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Woody encroachment is a widespread and acute phenomenon affecting grasslands and savannas worldwide. We performed a meta-analysis of 29 studies from 13 different grassland/savanna communities in North America to determine the consequences of woody encroachment on plant species richness. In all 13 communities, species richness declined with woody plant encroachment (average decline = 45%). Species richness declined more in communities with higher precipitation ( r 2 = 0.81) and where encroachment was associated with a greater change in annual net primary productivity (ANPP; r 2 = 0.69). Based on the strong positive correlation between precipitation and ANPP following encroachment ( r 2 = 0.87), we hypothesize that these relationships occur because water-limited woody plants experience a greater physiological and demographic release as precipitation increases. The observed relationship between species richness and ANPP provides support for the theoretical expectation that a trade-off occurs between richness and productivity in herbaceous communities. We conclude that woody plant encroachment leads to significant declines in species richness in North American grassland/savanna communities.

Ecosystems with alternative attractors are susceptible to abrupt regime shifts that are often difficult to predict and reverse. In this study, we quantify multiple system dynamics to determine whether the transition of mesic grassland to shrubland, a widespread phenomenon, represents a linear reversible process, a nonlinear but reversible threshold process, or a transition between alternative attractors that is nonlinear and prone to hysteresis. Using a 28-yr data set with annual resolution and extensive spatial replication, we found that shrub cover is correlated with distinct thresholds of fire and C 4 grass cover, resulting in temporal bimodality of shrub cover and abrupt shifts of shrub cover despite gradual changes in grass cover. These abrupt increases in shrub cover are the most rapid ever reported in grasslands, and illustrate internal thresholds that separate grasslands and shrublands. Nonlinear transitions from low to high shrub cover were also closely associated with positive feedback mechanisms that alter fire and competition ( r 2 = 0.65), suggesting that grasslands and shrublands could show hysteresis, and by definition exist as alternative attractors. Thus, the response of this ecosystem to anthropogenic activity should tend to be rapid, nonlinear, and perhaps difficult to reverse. Regime shifts in this mesic grassland were predictable: we found that grassland and shrubland attractors were differentiated by critical thresholds of ∼50-70% grass cover, 5-10% shrub cover, and a fire return interval of ∼3 yr. These thresholds may provide adaptive potential for managing nonlinear behavior in socio-ecological systems in a changing environment.

Summary Frequent fires are often proposed as a way of preventing woody encroachment in savannas, yet few studies have examined whether high‐intensity fires can effectively reverse woody encroachment. We applied successive fire treatments to examine the effect of fire intensity on woody vegetation structure. The treatments included early dry season, low‐intensity fires; late dry season, higher‐intensity fires; and an unburnt control. We used pre‐ and post‐fire airborne LiDAR to compare vegetation structural changes brought about by fires of different intensity. Early dry season fires were of lower intensity (1400–2100 kW m−1) than late dry season fires (2500–4300 kW m−1). The two treatments also differed in terms of fuel consumed, scorch heights and char heights, indicating that clear differences in fire intensity and severity were achieved. After 4 years and two fire applications, relative woody cover increased by between 20 and 110% in different height categories following low‐intensity and control treatments and declined by between 3 and 70% following high‐intensity fire treatments. Declines were markedly higher following two repeated high‐intensity fires than following a high and then a moderate‐intensity fire. Because woody shrubs in lower height classes can recover rapidly, repeated high‐intensity fires would be needed to maintain lower cover. Tall trees are often assumed to be unaffected by fires. However, we found that the rate of tree loss was directly related to fire intensity, where 36% of trees were lost following repeated high‐intensity fires, compared to 22% after a high‐ and then a moderate‐intensity fire and 6% after two low‐intensity fires (3% without fire). Synthesis and applications. Using LiDAR data we show that high‐intensity fires can, at least in the short term, significantly reduce woody cover in South African savannas. The use of repeated high‐intensity fires simultaneously causes both a positive (reduction in cover of short shrubs) and a negative (loss of tall trees) outcome, and managers need to make trade‐offs when contemplating the use of fire intensity to achieve specific goals. One potential solution may be to repeatedly apply high‐intensity treatments to some areas, and not to others. This could generate a heterogeneous landscape where grasses become dominant and tall trees become scarce in some places, but in others, tall trees persist (or at least decline at slower rates), and shorter woody shrubs increase in dominance. Whether this would be acceptable, or practical, remains to be tested. Using LiDAR data we show that high‐intensity fires can, at least in the short term, significantly reduce woody cover in South African savannas. The use of repeated high‐intensity fires simultaneously causes both a positive (reduction in cover of short shrubs) and a negative (loss of tall trees) outcome, and managers need to make trade‐offs when contemplating the use of fire intensity to achieve specific goals. One potential solution may be to repeatedly apply high‐intensity treatments to some areas, and not to others. This could generate a heterogeneous landscape where grasses become dominant and tall trees become scarce in some places, but in others, tall trees persist (or at least decline at slower rates), and shorter woody shrubs increase in dominance. Whether this would be acceptable, or practical, remains to be tested.

