Critical transitions in the Amazon forest system

<jats:title>Abstract</jats:title><jats:p>The possibility that the Amazon forest system could soon reach a tipping point, inducing large-scale collapse, has raised global concern<jats:sup>1–3</jats:sup>. For 65 million years, Amazonian forests remained relatively resilient to climatic variability. Now, the region is increasingly exposed to unprecedented stress from warming temperatures, extreme droughts, deforestation and fires, even in central and remote parts of the system<jats:sup>1</jats:sup>. Long existing feedbacks between the forest and environmental conditions are being replaced by novel feedbacks that modify ecosystem resilience, increasing the risk of critical transition. Here we analyse existing evidence for five major drivers of water stress on Amazonian forests, as well as potential critical thresholds of those drivers that, if crossed, could trigger local, regional or even biome-wide forest collapse. By combining spatial information on various disturbances, we estimate that by 2050, 10% to 47% of Amazonian forests will be exposed to compounding disturbances that may trigger unexpected ecosystem transitions and potentially exacerbate regional climate change. Using examples of disturbed forests across the Amazon, we identify the three most plausible ecosystem trajectories, involving different feedbacks and environmental conditions. We discuss how the inherent complexity of the Amazon adds uncertainty about future dynamics, but also reveals opportunities for action. Keeping the Amazon forest resilient in the Anthropocene will depend on a combination of local efforts to end deforestation and degradation and to expand restoration, with global efforts to stop greenhouse gas emissions.</jats:p>

section_content: Understanding the risk of such catastrophic behaviour requires addressing complex factors that shape ecosystem resilience 10. A major question is whether a large-scale collapse of the Amazon forest system could actually happen within the twenty-first century, and if this would be associated with a particular tipping point. Here we synthesize evidence from paleorecords, observational data and modelling studies of critical drivers of stress on the system. We assess potential thresholds of those drivers and the main feedbacks that could push the Amazon forest towards a tipping point. From examples of disturbed forests across the Amazon, we analyse the most plausible ecosystem trajectories that may lead to alternative stable states 10. Moreover, inspired by the framework of 'planetary boundaries' 11, we identify climatic and land use boundaries that reveal a safe operating space for the Amazon forest system in the Anthropocene epoch 12.
section_name: Analysis

section_content: Over time, environmental conditions fluctuate and may cause stress on ecosystems (for example, lack of water for plants). When stressing conditions intensify, some ecosystems may change their equilibrium state gradually, whereas others may shift abruptly between alternative stable states 10. A 'tipping point' is the critical threshold value of an environmental stressing condition at which a small disturbance may cause an abrupt shift in the ecosystem state 2, 3, 13, 14, accelerated by positive feedbacks 15 (see Extended Data Table 1 ). This type of behaviour in which the system gets into a phase of self-reinforcing (runaway) change is often referred to as 'critical transition' 16. As ecosystems approach a tipping point, they often lose resilience while still remaining close to equilibrium 17. Thus, monitoring changes in ecosystem resilience and in key environmental conditions may enable societies to manage and avoid critical transitions. We adopt the concept of 'ecological resilience' 18 (hereafter 'resilience'), which refers to the ability of an ecosystem to persist with similar structure, functioning and interactions, despite disturbances that push it to an alternative stable state. The possibility that alternative stable states (or bistability) may exist in a system has important implications, because the crossing of tipping points may be irreversible for the time scales that matter to societies 10. Tropical terrestrial ecosystems are a well-known case in which critical transitions between alternative stable states may occur (Extended Data Fig. 1 ).
section_name: Theory and concepts

section_content: The Amazon system has been mostly covered by forest throughout the Cenozoic era 19 (for 65 million years). Seven million years ago, the Amazon river began to drain the massive wetlands that covered most of the western Amazon, allowing forests to expand over grasslands in that region. More recently, during the drier and cooler conditions of the Last Glacial Maximum 20 (LGM) (around 21,000 years ago) and of the mid-Holocene epoch 21 (around 6,000 years ago), forests persisted even when humans were already present in the landscape 22. Nonetheless, savannas expanded in peripheral parts of the southern Amazon basin during the LGM and mid-Holocene 23, as well as in the northeastern Amazon during the early Holocene (around 11,000 years ago), probably influenced by drier climatic conditions and fires ignited by humans 24, 25. Throughout the core of the Amazon forest biome, patches of white-sand savanna also expanded in the past 20,000-7,000 years, driven by sediment deposition along ancient rivers 26, and more recently (around 800 years ago) owing to Indigenous fires 27. However, during the past 3,000 years, forests have been mostly expanding over savanna in the southern Amazon driven by increasingly wet conditions 28. 

Although palaeorecords suggest that a large-scale Amazon forest collapse did not occur within the past 65 million years 19, they indicate that savannas expanded locally, particularly in the more seasonal peripheral regions when fires ignited by humans were frequent 23, 24. Patches of white-sand savanna also expanded within the Amazon forest owing to geomorphological dynamics and fires 26, 27. Past drought periods were usually associated with much lower atmospheric CO 2 concentrations, which may have reduced water-use efficiency of trees 29 (that is, trees assimilated less carbon during transpiration). However, these periods also coincided with cooler temperatures 20, 21, which probably reduced water demand by trees 30. Past drier climatic conditions were therefore very different from the current climatic conditions, in which observed warming trends may exacerbate drought impacts on the forest by exposing trees to unprecedented levels of water stress 31, 32.
section_name: Past dynamics

section_content: Satellite observations from across the Amazon suggest that forest resilience has been decreasing since the early 2000s 33, possibly as a result of global changes. In this section, we synthesize three global change impacts that vary spatially and temporally across the Amazon system, affecting forest resilience and the risk of critical transitions.
section_name: Global change impacts on forest resilience

section_content: Within the twenty-first century, global warming may cause long-term changes in Amazonian climatic conditions 2. Human greenhouse gas emissions continue to intensify global warming, but the warming rate also depends on feedbacks in the climate system that remain uncertain 2, 3. Recent climate models of the 6th phase of the Coupled Model Intercomparison Project (CMIP6) agree that in the coming decades, rainfall conditions will become more seasonal in the eastern and southern Amazonian regions, and temperatures will become higher across the entire Amazon 1, 2. By 2050, models project that a significant increase in the number of consecutive dry days by 10-30 days and in annual maximum temperatures by 2-4 °C, depending on the greenhouse gas emission scenario 2. These climatic conditions could expose the forest to unprecedented levels of vapour pressure deficit 31 and consequently water stress 30. 

