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During the past century, the biomass of woody species has increased in many grassland and savanna ecosystems. As many of these species fix nitrogen symbiotically, they may alter not only soil nitrogen (N) conditions but also those of phosphorus (P). We studied the N-fixing shrub Dichrostachys cinerea in a mesic savanna in Zambia, quantifying its ef...
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... we present a study of the effects of woody encroachment on soil nutrient availabilities and carbon sequestration, as well as potential feedbacks upon encroaching shrubs. The work was conducted in a mesic savanna habitat in Zambia that was encroached to varying degrees by the leguminous thorny shrub Dichrostachys cine- rea (Fig. 1). This species is widely distributed in subtropical and tropical Africa and can be found in Arabia, tropical Asia, America, and Australia (PROTEA 2014). It is an important woody encroacher in many African savannas (Lock 1993;Roques et al. 2001;Moleele et al. 2002;Tobler et al. 2003;Hagenah et al. 2009;Eldridge et al. 2011). In the Kafue ...
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... The low amounts of soil C could be attributed to grazing and burning as these are major controls of carbon input quantity and quality in grasslands [82]. The soil carbon may initially increase with woody plant density, but then declines when woody plant density becomes high, which also inhibits understory grass growth [76,83]. This suggests that most soil C is derived from herbaceous plants such as grasses, which declines in abundance with increased density of woody plants. ...
Increasing densities of woody plants, known as woody plant encroachment, is a phenomenon affecting savannas and grasslands in many parts of the world. Yet, these ecosystems sustain a significant proportion of the human population through the provision of ecosystem services, such as forage for livestock and wildlife production. While low to medium altitude rangelands are encroached by many species of woody plants, high altitude rangelands in southern Africa show increasing densities of Leucosidea sericea, a woody shrub or small to medium-sized tree. Influences of this species on rangeland dynamics are unknown. This study aimed to determine the influence of L. sericea on rangeland functioning in the Vuvu communal area in the Eastern Cape, South Africa. Effects of L. sericea on plant species diversity and composition, rangeland condition and grazing capacity were measured in sites of variable densities of the species in topographical locations designated as plains, upland and stream sites, using a point-to-tuft method along 50-m long transects. Soil samples were collected to a depth of 5 cm from plains, streams, and upland sites, and analysed for organic carbon, nitrogen, phosphorus, magnesium, calcium, and pH. Plant species richness and abundance were similar among topographical locations, which was reflected by the similar Shannon-Weiner (H′) diversity indices among sites. Topographical locations differed significantly in species composition. The plains sites had a higher grazing capacity than stream sites, which had a grazing capacity similar to that of upland sites. Values of soil physicochemical properties were similar among the sites. Overall, soils were acidic (range in pH: 4.4–4.6) and had low amounts of organic carbon and total nitrogen. These findings suggest that L. sericea is not the primary cause of rangeland degradation as all sites were in poor condition as shown by the low grazing capacity, poor rangeland condition and depauperate species richness and diversity. Therefore, rangeland management should shift towards restoration strategies aimed to revitalise the rangeland.
... One intriguing observation is that nutrient availability is typically not a limiting factor for the enhanced productivity of woody plants in grasslands. Some woody plant species, such as honey mesquite, possess the capacity to acquire nitrogen through symbiotic fixation and are able to obtain nitrogen and phosphorus from deeper soil layers, which are inaccessible to grass (Blaser et al., 2014). This active cycling of nitrogen and phosphorus in encroached grasslands can contribute to enlarging the nutrient pool in soil (Zhou et al., 2018). ...
