Tree cover in sub-Saharan Africa: rainfall and fire constrain forest and savanna as alternative stable states.
TL;DR: This model demonstrates that, given relatively conservative and empirically supported assumptions about the establishment of trees in savannas, alternative stable states for the same set of environmental conditions are possible via a fire feedback mechanism and could improve the ability to predict changes in biome distributions and in carbon storage under climate and global change scenarios.
Abstract: Savannas are known as ecosystems with tree cover below climate-defined equilibrium values However, a predictive framework for understanding constraints on tree cover is lacking We present (a) a spatially extensive analysis of tree cover and fire distribution in sub-Saharan Africa, and (b) a model, based on empirical results, demonstrating that savanna and forest may be alternative stable states in parts of Africa, with implications for understanding savanna distributions Tree cover does not increase continuously with rainfall, but rather is constrained to low ( 75%, "forest") Intermediate tree cover rarely occurs Fire, which prevents trees from establishing, differentiates high and low tree cover, especially in areas with rainfall between 1000 mm and 2000 mm Fire is less important at low rainfall ( 2000 mm), where fire is rare This pattern suggests that complex interactions between climate and disturbance produce emergent alternative states in tree cover The relationship between tree cover and fire was incorporated into a dynamic model including grass, savanna tree saplings, and savanna trees Only recruitment from sapling to adult tree varied depending on the amount of grass in the system Based on our empirical analysis and previous work, fires spread only at tree cover of 40% or less, producing a sigmoidal fire probability distribution as a function of grass cover and therefore a sigmoidal sapling to tree recruitment function This model demonstrates that, given relatively conservative and empirically supported assumptions about the establishment of trees in savannas, alternative stable states for the same set of environmental conditions (ie, model parameters) are possible via a fire feedback mechanism Integrating alternative stable state dynamics into models of biome distributions could improve our ability to predict changes in biome distributions and in carbon storage under climate and global change scenarios
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TL;DR: Tree cover, climate, fire, and soils data sets are used to show that tree cover is globally discontinuous, and only fire differentiates between savanna and forest.
Abstract: Theoretically, fire–tree cover feedbacks can maintain savanna and forest as alternative stable states. However, the global extent of fire-driven discontinuities in tree cover is unknown, especially accounting for seasonality and soils. We use tree cover, climate, fire, and soils data sets to show that tree cover is globally discontinuous. Climate influences tree cover globally but, at intermediate rainfall (1000 to 2500 millimeters) with mild seasonality (less than 7 months), tree cover is bimodal, and only fire differentiates between savanna and forest. These may be alternative states over large areas, including parts of Amazonia and the Congo. Changes in biome distributions, whether at the cost of savanna (due to fragmentation) or forest (due to climate), will be neither smooth nor easily reversible.
1,043 citations
TL;DR: Empirical reconstruction of the basins of attraction indicates that the resilience of the states varies in a universal way with precipitation, and this results allow the identification of regions where forest or savanna may most easily tip into an alternative state.
Abstract: It has been suggested that tropical forest and savanna could represent alternative stable states, implying critical transitions at tipping points in response to altered climate or other drivers. So far, evidence for this idea has remained elusive, and integrated climate models assume smooth vegetation responses. We analyzed data on the distribution of tree cover in Africa, Australia, and South America to reveal strong evidence for the existence of three distinct attractors: forest, savanna, and a treeless state. Empirical reconstruction of the basins of attraction indicates that the resilience of the states varies in a universal way with precipitation. These results allow the identification of regions where forest or savanna may most easily tip into an alternative state, and they pave the way to a new generation of coupled climate models.
945 citations
TL;DR: Based on data from central Brazil, it is proposed that these interactions are governed by two critical thresholds; the fire-resistance threshold is reached when individual trees have accumulated sufficient bark to avoid stem death, and theFire-suppression threshold is reach when an ecosystem has sufficient canopy cover to suppress fire by excluding grasses.
Abstract: Fire shapes the distribution of savanna and forest through complex interactions involving climate, resources and species traits. Based on data from central Brazil, we propose that these interactions are governed by two critical thresholds. The fire-resistance threshold is reached when individual trees have accumulated sufficient bark to avoid stem death, whereas the fire-suppression threshold is reached when an ecosystem has sufficient canopy cover to suppress fire by excluding grasses. Surpassing either threshold is dependent upon long fire-free intervals, which are rare in mesic savanna. On high-resource sites, the thresholds are reached quickly, increasing the probability that savanna switches to forest, whereas low-resource sites are likely to remain as savanna even if fire is infrequent. Species traits influence both thresholds; saplings of savanna trees accumulate bark thickness more quickly than forest trees, and are more likely to become fire resistant during fire-free intervals. Forest trees accumulate leaf area more rapidly than savanna trees, thereby accelerating the transition to forest. Thus, multiple factors interact with fire to determine the distribution of savanna and forest by influencing the time needed to reach these thresholds. Future work should decipher multiple environmental controls over the rates of tree growth and canopy closure in savanna.