Satellite observations of climatic variability 31 confirm model projections 2, showing that since the early 1980s, the Amazonian region has been warming significantly at an average rate of 0. 27 °C per decade during the dry season, with the highest rates of up to 0. 6 °C per decade in the centre and southeast of the biome (Fig. 1a ). Only a few small areas in the west of the biome are significantly cooling by around 0. 1 °C per decade (Fig. 1a ). Dry season mean temperature is now more than 2 °C higher than it was 40 years ago in large parts of the central and southeastern Amazon. If trends continue, these areas could potentially warm by over 4 °C by 2050. Maximum temperatures during the dry season follow a similar trend, rising across most of the biome (Extended Data Fig. 2 ), exposing the forest 34 and local peoples 35 to potentially unbearable heat. Rising temperatures will increase thermal stress, potentially reducing forest productivity and carbon storage capacity 36 and causing widespread leaf damage 34. 

Since the early 1980s, rainfall conditions have also changed 31. Peripheral and central parts of the Amazon forest are drying significantly, such as in the southern Bolivian Amazon, where annual rainfall reduced by up to 20 mm yr -1 (Extended Data Fig. 3a ). By contrast, parts of the western and eastern Amazon forest are becoming wetter, with annual rainfall increasing by up to 20 mm yr -1. If these trends continue, ecosystem stability (as in Extended Data Fig. 1 ) will probably change in parts of the Amazon by 2050, reshaping forest resilience to disturbances (Fig. 1b and Extended Data Fig. 3b ). For example, 6% of the biome may change from stable forest to a bistable regime in parts of the southern and central Amazon. Another 3% of the biome may pass the critical threshold in annual rainfall into stable savanna in the southern Bolivian Amazon. Bistable areas covering 8% of the biome may turn into stable forest in the western Amazon (Peru and Bolivia), thus becoming more resilient to disturbances. For comparison with satellite observations, we used projections of ecosystem stability by 2050 based on CMIP6 model ensembles for a low (SSP2-4. 5) and a high (SSP5-8. 5) greenhouse gas emission scenario (Extended Data Fig. 4 and Supplementary Table 1 ). An ensemble with the 5 coupled models that include a dynamic vegetation module indicates that 18-27% of the biome may transition from stable forest to bistable and that 2-6% may transition to stable savanna (depending on the scenario), mostly in the northeastern Amazon. However, an ensemble with all 33 models suggests that 35-41% of the biome could become bistable, including large areas of the southern Amazon. The difference between both ensembles is possibly related to the forest-rainfall feedback included in the five coupled models, which increases total annual rainfall and therefore the stable forest area along the southern Amazon, but only when deforestation is not included in the simulations 4, 37. Nonetheless, both model ensembles agree that bistable regions will expand deeper into the Amazon, increasing the risk of critical transitions due to disturbances (as implied by the existence of alternative stable states; Extended Data Fig. 1 ).
section_name: Regional climatic conditions

section_content: Within the remaining Amazon forest area, 17% has been degraded by human disturbances 38, such as logging, edge effects and understory fires, but if we consider also the impacts from repeated extreme drought events in the past decades, 38% of the Amazon could be degraded 39. Increasing rainfall variability is causing extreme drought events to become more widespread and frequent across the Amazon (Fig. 1c ), together with extreme wet events and convective storms that result in more windthrow disturbances 40. Drought regimes are intensifying across the region 41, possibly due to deforestation 42 that continues to expand within the system (Extended Data Fig. 5 ). As a result, new fire regimes are burning larger forest areas 43, emitting more carbon to the atmosphere 44 and forcing IPLCs to readapt 45. Road networks (Fig. 1d ) facilitate illegal activities, promoting more deforestation, logging and fire spread throughout the core of the Amazon forest 38, 39. The impacts of these pervasive disturbances on biodiversity and on IPLCs will probably affect ecosystem adaptability (Box 1), and consequently forest resilience to global changes. 

Currently, 86% of the Amazon biome may be in a stable forest state (Extended Data Fig. 1b ), but some of these stable forests are showing signs of fragility 33. For instance, field evidence from long-term monitoring sites across the Amazon shows that tree mortality rates are increasing in most sites, reducing carbon storage 46, while favouring the replacement by drought-affiliated species 47. Aircraft measurements of vertical carbon flux between the forest and atmosphere reveal how southeastern forests are already emitting more carbon than they absorb, probably because of deforestation and fire 48. Transition potential rises with compounding disturbances and varies as follows: less than 0 (in blue) as low; between 1 and 2 as moderate (in yellow); more than 2 as high (orange-red). Transition potential represents the sum of: (1) slopes of dry season mean temperature (as in a, multiplied by 10);

(2) ecosystem stability classes estimated for year 2050 (as in b), with 0 for stable forest, 1 for bistable and 2 for stable savanna; (3) accumulated impacts from extreme drought events, with 0. 2 for each event; (4) road proximity as proxy for degrading activities, with 1 for pixels within 10 km from a road; (5) areas with higher governance within protected areas and Indigenous territories, with -1 for pixels inside these areas. For more details, see Methods.
section_name: Disturbance regimes

section_content: As bistable forests expand deeper into the system (Fig. 1b and Extended Data Fig. 4 ), the distribution of compounding disturbances may indicate where ecosystem transitions are more likely to occur in the coming decades (Fig. 1f ). For this, we combined spatial information on warming and drying trends, repeated extreme drought events, together with road networks, as proxy for future deforestation and degradation 38, 39. We also included protected areas and Indigenous territories as areas with high forest governance, where deforestation and fire regimes are among the lowest within the Amazon 49 (Fig. 1e ). This simple additive approach does not consider synergies between compounding disturbances that could trigger unexpected ecosystem transitions. However, by exploring only these factors affecting forest resilience and simplifying the enormous Amazonian complexity, we aimed to produce a simple and comprehensive map that can be useful for guiding future governance. We found that 10% of the Amazon forest biome has a relatively high transition potential (more than 2 disturbance types; Fig. 1f ), including bistable forests that could transition into a low tree cover state near savannas of Guyana, Venezuela, Colombia and Peru, as well as stable forests that could transition into alternative compositional states within the central Amazon, such as along the BR319 and Trans-Amazonian highways. Smaller areas with high transition potential were found scattered within deforestation frontiers, where most forests have been carved by roads 50, 51. Moreover, 47% of the biome has a moderate transition potential (more than 1 disturbance type; Fig. 1f ), including relatively remote parts of the central Amazon where warming trends and repeated extreme drought events overlap (Fig. 1a, c ). By contrast, large remote areas covering 53% of the biome have low transition potential, mostly reflecting the distribution of protected areas and Indigenous territories (Fig. 1e ). If these estimates, however, considered projections from CMIP6 models and their relatively broader areas of bistability (Extended Data Fig. 4 ), the proportion of the Amazon forest that could transition into a low tree cover state would be much larger.
section_name: Analysis

