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Dagbovie AS, Sherratt JA. Pattern selection and hysteresis in the Rietkerk model for banded vegetation in semi-arid environments. J R Soc Interface 2015; 11:rsif.2014.0465. [PMID: 25142517 DOI: 10.1098/rsif.2014.0465] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Banded vegetation is a characteristic feature of semi-arid environments. It occurs on gentle slopes, with alternating stripes of vegetation and bare ground running parallel to the contours. A number of mathematical models have been proposed to investigate the mechanisms underlying these patterns, and how they might be affected by changes in environmental conditions. One of the most widely used models is due to Rietkerk and co-workers, and is based on a water redistribution hypothesis, with the key feedback being that the rate of rainwater infiltration into the soil is an increasing function of plant biomass. Here, for the first time, we present a detailed study of the existence and stability of pattern solutions of the Rietkerk model on slopes, using the software package wavetrain (www.ma.hw.ac.uk/wavetrain). Specifically, we calculate the region of the rainfall-migration speed parameter plane in which patterns exist, and the sub-region in which these patterns are stable as solutions of the model partial differential equations. We then perform a detailed simulation-based study of the way in which patterns evolve when the rainfall parameter is slowly varied. This reveals complex behaviour, with sudden jumps in pattern wavelength, and hysteresis; we show that these jumps occur when the contours of constant pattern wavelength leave the parameter region giving stable patterns. Finally, we extend our results to the case in which a diffusion term for surface water is added to the model equations. The parameter regions for pattern existence and stability are relatively insensitive to small or moderate levels of surface water diffusion, but larger diffusion coefficients significantly change the subdivision into stable and unstable patterns.
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Affiliation(s)
- Ayawoa S Dagbovie
- Department of Mathematics and Maxwell Institute for Mathematical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
| | - Jonathan A Sherratt
- Department of Mathematics and Maxwell Institute for Mathematical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
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52
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Xu C, Van Nes EH, Holmgren M, Kéfi S, Scheffer M. Local Facilitation May Cause Tipping Points on a Landscape Level Preceded by Early-Warning Indicators. Am Nat 2015; 186:E81-90. [DOI: 10.1086/682674] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
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53
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Xu C, Vergnon R, Cornelissen JHC, Hantson S, Holmgren M, van Nes EH, Scheffer M. Temperate forest and open landscapes are distinct alternative states as reflected in canopy height and tree cover. Trends Ecol Evol 2015. [DOI: 10.1016/j.tree.2015.07.002] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
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54
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Staal A, Dekker SC, Hirota M, van Nes EH. Synergistic effects of drought and deforestation on the resilience of the south-eastern Amazon rainforest. ECOLOGICAL COMPLEXITY 2015. [DOI: 10.1016/j.ecocom.2015.01.003] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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55
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Luo Y, Keenan TF, Smith M. Predictability of the terrestrial carbon cycle. GLOBAL CHANGE BIOLOGY 2015; 21:1737-1751. [PMID: 25327167 DOI: 10.1111/gcb.12766] [Citation(s) in RCA: 66] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/16/2014] [Accepted: 10/09/2014] [Indexed: 06/04/2023]
Abstract
Terrestrial ecosystems sequester roughly 30% of anthropogenic carbon emission. However this estimate has not been directly deduced from studies of terrestrial ecosystems themselves, but inferred from atmospheric and oceanic data. This raises a question: to what extent is the terrestrial carbon cycle intrinsically predictable? In this paper, we investigated fundamental properties of the terrestrial carbon cycle, examined its intrinsic predictability, and proposed a suite of future research directions to improve empirical understanding and model predictive ability. Specifically, we isolated endogenous internal processes of the terrestrial carbon cycle from exogenous forcing variables. The internal processes share five fundamental properties (i.e., compartmentalization, carbon input through photosynthesis, partitioning among pools, donor pool-dominant transfers, and the first-order decay) among all types of ecosystems on the Earth. The five properties together result in an emergent constraint on predictability of various carbon cycle components in response to five classes of exogenous forcing. Future observational and experimental research should be focused on those less predictive components while modeling research needs to improve model predictive ability for those highly predictive components. We argue that an understanding of predictability should provide guidance on future observational, experimental and modeling research.