Woody Plant Encroachment (WPE) is a key driver of grassland collapse in the Southern Great Plain (SGP), resulting in a series of adverse ecological and socioeconomic consequences. Climate change will interact with ongoing WPE as it will likely shift the potential ranges of WPE species. In this study, we employed an ensemble approach integrating results from multiple Species Distribution Models to project future distribution ranges of four major WPE species (Ashe juniper, honey mesquite, post oak, and eastern redcedar) in the SGP across the 21st century. The findings highlighted a noteworthy trend: under future climate conditions, the distribution ranges for these WPE species were projected to shift northward and eastward. Of particular concern is honey mesquite with significant expansion in distribution range, potentially covering up to two‐thirds of the SGP's non‐agricultural area by the end of the 21st century. Conversely, the other three WPE species were expected to experience a contraction in their distribution ranges. Ashe juniper may experience a decline in its current habitats in central Texas but gain new habitats in northern Texas, Oklahoma, and Kansas. The suitable ranges of post oak and eastern redcedar were projected to shrink eastward, primarily being restricted to eastern portions of Oklahoma and Texas under the RCP4.5 and a smaller area in eastern Oklahoma under the RCP8.5. The projected shift in WPE ranges provides a scientific basis for governments to optimize the allocation of management resources and implement timely practices to control the spread of woody plants during the early encroachment stage. Our study methodology is applicable to other regions and continents with WPE issues, including Africa, South America, and Australia.
... Ecological studies often rely on chronosequences (i.e., space for time substitutions) to investigate succession of plant communities, diversity, primary production, soil nutrients, soil microbial activity, and fungal communities over long timeframes (Springsteen et al. 2010;Hollingsworth et al. 2010;Blaser et al. 2014;Cava et al. 2018;Yang et al. 2019;Li et al. 2020). Chronosequences enable long-term studies in a more compressed time scale. ...
Drivers of shrub primary production and associated landscape impacts of encroachment are well known in drylands but have not been thoroughly studied in mesic and coastal habitats. The native, nitrogen-fixing shrub, Morella cerifera, has expanded into coastal grassland along the US Atlantic coast due to warming temperatures, but impacts on ecosystem function are not well known. Annual net primary production (ANPP) of Morella cerifera and key environmental drivers were measured long-term (1990 – 2007) across a chronosequence of shrub age on a mid-Atlantic barrier island. Soil and groundwater nutrients were compared with un-encroached grassland soil to evaluate impacts of vegetation on nutrient dynamics. Shrub ANPP declined with age at the same rate among all thickets, but there was variability from year to year. When climate variables were included in models, shrub age, precipitation, and freshwater table depth were consistent predictors of ANPP. Water table depth decreased over time, reducing ANPP. This may be due to rising sea-level, as well as to feedbacks with shrub age and evapotranspiration. Soil N and C increased with shrub age and were higher than adjacent grassland sites; however, there was a significant loss of N and C to groundwater. Our results demonstrate that drivers influencing the encroachment of shrubs in this coastal system (i.e., warming temperature) are not as important in predicting shrub primary production. Rather, interactions between shrub age and hydrological properties impact ANPP, contributing to coastal carbon storage.
... Different letters represent significant differences between vegetation types (p < 0.05) within specific geological formations. al., 2017; Barger et al., 2011;Blaser et al., 2014;Eldridge et al., 2011;Zhou et al., 2017Zhou et al., , 2018bZhou et al., , 2021. ...
Woody-plant encroachment into grasslands and savannas has been globally widespread during the past century, likely driven by interactions between grazing, fire suppression, rising atmospheric CO2, and climate change. In the southernmost US Great Plains, Ashe juniper and live oak have increased in abundance. To evaluate potential interactions between this vegetation change and the underlying soil parent material on ecosystem biogeochemistry, we quantified soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and δ13C of SOC in soils obtained from trenches passing through grassland, juniper, and oak patches on soils lying atop the respective Edwards and Buda limestone formations in central Texas. Soils on the Edwards formation are more shallow and have more rock outcropping than those on Buda. The δ13C values of SOC under grasslands was -19 ‰, whereas those under woody patches were -21 ‰ to -24 ‰, indicating that wooded areas were relatively recent components of the landscape. Compared with grasslands, areas now dominated by juniper or oak had elevated SOC, TN, and TP storage in soils lying atop Edwards limestone. In Buda soils, only oak patches had increased SOC, TN, and TP storage compared with grasslands. Woody encroachment effects on soil nutrients were higher in soils on the Edwards formation, perhaps because root and litter inputs were more concentrated in the relatively shallow layer of soil atop the Edwards bedrock. Our findings suggest that geological factors should be considered when predicting nutrient store responses in savannas following vegetation change. Given that woody encroachment is occurring globally, our results have important implications for the management and conservation of these ecosystems. The potential interactive effects between vegetation change and soil parent material on C, N, and P storage warrant attention in future studies aimed at understanding and modeling the global consequences of woody encroachment.