section_content: Rising atmospheric CO 2 concentrations are expected to increase the photosynthetic rates of trees, accelerating forest growth and biomass accumulation on a global scale 52. In addition, CO 2 may reduce water stress by increasing tree water-use efficiency 29. As result, a 'CO 2 fertilization effect' could increase forest resilience to climatic variability 53, 54. However, observations from across the Amazon 46 suggest that CO 2 -driven accelerations of tree growth may have contributed to increasing tree mortality rates (trees grow faster but also die earlier), which could eventually neutralize the forest carbon sink in the coming decades 55. Moreover, increases in tree water-use efficiency may reduce forest transpiration and consequently atmospheric moisture flow across the Amazon 53, 56, potentially reducing forest resilience in the southwest of the biome 4, 37. Experimental evidence suggests that CO 2 fertilization also depends on soil nutrient availability, particularly nitrogen and phosphorus 57, 58. Thus, it is possible that in the fertile soils of the western Amazon and Várzea floodplains, forests may gain resilience from increasing atmospheric CO 2 (depending on how it affects tree mortality rates), whereas on the weathered (nutrient-poor) soils across most of the Amazon basin 59, forests might not respond to atmospheric CO 2 increase, particularly on eroded soils within deforestation frontiers 60. In sum, owing to multiple interacting factors, potential
section_name: CO 2 fertilization

section_name: Box 1

section_content: We define 'ecosystem adaptability' as the capacity of an ecosystem to reorganize and persist in the face of environmental changes. In the past, many internal mechanisms have probably contributed to ecosystem adaptability, allowing Amazonian forests to persist during times of climate change. In this section we synthesize two of these internal mechanisms, which are now being undermined by global change.
section_name: Ecosystem adaptability

section_content: Amazonian forests are home to more than 15,000 tree species, of which 1% are dominant and the other 99% are mostly rare 107. A single forest hectare in the central and northwestern Amazon can contain more than 300 tree species (Extended Data Fig. 7a ). Such tremendous tree species diversity can increase forest resilience by different mechanisms. Tree species complementarity increases carbon storage, accelerating forest recovery after disturbances 108. Tree functional diversity increases forest adaptability to climate chance by offering various possibilities of functioning 99. Rare species provide 'ecological redundancy', increasing opportunities for replacement of lost functions when dominant species disappear 109. Diverse forests are also more likely to resist severe disturbances owing to 'response diversity' 110 -that is, some species may die, while others persist. For instance, in the rainy western Amazon, drought-resistant species are rare but present within tree communities 111, implying that they could replace the dominant drought-sensitive species in a drier future. Diversity of other organisms, such as frugivores and pollinators, also increases forest resilience by stabilizing ecological networks 15, 112. Considering that half of Amazonian tree species are estimated to become threatened (IUCN Red list) by 2050 owing to climate change, deforestation and degradation 8, biodiversity losses could contribute to further reducing forest resilience.
section_name: Biodiversity

section_content: Globally, Indigenous peoples and local communities (IPLCs) have a key role in maintaining ecosystems resilient to global change 113. Humans have been present in the Amazon for at least 12,000 years 114 and extensively managing landscapes for 6,000 years 22. Through diverse ecosystem management practices, humans built thousands of earthworks and 'Amazon Dark Earth' sites, and domesticated plants and landscapes across the Amazon forest 115, 116. By creating new cultural niches, humans partly modified the Amazonian flora 117, 118, increasing their food security even during times of past climate change 119, 120 without the need for large-scale deforestation 117. Today, IPLCs have diverse ecological knowledge about Amazonian plants, animals and landscapes, which allows them to quickly identify and respond to environmental changes with mitigation and adaptation practices 68, 69. IPLCs defend their territories against illegal deforestation and land use disturbances 49, 113, and they also promote forest restoration by expanding diverse agroforestry systems 121, 122. Amazonian regions with the highest linguistic diversity (a proxy for ecological knowledge diversity 123 ) are found in peripheral parts of the system, particularly in the north-west (Extended Data Fig. 7b ). However, consistent loss of Amazonian languages is causing an irreversible disruption of ecological knowledge systems, mostly driven by road construction 7. Continued loss of ecological knowledge will undermine the capacity of IPLCs to manage and protect Amazonian forests, further reducing their resilience to global changes 9. 

responses of Amazonian forests to CO 2 fertilization are still poorly understood. Forest responses depend on scale, with resilience possibly increasing at the local scale on relatively more fertile soils, but decreasing at the regional scale due to reduced atmospheric moisture flow.
section_name: Indigenous peoples and local communities

section_name: Local versus systemic transition

section_content: Environmental heterogeneity can reduce the risk of systemic transition (large-scale forest collapse) because when stressing conditions intensify (for example, rainfall declines), heterogeneous forests may transition gradually (first the less resilient forest patches, followed by the more resilient ones), compared to homogeneous forests that may transition more abruptly 17 (all forests transition in synchrony). Amazonian forests are heterogeneous in their resilience to disturbances, which may have contributed to buffering large-scale transitions in the past 37, 61, 62. At the regional scale, a fundamental heterogeneity factor is rainfall and how it translates into water stress. Northwestern forests rarely experience water stress, which makes them relatively more resilient than southeastern forests that may experience water stress in the dry season, and therefore are more likely to shift into a low tree cover state. As a result of low exposure to water deficit, most northwestern forests have trees with low drought resistance and could suffer massive mortality if suddenly exposed to severe water stress 32. However, this scenario seems unlikely to occur in the near future (Fig. 1 ). By contrast, most seasonal forest trees have various strategies to cope with water deficit owing to evolutionary and adaptive responses to historical drought events 32, 63. These strategies may allow seasonal forests to resist current levels of rainfall fluctuations 32, but seasonal forests are also closer to the critical rainfall thresholds (Extended Data Fig. 1 ) and may experience unprecedented water stress in the coming decades (Fig. 1 ). 