This review synthesizes evidence that altered fire frequency drives discontinuous ecosystem transitions from mesic grasslands to shrublands or woodlands in the Central Great Plains, USA. Long‐term fire manipulations reveal that grassland to shrubland transitions are triggered when fire‐free intervals increase from 1–3 years to ≥ 3–8 years, and longer fire returns (˜10 years or more) result in transitions to woodlands. Grazing and soil properties alter these fire thresholds. Grassland to shrubland transitions are abrupt and exhibit nonlinear relationships between driver and state variables. Transitions to shrublands and woodlands exhibit hysteresis, where reintroducing frequent fires does not reverse transitions in management‐relevant time‐scales (decades). Nonlinear transitions and hysteresis emerge because grasses generate positive feedbacks with fire that create strong demographic barriers for shrub and tree establishment. Fire‐free intervals allow shrubs and trees to reach a size sufficient to survive fire, reproduce and disrupt the fire feedback loop through competition. Synthesis. Mesic grasslands, shrublands and woodlands constitute self‐reinforcing states (alternative attractors) separated by critical fire frequency thresholds. Even without major shifts in climate, altered fire frequency can produce dramatic state changes, highlighting the importance of fire for predicting future ecosystem states. Local management should focus on prevention of unwanted transitions rather than post hoc restoration.

Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help maintain grassy vegetation where the climate is suitable for woodlands or forests. Saplings in savannahs are particularly vulnerable to topkill of above-ground biomass. Larger trees are more fire-resistant and suffer little damage when burnt. Recruitment to large mature tree size classes depends on sapling growth rates to fire-resistant sizes and the time between fires. Carbon dioxide (CO 2 ) can influence the growth rate of juvenile plants, thereby affecting tree recruitment and the conversion of open savannahs to woodlands. Trees have increased in many savannahs throughout the world, whereas some humid savannahs are being invaded by forests. CO 2 has been implicated in this woody increase but attribution to global drivers has been controversial where changes in grazing and fire have also occurred. We report on diverse tests of the magnitude of CO 2 effects on both ancient and modern ecosystems with a particular focus on African savannahs. Large increases in trees of mesic savannahs in the region cannot easily be explained by land use change but are consistent with experimental and simulation studies of CO 2 effects. Changes in arid savannahs seem less obviously linked to CO 2 effects and may be driven more by overgrazing. Large-scale shifts in the tree—grass balance in the past and the future need to be better understood. They not only have major impacts on the ecology of grassy ecosystems but also on Earth—atmosphere linkages and the global carbon cycle in ways that are still being discovered.