... Woody encroachment often alters above-and belowground primary productivity as well as soil C, N, and P storage and cycling rates (Barger et al., 2011;Blaser et al., 2014;Boutton et al., 2009;Jackson et al., 2002;Jackson et al., 2000;Li et al., 2016;O'Donnell and Caylor, 2012;Saintilan and Rogers, 2015;Zhou et al., 2018). As both increases and decreases in soil organic C and total N have been observed in response to woody encroachment (Barger et al., 2011;Eldridge et al., 2011;Jackson et al., 2002;Eldridge et al., 2016;Li et al., 2016;Mureva et al., 2018;Zhou et al., 2023), this widespread land cover change represents an uncertainty in regional and global C cycling models (Asner et al., 2004;Barger et al., 2011;Goodale and Davidson, 2002;Pacala et al., 2001) that needs to be better constrained to improve the accuracy of future carbon budget projections and climate models. ...
Semi-arid grasslands and savannas in the southern Great Plains USA are extensively used for livestock grazing. Over the past century, Juniperus (juniper) and Quercus (oak) species abundance have increased due to intensive grazing and reduced fire frequency. We investigated the interactions between livestock grazing history (none, moderate, heavy) and vegetation cover (grassland, juniper, oak) using a ∼ 70-year grazing experiment in west-central Texas. We explored effects on soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), microbial community composition, and function. SOC and TN were 50–150 % higher under juniper and oak compared to grasslands, and 10–30 % lower in grazed vs. ungrazed areas. Vegetation × grazing interaction showed greater SOC and TN loss under oak than juniper or grasslands. Ungrazed controls had higher soil TP than grazed areas, with oak and juniper soils having more TP than grasslands. Bacterial and fungal communities differed between grassland and woody vegetation. Grazing affected only bacterial communities. SOC, TN, TP accounted for differences in community structure. Abundances of genes related to methane, nitrogen, sulfur metabolisms, and dominant fungal trophic modes were linked to soil C, N, P ratios. These findings highlight how long-term livestock grazing and woody plant encroachment influence soil C, N, P cycles, altering soil microbial community structure and function. This study provides insights for savanna ecosystem management and integrating land cover effects into biogeochemical models for global change scenarios.
... WPE can be a significant pathway for increased ecosystem C sequestration (Bastin et al., 2019;Blaser et al., 2014;Cook-Patton et al., 2021;Geesing et al., 2000;Griscom et al., 2017;Hibbard et al., 2001;Hudak et al., 2003); however, these effects depend on climatic conditions and plant rooting profiles (Jackson et al., 2002). ...
Woody plant encroachment (WPE) is a global trend that occurs in many biomes, including savannas, and accelerates with fire suppression. Since WPE can result in increased storage of soil organic carbon (SOC), fire management, which may include fire suppression, can improve ecosystem carbon (C) sequestration in savannas.
At our study site in Kruger National Park, South Africa, we used a long‐term (~70 year) fire experiment to study the drivers and consequences of changes in woody cover (trees and shrubs) on SOC sequestration. We surveyed four fire manipulation treatments, replicated at eight locations within the park: annual high‐intensity burns, triennial high (dry season) and low‐intensity (wet season) burns, and fire exclusion, to capture the range of fire management scenarios under consideration. The changes in woody cover were calculated over a period similar to the experiment's duration (~80 years) using aerial photographs (1944–2018). Soils were analysed to 30 cm depth for SOC and δ¹³C, under and away from the tree canopy to isolate local‐ and landscape‐level effects of WPE on SOC.