Other key heterogeneity factors (Extended Data Fig. 6 ) include topography, which determines plant access to groundwater 64, and seasonal flooding, which increases forest vulnerability to wildfires 65. Future changes in rainfall regimes will probably affect hydrological regimes 66, exposing plateau (hilltop) forests to unprecedented water stress, and floodplain forests to extended floods, droughts and wildfires. Soil fertility is another heterogeneity factor that may affect forest resilience 59, and which may be undermined by disturbances that cause topsoil erosion 60. Moreover, as human disturbances intensify throughout the Amazon (Fig. 1 ), the spread of invasive grasses and fires can make the system increasingly homogeneous. Effects of heterogeneity on Amazon forest resilience have been poorly investigated so far (but see refs. 37,61,62) and many questions remain open, such as how much heterogeneity exists in the system and whether it can mitigate a systemic transition.
section_name: Environmental heterogeneity

section_content: Connectivity across Amazonian landscapes and regions can contribute to synchronize forest dynamics, causing different forests to behave more similarly 17. Depending on the processes involved, connectivity can either increase or decrease the risk of systemic transition 17. For instance, connectivity may facilitate forest recovery after disturbances through seed dispersal, but also it may spread disturbances, such as fire. In the Amazon, an important source of connectivity enhancing forest resilience is atmospheric moisture flow westward (Fig. 2 ), partly maintained by forest evapotranspiration 4, 37, 67. Another example of connectivity that may increase social-ecological resilience is knowledge exchange among IPLCs about how to adapt to global change 68, 69 (see Box 1). However, complex systems such as the Amazon can be particularly vulnerable to sources of connectivity that spread disturbances and increase the risk of systemic transition 70. For instance, roads carving through the forest are well-known sources of illegal activities, such as logging and burning, which increase forest flammability 38, 39.
section_name: Sources of connectivity

section_name: Five critical drivers of water stress

section_content: Most CMIP6 models agree that a large-scale dieback of the Amazon is unlikely in response to global warming above pre-industrial levels 2, but this ecosystem response is based on certain assumptions, such as a large CO 2 -fertilization effect 53. Forests across the Amazon are already responding with increasing tree mortality rates that are not simulated by these models 46, possibly because of compounding disturbance regimes (Fig. 1 ). Nonetheless, a few global climate models 3, 14, [71] [72] [73] [74] indicate a broad range for a potential critical threshold in global warming between 2 and 6 °C (Fig. 3a ). These contrasting results can be explained by general differences between numerical models and their representation of the complex Amazonian system. While some models with dynamic vegetation indicate local-scale tipping events in peripheral parts of the Amazon 5, 6, other models suggest an increase in biomass and forest cover (for example, in refs. 53,54). For instance, a study found that when considering only climatic variability, a large-scale Amazon forest dieback is unlikely, even under a high greenhouse gas emission scenario 75. However, most updated CMIP6 models agree that droughts in the Amazon region will increase in length and intensity, and that exceptionally hot droughts will become more common 2, creating conditions that will probably boost other types of disturbances, such as large and destructive forest fires 76, 77. To avoid broad-scale ecosystem transitions due to synergies between climatic and land use disturbances (Fig. 3b ), we suggest a safe boundary for the Amazon forest at 1. 5 °C for global warming above pre-industrial levels, in concert with the Paris Agreement goals.
section_name: Global warming

section_content: Satellite observations of tree cover distributions across tropical South America suggest a critical threshold between 1,000 and 1,250 mm of Brazil holds 60% of the Amazon forest biome and has a major responsibility towards its neighbouring countries in the west. Brazil is the largest supplier of rainfall to western Amazonian countries. Up to one-third of the total annual rainfall in Amazonian territories of Bolivia, Peru, Colombia and Ecuador depends on water originating from Brazil's portion of the Amazon forest. This international connectivity illustrates how policies related to deforestation, especially in the Brazilian Amazon, will affect the climate in other countries. Arrow widths are proportional to the percentage of the annual rainfall received by each country within their Amazonian areas. We only show flows with percentages higher than 10% (see Methods for details).
section_name: Annual rainfall

section_content: annual rainfall 78, 79. On the basis of our reanalysis using tree cover data from the Amazon basin (Extended Data Fig. 1a ), we confirm a potential threshold at 1,000 mm of annual rainfall (Fig. 3a ), below which forests become rare and unstable. Between 1,000 and 1,800 mm of annual rainfall, high and low tree cover ecosystems exist in the Amazon as two alternative stable states (see Extended Data Table 2 for uncertainty ranges). Within the bistability range in annual rainfall conditions, forests are relatively more likely to collapse when severely disturbed, when compared to forests in areas with annual rainfall above 1,800 mm (Extended Data Fig. 1a ). For floodplain ecosystems covering 14% of the forest biome, a different critical threshold has been estimated at 1,500 mm of annual rainfall 65, implying that floodplain forests may be the first to collapse in a drier future. To avoid local-scale ecosystem transitions due to compounding disturbances, we suggest a safe boundary in annual rainfall conditions at 1,800 mm.
section_name: Analysis

section_content: Satellite observations of tree cover distributions across tropical South America suggest a critical threshold in rainfall seasonality intensity at -400 mm of the maximum cumulative water deficit 37, 80 (MCWD). Our reanalysis of the Amazon basin (Extended Data Fig. 1c ) confirms the critical threshold at approximately -450 mm in the MCWD (Fig. 3a ), and suggests a bistability range between approximately -350 and -450 mm (see Extended Data Table 2 for uncertainty ranges), in which forests are more likely to collapse when severely disturbed than forests in areas with MCWD below -350 mm. To avoid local-scale ecosystem transitions due to compounding disturbances, we suggest a safe boundary of MCWD at -350 mm.
section_name: Rainfall seasonality intensity

section_content: Satellite observations of tree cover distributions across tropical South America suggest a critical threshold at 7 months of dry season length 79 (DSL). Our reanalysis of the Amazon basin (Extended Data Fig. 1d ) suggests a critical threshold at eight months of DSL (Fig. 3a ), with a bistability range between approximately five and eight months (see Extended Data Table 2 for uncertainty ranges), in which forests are more likely to collapse when severely disturbed than forests in areas with DSL below five months. To avoid local-scale ecosystem transitions due to compounding disturbances, we suggest a safe boundary of DSL at five months.
section_name: Dry season length