1. State transitions are changes in ecosystem structure and self-reinforcing feedbacks that are initiated when an exogenous driver variable crosses a threshold. Reversing state transitions is difficult and costly. While some state transitions are relatively rapid, many take years to decades. Outside of theoretical models, very little is known about slower state transitions and how they unfold in time and space. 2. We quantified changes in spatial variance as a mesic grassland ecosystem shifts to a shrub-dominated state, using long-term experiments and simulations that maintain grasslands with annual fires or initiate a state transition to shrub dominance by decreasing fire frequency. 3. In the experiments, the susceptibility to state transitions varied substantially in space. In the less frequent fire treatment, some plots became shrub-dominated around year 20 and grass extirpations began in year 25, but a third of the plots were still grass-dominated in year 37. Variable rates of state transition resulted in increasing spatial variance of grass cover over time, whereas shrub cover variance decreased. In the annually burned treatment, grasses remained dominant and the spatial variance of grass cover declined. 4. In a separate experiment, less frequent fires were maintained for 23 years and then switched to annual fires. The switch to annual fires occurred shortly after grass variance started to increase and a majority of these plots quickly returned to a grass dominated state. 5. In simulations, spatial variance remained low and average grass cover was high under frequent fires. If fire frequency decreased below a threshold, the ecosystem transitioned to shrubland, with a transient increase in the spatial variance of grass cover during the transition between states. 6. Synthesis. Spatial variability in the rate and susceptibility to state transitions is indicative of a system with a patchy spatial structure, high spatial heterogeneity and low connectivity between patches. Increases in spatial variance can serve as an indication that some patches have begun a state transition and that management interventions are needed to avoid widespread transitions. This is one of the first empirical examples where altering management after an increase in spatial variance prevented state transitions.

Context The woody vegetation encroachment into natural grasslands is a significant global concern in nature reserves and other protected and conserved landscapes. Bush encroachment remains one of the major contributors of land degradation and landscape alterations. The phenomenon adversely affects biodiversity, conservation efforts, landscape productivity and recreational value. Objectives To understand the progression and threat of woody vegetation invasion into nature reserves, this study aimed to monitor bush encroachment and associated land use-land cover types in a nature reserve using high spatial resolution multi-temporal data within the Google Earth Engine (GEE) platform. Methods The study employed RapidEye and PlanetScope data spanning the period from 2009 to 2023 to estimate the changing extent of woody vegetation, grassland cover and bare areas, providing a comprehensive analysis of their dynamics over the 14-year study period. Results The results indicated that over the study period, approximately 130.69 hectares of grassland underwent a transition to woody vegetation, while approximately 2.78 hectares of woody vegetation was converted to grassland. The study revealed a net increase of 127.91 hectares in the total area covered by woody vegetation. The analysis revealed a notable upward trend in woody vegetation expansion during the 14 years of study, with percentage coverage of 37.69%, 51.18%, 64.52% and 74.02% in 2009, 2014, 2019 and 2023, respectively. Conclusions Considering the outcomes of this study, improvement of management schemes in the Bisley Nature Reserve is critical for the management and restoration of grasslands. Overall, the study provides valuable insights on the threat of bush encroachment in nature reserves and aid decision making for management of these landscapes.

Large mammalian herbivores (LMH) exert strong effects on plants in tropical savannas, and many wild LMH populations are declining. However, predicting the impacts of these declines on vegetation structure remains challenging. Experiments suggest that tree cover can increase rapidly following LMH exclusion. Yet it is unclear whether these results scale up to predict ecosystem‐level impacts of LMH declines, which often alter fire regimes, trigger compensatory responses of other herbivores and accompany anthropogenic land‐use changes. Moreover, theory predicts that grazers and browsers should have opposing effects on tree cover, further complicating efforts to forecast the outcomes of community‐wide declines. We used the near‐extirpation of grazing and browsing LMH from Gorongosa National Park during the Mozambican Civil War (1977–1992) as a natural experiment to test whether megafaunal collapse increased tree cover. We classified herbaceous and tree cover in satellite images taken (a) at the onset of war in 1977 and (b) in 2012, two decades after hostilities ceased. Throughout the 3620‐km² park, proportional tree cover increased by 34% (from 0.29 to 0.39) – an addition of 362 km². Four of the park's five major habitat zones (including miombo woodland, Acacia–Combretum–palm savanna, and floodplain grassland) showed even greater increases in tree cover (51–134%), with an average increase of 94% in ecologically critical Rift Valley habitats. Only in the eastern Cheringoma Plateau, which had historically low wildlife densities, did tree cover decrease (by 5%). The most parsimonious explanation for these results is that reduced browsing pressure enhanced tree growth, survival and/or recruitment; we found no directional trends in rainfall or fire that could explain increased tree cover. Synthesis. Catastrophic large‐herbivore die‐offs in Mozambique's flagship national park were followed by 35 years of woodland expansion, most severely in areas where pre‐war wildlife biomass was greatest. These findings suggest that browsing release supersedes grazer–grass–fire feedbacks in governing ecosystem‐level tree cover, consistent with smaller‐scale experimental results, although the potentially complementary effect of CO₂ fertilization cannot be definitively ruled out. Future work in Gorongosa will reveal whether recovering LMH populations reverse this trend, or alternatively whether woody encroachment hinders ongoing restoration efforts.