628 citations
Vrije Universiteit Brussel1, University of Copenhagen2, University of Gothenburg3, New York University4, University of Pennsylvania5, Wageningen University and Research Centre6, University of Helsinki7, King's College London8, Institut national de la recherche agronomique9, University of Maryland, Baltimore10, Waseda University11, Harvard University12, University College Cork13, University of Minnesota14, University of Texas at Austin15, University of Warwick16, Stanford University17, Veterans Health Administration18, ETH Zurich19, Shanghai Jiao Tong University20, Katholieke Universiteit Leuven21, Molecular Medicine Partnership Unit22, Max Delbrück Center for Molecular Medicine23
TL;DR: The concept of enterotypes and their use to characterize the gut microbiome are debated, a classifier and standardized methodology is provided to aid cross-study comparisons, and a balanced application of the concept is encouraged.
Abstract: Population stratification is a useful approach for a better understanding of complex biological problems in human health and wellbeing. The proposal that such stratification applies to the human gut microbiome, in the form of distinct community composition types termed enterotypes, has been met with both excitement and controversy. In view of accumulated data and re-analyses since the original work, we revisit the concept of enterotypes, discuss different methods of dividing up the landscape of possible microbiome configurations, and put these concepts into functional, ecological and medical contexts. As enterotypes are of use in describing the gut microbial community landscape and may become relevant in clinical practice, we aim to reconcile differing views and encourage a balanced application of the concept.
622 citations
TL;DR: Recent developments in modeling social-ecological systems are presented, some of these challenges are illustrated with examples related to coral reefs and grasslands, and the implications for economic and policy analysis are identified.
Abstract: Systems linking people and nature, known as social-ecological systems, are increasingly understood as complex adaptive systems. Essential features of these complex adaptive systems – such as nonlinear feedbacks, strategic interactions, individual and spatial heterogeneity, and varying time scales – pose substantial challenges for modeling. However, ignoring these characteristics can distort our picture of how these systems work, causing policies to be less effective or even counterproductive. In this paper we present recent developments in modeling social-ecological systems, illustrate some of these challenges with examples related to coral reefs and grasslands, and identify the implications for economic and policy analysis.
555 citations
Cites background from "Tree cover in sub-Saharan Africa: r..."
...Linear models cannot explain why coral reefs suddenly flip between a clear and a turbid state (Crépin, 2007), why forests may rapidly turn into grasslands (Hirota et al., 2011; Staver et al., 2011a, b) or why stock markets crash....
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2,501 citations
TL;DR: The reasons why alternative equilibria are theoretically expected in shallow lakes are discussed, evidence from the field is reviewed, and recent applications of this insight in lake management are evaluated.
Abstract: The turbidity of lakes is generally considered to be a smooth function of their nutrient status. However, recent results suggest that over a range of nutrient concentrations, shallow lakes can have two alternative equilibria: a clear state dominated by aquatic vegetation, and a turbid state characterized by high algal biomass. This bi-stability has important implications for the possibilities of restoring eutrophied shallow lakes. Nutrient reduction alone may have little impact on water clarity, but an ecosystem disturbance like foodweb manipulation can bring the lake back to a stable clear state. We discuss the reasons why alternative equilibria are theoretically expected in shallow lakes, review evidence from the field and evaluate recent applications of this insight in lake management.
2,474 citations
TL;DR: In this paper, the authors review emerging ways to link theory to observation, and conclude that although, field observations can provide hints of alternative stable states, experiments and models are essential for a good diagnosis.
Abstract: Occasionally, surprisingly large shifts occur in ecosystems. Theory suggests that such shifts can be attributed to alternative stable states. Verifying this diagnosis is important because it implies a radically different view on management options, and on the potential effects of global change on such ecosystems. For instance, it implies that gradual changes in temperature or other factors might have little effect until a threshold is reached at which a large shift occurs that might be difficult to reverse. Strategies to assess whether alternative stable states are present are now converging in fields as disparate as desertification, limnology, oceanography and climatology. Here, we review emerging ways to link theory to observation, and conclude that although, field observations can provide hints of alternative stable states, experiments and models are essential for a good diagnosis.
2,464 citations