section_content: A potential vegetation model 81 found a critical threshold at 20% of accumulated deforestation (Fig. 3a ) by simulating Amazon forest responses to different scenarios of accumulated deforestation (with associated fire events) and of greenhouse gas emissions, and by considering a CO 2 fertilization effect of 25% of the maximum photosynthetic assimilation rate. Beyond 20% deforestation, forest mortality accelerated, causing large reductions in regional rainfall and consequently an ecosystem transition of 50-60% of the Amazon, depending on the emissions scenario. Another study using a climate-vegetation model found that with accumulated deforestation of 30-50%, rainfall in non-deforested areas downwind would decline 67 by 40% (ref. 67), potentially causing more forest loss 4, 37. Other more recent models incorporating fire disturbances support a potential broad-scale transition of the Amazon forest, simulating a biomass loss of 30-40% under a high-emission scenario 5, 82 (SSP5-8. 5 at 4 °C). The Amazon biome has already lost 13% of its original forest area due to deforestation 83 (or 15% of the biome if we consider also young secondary forests 83 that provide limited contribution to moisture flow 84 ). Among the remaining old-growth forests, at least 38% have been degraded by land use disturbances and repeated extreme droughts 39, with impacts on moisture recycling that are still uncertain. Therefore, to avoid broad-scale ecosystem transitions due to runaway forest loss (Fig. 3b ), we suggest a safe boundary of accumulated deforestation of 10% of the original forest biome cover, which requires ending large-scale deforestation and restoring at least 5% of the biome.
section_name: Accumulated deforestation

section_name: Three alternative ecosystem trajectories

section_content: In stable forest regions of the Amazon with annual rainfall above 1,800 mm (Extended Data Fig. 1b ), forest cover usually recovers within a few years or decades after disturbances, yet forest composition and functioning may remain degraded for decades or centuries [84] [85] [86] [87]. Estimates from across the Amazon indicate that approximately 30% of areas previously deforested are in a secondary forest state 83 (covering 4% of the biome). An additional 38% of the forest biome has been damaged by extreme droughts, fires, logging and edge effects 38, 39. These forests may naturally regrow through forest succession, yet because of Fig. 3 | Drivers of water stress on the Amazon forest, their critical thresholds, safe boundaries and interactions. a, Five critical drivers of water stress on Amazonian forests affect (directly or indirectly) the underlying tipping point of the system. For each driver, we indicate potential critical thresholds and safe boundaries that define a safe operating space for keeping the Amazon forest resilient 11, 12. We followed the precautionary principle and considered the most conservative thresholds within the ranges, when confidence was low. b, Conceptual model showing how the five drivers may interact (arrows indicate positive effects) and how these interactions may strengthen a positive feedback between water stress and forest loss. These emerging positive feedback loops could accelerate a systemic transition of the Amazon forest 15. At global scales, driver 1 (global warming) intensifies with greenhouse gas emissions, including emissions from deforestation. At local scales, driver 5 (accumulated deforestation) intensifies with land use changes. Drivers 2 to 4 (regional rainfall conditions) intensify in response to drivers 1 and 5. The intensification of these drivers may cause widespread tree mortality for instance because of extreme droughts and fires 76. Water stress affects vegetation resilience globally 79, 104, but other stressors, such as heat stress 34, 36, may also have a role. In the coming decades, these five drivers could change at different rates, with some approaching a critical threshold faster than others. Therefore, monitoring them separately can provide vital information to guide mitigation and adaptation strategies. 

feedbacks 15, succession can become arrested, keeping forests persistently degraded (Fig. 4 ). Different types of degraded forests have been identified in the Amazon, each one associated with a particular group of dominant opportunistic plants. For instance, Vismia forests are common in old abandoned pastures managed with fire 85, and are relatively stable, because Vismia trees favour recruitment of Vismia seedlings in detriment of other tree species 88, 89. Liana forests can also be relatively stable, because lianas self-perpetuate by causing physical damage to trees, allowing lianas to remain at high density 90, 91. Liana forests are expected to expand with increasing aridity, disturbance regimes and CO 2 fertilization 90. Guadua bamboo forests are common in the southwestern Amazon 92, 93. Similar to lianas, bamboos self-perpetuate by causing physical damage to trees and have been expanding over burnt forests in the region 92. Degraded forests are usually dominated by native opportunistic species, and their increasing expansion over disturbed forests could affect Amazonian functioning and resilience in the future.
section_name: Degraded forest

section_content: White-sand savannas are ancient ecosystems that occur in patches within the Amazon forest biome, particularly in seasonally waterlogged or flooded areas 94. Their origin has been attributed to geomorphological dynamics and past Indigenous fires 26, 27, 94. In a remote landscape far from large agricultural frontiers, within a stable forest region of the Amazon (Extended Data Fig. 1b ), satellite and field evidence revealed that white-sand savannas are expanding where floodplain forests were repeatedly disturbed by fires 95. After fire, the topsoil of burnt forests changes from clayey to sandy, favouring the establishment of savanna trees and native herbaceous plants 95. Shifts from forest to white-sand savanna (Fig. 4 ) are probably stable (that is, the ecosystem is unlikely to recover back to forest within centuries), based on the relatively long persistence of these savannas in the landscape 94. Although these ecosystem transitions have been confirmed only in the Negro river basin (central Amazon), floodplain forests in other parts of the Amazon were shown to be particularly vulnerable to collapse 45, 64, 65.
section_name: White-sand savanna

section_content: In bistable regions of the Amazon forest with annual rainfall below 1,800 mm (Extended Data Fig. 1b ), shifts to degraded open-canopy ecosystems are relatively common after repeated disturbances by fire 45, 96. The ecosystem often becomes dominated by fire-tolerant tree and palm species, together with alien invasive grasses and opportunistic herbaceous plants 96, 97, such as vines and ferns. Estimates from the southern Amazon indicate that 5-6% of the landscape has already shifted into degraded open-canopy ecosystems due to deforestation and fires 45, 96. It is still unclear, however, whether degraded open-canopy ecosystems are stable or transient (Fig. 4 ). Palaeorecords from the northern Amazon 98 show that burnt forests may spend centuries in a degraded open-canopy state before they eventually shift into a savanna. Today, invasion by alien flammable grasses is a novel stabilizing mechanism 96, 97, but the long-term persistence of these grasses in the ecosystem is also uncertain. From examples of disturbed forests across the Amazon, we identify the three most plausible ecosystem trajectories related to the types of disturbances, feedbacks and local environmental conditions. These alternative trajectories may be irreversible or transient depending on the strength of the novel interactions 15. Particular combinations of interactions (arrows show positive effects described in the literature) may form feedback loops 15 that propel the ecosystem through these trajectories. In the 'degraded forest' trajectory, feedbacks often involve competition between trees and other opportunistic plants 85, 90, 92, as well as interactions between deforestation, fire and seed limitation 84, 87, 105. At the landscape scale, secondary forests are more likely to be cleared than mature forests, thus keeping forests persistently young and landscapes fragmented 83. 