Management of tree cover, either to curb bush encroachment or to mitigate losses of woody cover to over-browsing, is a major concern in savanna ecosystems. Once established, trees are often “trapped” as saplings, since interactions among disturbance, plant competition, and precipitation delay sapling recruitment into adult size classes. Saplings can be directly suppressed by wildlife browsing and competition from adjacent plants, and indirectly facilitated by grazers, such as cattle, which feed on neighboring grasses. Yet few experimental studies have simultaneously quantified the effects of cattle and wildlife on sapling growth, particularly over long time scales. We used a series of replicated 4-ha herbivore-manipulation plots to investigate the net effects of wildlife and moderate cattle grazing on Acacia drepanolobium sapling growth over 10 years that encompassed extended wet and dry periods. We also simulated more intense cattle grazing using grass removal treatments (0.5-m radius around saplings), and we quantified the role of intraspecific tree competition using neighborhood tree surveys (trees within a 3-m radius). Wildlife, which included elephants, had a positive effect on sapling growth. Wildlife also reduced neighbor tree density during the 10-yr study, which likely caused the positive effect of wildlife on saplings. Although moderate cattle grazing did not affect sapling growth, grass removal treatments simulating heavy grazing increased sapling growth. Both grass removal and neighbor tree effects on saplings were strongest during above-average rainfall years following drought. This highlights that livestock-driven reductions in grass cover and catastrophic wildlife damage to trees during droughts present a need, or an opportunity, for targeted management of sapling growth and woody plant cover during ensuing wet periods.

Woody plant encroachment (WPE) is transforming grasslands globally, yet accurately mapping this process remains challenging. State-funded, publicly available high-resolution aerial imagery offers a potential solution, including the USDA’s National Agriculture Imagery Program (NAIP) and NSF’s National Ecological Observatory Network (NEON) Aerial Observation Platform (AOP). We evaluated the accuracy of land cover classification using NAIP, NEON, and both sources combined. We compared two machine learning models—support vector machines and random forests—implemented in R using large training and evaluation data sets. Our study site, Konza Prairie Biological Station, is a long-term experiment in which variable fire and grazing have created mosaics of herbaceous plants, shrubs, deciduous trees, and evergreen trees (Juniperus virginiana). All models achieved high overall accuracy (>90%), with NEON slightly outperforming NAIP. NAIP underperformed in detecting evergreen trees (52–78% vs. 83–86% accuracy with NEON). NEON models relied on LiDAR-based canopy height data, whereas NAIP relied on multispectral bands. Combining data from both platforms yielded the best results, with 97.7% overall accuracy. Vegetation indices contributed little to model accuracy, including NDVI (normalized digital vegetation index) and EVI (enhanced vegetation index). Both machine learning methods achieved similar accuracy. Our results demonstrate that free, high-resolution imagery and open-source tools can enable accurate, high-resolution, landscape-scale WPE monitoring. Broader adoption of such approaches could substantially improve the monitoring and management of grassland biodiversity, ecosystem function, ecosystem services, and environmental resilience.