronata as a medicinal plant in Africa, while the closely related species of Z. jujuba and Z. mauritiana are cultivated in India and China for medical uses and for food. The phenomenon of WPE presents African scientists and pharmaceutical industry an opportunity and a challenge to develop commercial drugs from the medicinal plants such as Z. mucronata and T. sericea, which nature is proliferating in savanna and grasslands. This observation also applies to all developing countries outside Africa that have a weak technological infrastructure for drug manufacture and are highly dependent on imported drugs.

Finally, it would seem that WPE is a mechanism used by nature to reduce anthropologically induced build-up of carbon dioxide in the atmosphere and delay the impending apocalypse of global warming and climate change. Thus far, global initiatives to substantially reduce greenhouse gases emission, especially carbon dioxide, have failed. Nature appears to be responding to the unparalleled emission of carbon dioxide into the atmosphere, particularly since about the 20th century, by using WPE to enhance the process of carbon sequestration in the soil and biomass of woody vegetation to decelerate the rate of atmospheric carbon dioxide build-up and reduce the rate of progression towards climate change of catastrophic dimension.