In the 'degraded open-canopy ecosystem' trajectory, feedbacks involve interactions among low tree cover and fire 97, soil erosion 60, seed limitation 105, invasive grasses and opportunistic plants 96. At the regional scale, a self-reinforcing feedback between forest loss and reduced atmospheric moisture flow may increase the resilience of these open-canopy degraded ecosystems 42. In the 'white-sand savanna' trajectory, the main feedbacks result from interactions among low tree cover and fire, soil erosion, and seed limitation 106. Bottom left, floodplain forest transition to white-sand savanna after repeated fires (photo credit: Bernardo Flores); bottom centre, forest transition to degraded opencanopy ecosystem after repeated fires (photo credit: Paulo Brando); bottom right, forest transition to Vismia degraded forest after slash-and-burn agriculture (photo credit: Catarina Jakovac).
section_name: Degraded open-canopy ecosystem

section_name: Analysis

section_content: Several aspects of the Amazon forest system may help improve earth system models (ESMs) to more accurately simulate ecosystem dynamics and feedbacks with the climate system. Simulating individual trees can improve the representation of growth and mortality dynamics, which ultimately affect forest dynamics (for example, refs. 61,62,99). Significant effects on simulation results may emerge from increasing plant functional diversity, representation of key physiological trade-offs and other features that determine water stress on plants, and also allowing for community adjustment to environmental heterogeneity and global change 32, 55, 62, 99. For now, most ESMs do not simulate a dynamic vegetation cover (Supplementary Table 1 ) and biomes are represented based on few plant functional types, basically simulating monocultures on the biome level. In reality, tree community adaptation to a heterogenous and dynamic environment feeds into the whole-system dynamics, and not covering such aspects makes a true Amazon tipping assessment more challenging. 

Our findings also indicate that Amazon forest resilience is affected by compounding disturbances (Fig. 1 ). ESMs need to include different disturbance scenarios and potential synergies for creating more realistic patterns of disturbance regimes. For instance, logging and edge effects can make a forest patch more flammable 39, but these disturbances are often not captured by ESMs. Improvements in the ability of ESMs to predict future climatic conditions are also required. One way is to identify emergent constraints 100, lowering ESMs variations in their projections of the Amazonian climate. Also, fully coupled ESMs simulations are needed to allow estimates of land-atmosphere feedbacks, which may adjust climatic and ecosystem responses. Another way to improve our understanding of the critical thresholds for Amazonian resilience and how these link to climatic conditions and to greenhouse gas concentrations is through factorial simulations with ESMs. In sum, although our study may not deliver a set of reliable and comprehensive equations to parameterize processes impacting Amazon forest dynamics, required for implementation in ESMs, we highlight many of the missing modelled processes.
section_name: Prospects for modelling Amazon forest dynamics

section_content: Forest resilience is changing across the Amazon as disturbance regimes intensify (Fig. 1 ). Although most recent models agree that a large-scale collapse of the Amazon forest is unlikely within the twenty-first century 2, our findings suggest that interactions and synergies among different disturbances (for example, frequent extreme hot droughts and forest fires) could trigger unexpected ecosystem transitions even in remote and central parts of the system 101. In 2012, Davidson et al. 102 demonstrated how the Amazon basin was experiencing a transition to a 'disturbance-dominated regime' related to climatic and land use changes, even though at the time, annual deforestation rates were declining owing to new forms of governance 103. Recent policy and approaches to Amazon development, however, accelerated deforestation that reached 13,000 km 2 in the Brazilian Amazon in 2021 (http:// terrabrasilis. dpi. inpe. br). The southeastern region has already turned into a source of greenhouse gases to the atmosphere 48. The consequences of losing the Amazon forest, or even parts of it, imply that we must follow a precautionary approach-that is, we must take actions that contribute to maintain the Amazon forest within safe boundaries 12. Keeping the Amazon forest resilient depends firstly on humanity's ability to stop greenhouse gas emissions, mitigating the impacts of global warming on regional climatic conditions 2. At the local scale, two practical and effective actions need to be addressed to reinforce forest-rainfall feedbacks that are crucial for the resilience of the Amazon forest 4, 37 : (1) ending deforestation and forest degradation; and (2) promoting forest restoration in degraded areas. Expanding protected areas and Indigenous territories can largely contribute to these actions. Our findings suggest a list of thresholds, disturbances and feedbacks that, if well managed, can help maintain the Amazon forest within a safe operating space for future generations.
section_name: Implications for governance

section_content: Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi. org/10. 1038/s41586-023-06970-0.
section_name: Online content

section_name: Methods

section_content: Our study site was the area of the Amazon basin, considering large areas of tropical savanna biome along the northern portion of the Brazilian Cerrado, the Gran Savana in Venezuela and the Llanos de Moxos in Bolivia, as well as the Orinoco basin to the north, and eastern parts of the Andes to the west. The area includes also high Andean landscapes with puna and paramo ecosystems. We chose this contour to allow better communication with the MapBiomas Amazonian Project (2022; https:// amazonia. mapbiomas. org). For specific interpretation of our results, we considered the contour of the current extension of the Amazon forest biome, which excludes surrounding tropical savanna biomes. 

We used the Moderate Resolution Imaging Spectroradiometer (MODIS) Vegetation Continuous Fields (VCF) data (MOD44B version 6; https://lpdaac. usgs. gov/products/mod44bv006/) for the year 2001 at 250-m resolution 124 to reanalyse tree cover distributions within the Amazon basin, refining estimates of bistability ranges and critical thresholds in rainfall conditions from previous studies. Although MODIS VCF can contain errors within lower tree cover ranges and should not be used to test for bistability between grasslands and savannas 125, the dataset is relatively robust for assessing bistability within the tree cover range of forests and savannas 126, as also shown by low uncertainty (standard deviation of tree cover estimates) across the Amazon (Extended Data Fig. 8 ). 