The largest increases in woody cover occurred with fire exclusion. We found that plots with higher increases in woody cover also had higher SOC. However, trees were not the only contributor to SOC gains, sustained high inputs of C4‐derived C (grasses), even under canopies in fire suppression plots, contributed significantly to SOC. We observed little difference in SOC sequestration between cooler triennial (wet season) burns and fire suppression.
Synthesis. Grass input to soil organic carbon (SOC) remained high across the full range of woody cover created by varying burning regimes. The total SOC stocks stored from tree input only matched grass‐derived SOC stocks after almost 70 years of fire exclusion. Our results point to C4 grasses as a resilient contributor to SOC under altered fire regimes and further challenge the assumption that increasing tree cover, either through afforestation schemes or fire suppression, will result in large gains in C sequestration in savanna soils, even after 70 years.
... Woody plant encroachment into savanna ecosystems is a geographically widespread phenomenon that appears to be driven by fire suppression, livestock grazing, climate change, and rising atmospheric CO 2 concentration (Archer et al., 2017;Boutton et al., 1998;Buitenwerf et al., 2012;Sankaran et al., 2005;Stevens et al., 2017). Woody encroachment generally increases soil organic C, nitrogen (N), and P storage in soils in arid/semi-arid regions (Blaser et al., 2014;Eldridge et al., 2011;Krull et al., 2005;Sitters et al., 2013), which likely increase the source of colloidal OC and P (Krause et al., 2020;Zhang et al., 2021). Furthermore, other soil properties may be altered following woody encroachment, such as porosity (Yu et al., 2018) and hydrologic characteristics (Holdo et al., 2020). ...
... Accelerated sea level rise can lead to intramarsh range expansion of burrowing crabs (Luk and Zajac, 2013), which can further exacerbate the demise of the floodingsensitive, long-lived species such as I. frutescens. The soil carbon storage potential of woody shrubs typically exceeds that of grasses (Blaser et al., 2014), so declines in I. frutescens, along with reduced vegetation cover overall, indicate that high densities of burrowing crabs can negatively affect the carbon sequestration value of salt marsh shrub zones. In a California marsh, Beheshti et al. (2021) excluded burrowing crabs for five years and similarly found increases in above and belowground biomass of a long lived, woody perennial species. ...
Emergent vegetation in coastal marshes across the Northeastern United States have been declining and changing in relative species composition due to abiotic factors such as accelerated sea level rise, storm intensity, excess nutrients, and other anthropogenic disturbances. A possible biotic factor in marsh decline is increased perturbation from burrowing crabs. Uca pugilator and Minuca pugnax (fiddler crabs) whose populations are increasing in many marshes in the eastern US, likely due to disruptions in food webs and expanding habitat from sea level rise. High densities of crab burrows contribute to creek bank erosion and possibly in other factors of marsh peat degradation. When burrowing crabs were excluded from large scale (9 m2) plots in a Cape Cod, MA, marsh for five months, vegetation species richness increased significantly, as did abundance of woody shrubs and the forb, Suaeda maritima (L.). Conversely, burrowing crab exclusion resulted in less Spartina alterniflora (Loisel) and less bare ground - indicating that the other types of vegetation were able to occupy more space in the absence of crab burrowing and sediment sorting activity. The experimental enclosure results were compared with vegetation and burrow density monitoring data taken from nearby long-term monitoring plots. Of the sixteen analyses, we found only one similar result between the 5 month exclusions and monitoring plots; high burrow densities were associated with higher density of S. alterniflora. In contrast to the exclusions, high burrow densities were positively correlated with abundance of Iva frutescens and Suaeda maritima. Six other species (or groupings of rare plants) were negatively associated with burrow densities, four of these are sensitive to inundation and are considered characteristic of the high marsh zone. Different interpretations of the effects of burrowing crabs will result depending on whether correlations are observed from monitoring plots or longer term, large-scale exclusions are conducted; both are needed to assess the rapid vegetation and faunal shifts that are occurring in temperate salt marshes. We suggest that Uca pugilator and Minuca sp. will benefit from sea level rise induced habitat expansion at the expense of the abundance and species richness of flooding-sensitive plants. The erosion and vegetation disturbance that high burrowing crab densities lead to will further exacerbate the decline of these species that are receding at the seaward edge of their distributions.