We used the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS; https://www. chc. ucsb. edu/data/chirps) 127 to estimate mean annual rainfall and rainfall seasonality for the present across the Amazon basin, based on monthly means from 1981 to 2020, at a 0. 05° spatial resolution. 

We used the Climatic Research Unit (CRU; https://www. uea. ac. uk/ groups-and-centres/climatic-research-unit) 128 to estimate mean annual temperature for the present across the Amazon basin, based on monthly means from 1981 to 2020, at a 0. 5° spatial resolution. 

To mask deforested areas until 2020, we used information from the MapBiomas Amazonia Project (2022), collection 3, of Amazonian Annual Land Cover and Land Use Map Series (https://amazonia. mapbiomas. org). 

To assess forest fire distribution across the Amazon forest biome and in relation to road networks, we used burnt area fire data obtained from the AQUA sensor onboard the MODIS satellite. Only active fires with a confidence level of 80% or higher were selected. The data are derived from MODIS MCD14ML (collection 6) 129, available in Fire Information for Resource Management System (FIRMS). The data were adjusted to a spatial resolution of 1 km.
section_name: Datasets

section_content: Using potential analysis 130, an empirical stability landscape was constructed based on spatial distributions of tree cover (excluding areas deforested until 2020; https://amazonia. mapbiomas. org) against mean annual precipitation, MCWD and DSL. Here we followed the methodology of Hirota et al. 104. For bins of each of the variables, the probability density of tree cover was determined using the MATLAB function ksdensity. Local maxima of the resulting probability density function are considered to be stable equilibria, in which local maxima below a threshold value of 0. 005 were ignored. Based on sensitivity tests (see below), we chose the intermediate values of the sensitivity parameter for each analysis, which resulted in the critical thresholds most similar to the ones previously published in the literature.
section_name: Potential analysis

section_content: We smoothed the densities of tree cover with the MATLAB kernel smoothing function ksdensity. Following Hirota et al. 104, we used a flexible bandwidth (h) according to Silverman's rule of thumb 131 : h = 1. 06σn 1/5, where σ is the standard deviation of the tree cover distribution and n is the number of points. To ignore small bumps in the frequency distributions, we used a dimensionless sensitivity parameter. This parameter filters out weak modes in the distributions such that a higher value implies a stricter criterion to detect a significant mode. In the manuscript, we used a value of 0. 005. For different values of this sensitivity parameter, we here test the estimated critical thresholds and bistability ranges (Extended Data Table 2 ). We inferred stable and unstable states of tree cover (minima and maxima in the potentials) for moving windows of the climatic variables. For mean annual precipitation, we used increments of 10 mm yr -1 between 0 and 3500 mm yr -1. For dry season length, we used increments of 0. 1 months between 0 and 12 months. For MCWD, we used increments of 10 mm between -800 mm and 0 mm.
section_name: Sensitivity tests of the potential analysis

section_content: We quantified a relative ecosystem transition potential across the Amazon forest biome (excluding accumulated deforestation; https:// amazonia. mapbiomas. org) to produce a simple spatial measure that can be useful for governance. For this, we combined information per pixel, at 5 km resolution, about different disturbances related to climatic and human disturbances, as well as high-governance areas within protected areas and Indigenous territories. We used values of significant slopes of the dry season ( July-October) mean temperature between 1981 and 2020 (P < 0. 1), estimated using simple linear regressions (at 0. 5° resolution from CRU) (Fig. 1a ). Ecosystem stability classes (stable forest, bistable and stable savanna as in Extended Data Fig. 1 ) were estimated using simple linear regression slopes of annual rainfall between 1981 and 2020 (P < 0. 1) (at 0. 05° resolution from CHIRPS), which we extrapolated to 2050 (Fig. 1b and Extended Data Fig. 3 ). Distribution of areas affected by repeated extreme drought events (Fig. 1c ) were defined when the time series (2001-2018) of the MCWD reached two standard deviation anomalies from historical mean. Extreme droughts were obtained from Lapola et al. 39, based on Climatic Research Unit gridded Time Series (CRU TS 4. 0) datasets for precipitation and evapotranspiration. The network of roads (paved and unpaved) across the Amazon forest biome (Fig. 1d ) was obtained from the Amazon Network of Georeferenced Socio-Environmental Information (RAISG; https://geo2. socioambiental. org/raisg). Protected areas (PAs) and Indigenous territories (Fig. 1e ) were also obtained from RAISG, and include both sustainable-use and restricted-use protected areas managed by national or sub-national governments, together with officially recognized and proposed Indigenous territories. We combined these different disturbance layers by adding a value for each layer in the following way: (1) slopes of dry season temperature change (as in Fig. 1a, multiplied by 10, thus between -0. 1 and +0. 6); (2) ecosystem stability classes estimated for year 2050 (as in Fig. 1b ), with 0 for stable forest, +1 for bistable and +2 for stable savanna; (3) accumulated impacts from repeated extreme drought events (from 0 to 5 events), with +0. 2 for each event; (4) road-related human impacts, with +1 for pixels within 10 km from a road; and (5) protected areas and Indigenous territories as areas with lower exposure to human (land use) disturbances, such as deforestation and forest fires, with -1 for pixels inside these areas. The sum of these layers revealed relative spatial variation in ecosystem transition potential by 2050 across the Amazon (Fig. 1f ), ranging from -1 (low potential) to 4 (very high potential).
section_name: Transition potential

section_content: To determine the atmospheric moisture flows between the Amazonian countries, we use the Lagrangian atmospheric moisture tracking model UTrack 132. The model tracks the atmospheric trajectories of parcels of moisture, updates their coordinates at each time step of 0. 1 h and allocates moisture to a target location in case of precipitation. For each millimetre of evapotranspiration, 100 parcels are released into the atmosphere. Their trajectories are forced with evaporation, precipitation, and wind speed estimates from the ERA5 reanalysis product at 0. 25° horizontal resolution for 25 atmospheric layers 133. Here we use the Extended Data Fig. 6 | Environmental heterogeneity in the Amazon forest system. Heterogeneity involves myriad factors, but two in particular, related to water availability, were shown to contribute to landscape-scale heterogeneity in forest resilience; topography shapes fine-scale variations of forest droughttolerance 135, 136, and floodplains may reduce forest resilience by increasing vulnerability to wildfires 65. Datasets: topography is shown by the Shuttle Radar Topography Mission (SRTM; https://earthexplorer. usgs. gov/) 137
section_name: Atmospheric moisture tracking