... Similar studies in other portions of the southern Great Plains and the western USA have also shown that juniper and/or oak encroachment into grasslands resulted in higher soil nutrient storage (Fernandez et al., 2013;Jessup et al., 2003;McKinley and Blair, 2008;Shawver et al., 2018). Furthermore, our results are broadly consistent with prior studies around the world showing that tree/shrub encroachment into grasslands, savannas, deserts, and other arid/semiarid ecosystems generally results 315 in increased concentrations and pools sizes of soil C, N, P, and other essential elements (Archer et al., 2017;Barger et al., 2011;Blaser et al., 2014;Eldridge et al., 2011;Zhou et al., 2017Zhou et al., , 2018bZhou et al., , 2021. ...
Woody plant encroachment into grasslands and savannas has been globally widespread during the past century, likely driven by interactions between grazing, fire suppression, rising atmospheric CO2, and climate change. In the southernmost U.S. Great Plains, Ashe juniper and live oak have increased in abundance. To evaluate potential interactions between this vegetation change and the underlying soil parent material on ecosystem biogeochemistry, we quantified soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and δ13C of SOC in soils obtained from trenches passing through grassland, juniper, and oak patches on soils lying atop Edwards vs. Buda limestone formations in central Texas. Soils on the Edwards formation are more shallow and have more rock outcropping than those on Buda. The δ13C of SOC under grasslands was -19 ‰, while those under woody patches were -21 to -24 ‰, indicating wooded areas were relatively recent components of the landscape. Compared to grasslands, areas now dominated by juniper or oak had elevated SOC, TN, and TP storage in soils lying atop Edwards limestone. In Buda soils, only oak patches had increased SOC, TN, and TP storage compared to grasslands. Woody encroachment effects on soil nutrients were higher in soils on the Edwards formation, perhaps because root and litter inputs were more concentrated in the relatively shallow layer of soil atop the Edwards bedrock. Our findings suggest geological factors should be considered in predicting responses of nutrient stores in savannas following vegetation change. Given that woody encroachment is occurring globally, our results have important implications for the management and conservation of these ecosystems. The potential interactive effects between vegetation change and soil parent material on C, N, and P storage warrant attention in future studies aimed at understanding and modeling the global consequences of woody encroachment.
... Due to the combined effect of grazing and tree cutting, these types of changes in vegetation formation have increased since the beginning of the 20th century (Archer, 2009;Pacala et al., 2001;Knapp et al., 2008). Recent changes in the dynamics of the vegetation in sub-Saharan Africa are being caused by the increased dominance of shrub-like plants over grass and tall tree cover (Blaser et al., 2014). Shrub encroachment threatens 13 million hectares in sub-Saharan Africa, and the loss of savannah systems is thought to have an impact on more than two billion people globally (Adeel, 2008;Archer, 2009). ...
Land Change Science (LCS), as a coupled human-environment system, is a multidisciplinary area that explores the dynamics of land use and land cover to understand key theories, problems, methodologies, and model applications. The present review integrated the research that has been conducted in the savanna ecosystem of Sub-Saharan Africa from a geographical perspective. The present study elaborates contemporary issues and thoughts in terms of several key aspects: (1) the Impacts of protected areas on the surrounding natural environment, wildlife, and socioeconomic activities of humans, (2) With the impact of new developments in remote sensing (RS) technology, observations of land change and the changes in the relationship between ecology and RS, (3) The effects of developments in RS on our environmental perspective and new connection opportunities for interrelated scientific disciplines, (4) The vulnerability of the savanna vegetation due to its multilayered and complex structure. Due to the changing climatic conditions, it is inevitable that ecosystems will encounter various problems in the near future, especially in Sub-Saharan Africa. Understanding the complex savanna ecosystem remains a challenge for researchers. Therefore, it is very essential to observe better and understand the nature and socio-economic cycle of humans for a sustainable future of savanna ecosystems.