section_content: Acknowledgements This work was inspired by the Science Panel for the Amazon (SPA) initiative (https://www. theamazonwewant. org/)that produced the first Amazon Assessment Report (2021). The authors thank C. Smith for providing deforestation rates data used in Extended Data Fig. 5b. B. M. F. and M. H. were supported by Instituto Serrapilheira ( Serra-1709-18983 ) and C. J. ( R-2111-40341 ). A. S. acknowledges funding from the Dutch Research Council (NWO) under the Talent Program Grant VI. Veni. 202. 170. R. A. B. and D. M. L. were supported by the AmazonFACE programme funded by the UK Foreign, Commonwealth and Development Office (FCDO) and Brazilian Ministry of Science, Technology and Innovation (MCTI). R. A. B. was additionally supported by the Met Office Climate Science for Service Partnership (CSSP) Brazil project funded by the UK Department for Science, Innovation and Technology (DSIT), and D. M. L. was additionally supported by FAPESP (grant no. 2020/08940-6 ) and CNPq (grant no. 309074/2021-5 ). C. L. thanks CNPq (proc. 159440/2018-1 and 400369/2021-4 ) and Brazil LAB ( Princeton University ) for postdoctoral fellowships. A. E. -M. is supported by the UKRI TreeScapes MEMBRA ( NE/V021346/1 ), the Royal Society ( RGS\R1\221115 ), the ERC TreeMort project ( 758873 ) and the CESAB Syntreesys project. R. S. O. received a CNPq productivity scholarship and funding from NERC - FAPESP 2019/07773-1. S. B. H. is supported by the Geneva Graduate Institute research funds, and UCLA 's committee on research. J. A. M. is supported by the National Institute of Science and Technology for Climate Change Phase 2 under CNPq grant 465501/2014-1 ; FAPESP grants 2014/50848-9, the National Coordination for Higher Education and Training (CAPES) grant 88887. 136402-00INCT. L. S. B. received FAPESP grant 2013/50531-0. D. N. and N. B. acknowledge funding from the European Union 's Horizon 2020 research and innovation programme under grant agreement no. 820970. N. B. has received further funding from the Volkswagen foundation, the European Union 's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 956170, as well as from the German Federal Ministry of Education and Research under grant no. 01LS2001A.

section_name: Data availability

section_content: All data supporting the findings of this study are openly available and their sources are presented in the Methods.
section_name: Data availability

section_content: Global and regional feedbacks are more likely to propel a large-scale tipping point. Adapted from 15.
section_name: Extended Data Table 1 | Examples of positive feedbacks that may affect Amazon forest resilience

section_name: Reporting summary

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Open Access This article is licensed under a Creative Commons Attribution 4. 0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons. org/licenses/by/4. 0/.

section_content: Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
section_name: Reporting summary

section_content: The authors declare no competing interests.
section_name: Competing interests

section_content: The online version contains supplementary material available at https://doi. org/10. 1038/s41586-023-06970-0. Correspondence and requests for materials should be addressed to Bernardo M. Flores or Marina Hirota. Peer review information Nature thanks Chris Huntingford and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer review reports are available. 

Reprints and permissions information is available at http://www. nature. com/reprints. Extended Data Fig. 1 | Alternative stable states in Amazonian tree cover relative to rainfall conditions. Potential analysis of tree cover distributions across rainfall gradients in the Amazon basin suggest the existence of critical thresholds and alternative stable states in the system. For this, we excluded accumulated deforestation until 2020 and included large areas of tropical savanna biome in the periphery of the Amazon basin (see Methods). Solid black lines indicate two stable equilibria. Small grey arrows indicate the direction towards equilibrium. (a) The overlap between ~ 1,000 and 1,800 mm of annual rainfall suggests that two alternative stable states may exist (bistability): a high tree cover state ~ 80 % (forests), and a low tree cover state ~ 20% (savannas). Tree cover around 50 % is rare, indicating an unstable state. Below 1,000 mm of annual rainfall, forests are rare, indicating a potential critical threshold for abrupt forest transition into a low tree cover state 79, 104 (arrow 1). Between 1,000 and 1,800 mm of annual rainfall, the existence of alternative stable states implies that forests can shift to a low tree cover stable state in response to disturbances (arrow 2). Above 1,800 mm of annual rainfall, low tree cover becomes rare, indicating a potential critical threshold for an abrupt transition into a high tree cover state. In this stable forest state, forests are expected to always recover after disturbances (arrow 3), although composition may change 47, 85. (b) Currently, the stable savanna state covers 1 % of the Amazon forest biome, bistable areas cover 13 % of the biome (less than previous analysis using broader geographical ranges 78 ) and the stable forest state covers 86 % of the biome. Similar analyses using the maximum cumulative water deficit (c) and the dry season length (d) also suggest the existence of critical thresholds and alternative stable states. When combined, these critical thresholds in rainfall conditions could result in a tipping point of the Amazon forest in terms of water stress, but other factors may play a role, such as groundwater availability 64. MODIS VCF may contain some level of uncertainty for low tree cover values, as shown by the standard deviation of tree cover estimates across the Amazon (Extended Data Fig. 8 ). However, the dataset is relatively robust for assessing bistability within the tree cover range between forest and savanna 126. In (b), annual deforestation rates for the entire Amazon biome were adapted with permission from Smith et al. 83.
section_name: Additional information Supplementary information

section_content: Extended Data Fig. 8 | Uncertainty of the MODIS VCF dataset across the Amazon basin. Map shows standard deviation (SD) of tree cover estimates from MODIS VCF 124. We masked deforested areas until 2020 using the MapBiomas Amazonia Project (2022; https://amazonia. mapbiomas. org).
section_name: Analysis

section_content: A range of possible thresholds results from different values of the sensitivity parameter in the potential analysis, for mean annual precipitation (MAP in mm/year), dry season length (DSL in months) and maximum climatological water deficit (MCWD in mm). F to S means forest to savanna threshold. S to F means savanna to forest threshold.
section_name: Analysis Extended Data Table 2 | Uncertainty ranges in the estimates of critical thesholds for transitions between forest and savanna