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Affiliation(s)
- Yiqi Luo
- Department of Microbiology and Plant Biology, University of Oklahoma, Norman, OK, USA; Center for Earth System Science, Tsinghua University, Beijing, 100084, China
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56
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Abstract
Transitions between regimes with radically different properties are ubiquitous in nature. Such transitions can occur either smoothly or in an abrupt and catastrophic fashion. Important examples of the latter can be found in ecology, climate sciences, and economics, to name a few, where regime shifts have catastrophic consequences that are mostly irreversible (e.g., desertification, coral reef collapses, and market crashes). Predicting and preventing these abrupt transitions remains a challenging and important task. Usually, simple deterministic equations are used to model and rationalize these complex situations. However, stochastic effects might have a profound effect. Here we use 1D and 2D spatially explicit models to show that intrinsic (demographic) stochasticity can alter deterministic predictions dramatically, especially in the presence of other realistic features such as limited mobility or spatial heterogeneity. In particular, these ingredients can alter the possibility of catastrophic shifts by giving rise to much smoother and easily reversible continuous ones. The ideas presented here can help further understand catastrophic shifts and contribute to the discussion about the possibility of preventing such shifts to minimize their disruptive ecological, economic, and societal consequences.
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Affiliation(s)
- Paula Villa Martín
- Departamento de Electromagnetismo y Física de la Materia, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
| | - Juan A Bonachela
- Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544-1003; and
| | - Simon A Levin
- Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544-1003; and
| | - Miguel A Muñoz
- Departamento de Electromagnetismo y Física de la Materia and Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
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57
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Tredennick AT, Hanan NP. Effects of tree harvest on the stable-state dynamics of savanna and forest. Am Nat 2015; 185:E153-65. [PMID: 25905514 DOI: 10.1086/680475] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
Contemporary theory on the maintenance and stability of the savanna biome has focused extensively on how climate and disturbances interact to affect tree growth and demography. In particular, the role of fire in reducing tree cover from climatic maxima is now well appreciated, and in certain cases, herbivory also strongly affects tree cover. However, in African savannas and forests, harvest of trees by humans for cooking and heating is an oft overlooked disturbance. Thus, we incorporate tree harvest into a population dynamic model of grasses, savanna saplings, savanna trees, and forest trees. We use assumptions about the differential demographic responses of savanna trees and forest trees to harvest to show how tree harvest influences tree cover, demography, and community composition. Tree harvest can erode the intrinsic basin of attraction for forest and make a state transition via fire to savanna more likely. The savanna state is generally resilient to all but high levels of tree harvest because of the resprouting abilities of savanna trees. In the absence of active fire suppression, our analysis suggests that we can expect to see large and potentially irreversible shifts from forest to savanna as demand increases for charcoal in sub-Saharan Africa. On the other hand, savanna tree species' traits promote savanna stability in the face of low to moderate harvest pressure.
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Affiliation(s)
- Andrew T Tredennick
- Natural Resource Ecology Laboratory and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado 80523
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58
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Blanco CC, Scheiter S, Sosinski E, Fidelis A, Anand M, Pillar VD. Feedbacks between vegetation and disturbance processes promote long-term persistence of forest–grassland mosaics in south Brazil. Ecol Modell 2014. [DOI: 10.1016/j.ecolmodel.2014.07.024] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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59
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Ursino N. Eco-hydrology driven fire regime in savanna. J Theor Biol 2014; 355:68-76. [PMID: 24727188 DOI: 10.1016/j.jtbi.2014.04.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2013] [Revised: 03/27/2014] [Accepted: 04/01/2014] [Indexed: 11/28/2022]
Abstract
Fire is an important evolutionary force and ecosystem consumer that shapes savanna composition. However, ecologists have not comprehensively explained the functioning and maintenance of flammable savannas. A new minimal model accounting for the interdependence between soil saturation, biomass growth, fuel availability and fire has been used to predict the increasing tree density and fire frequency along a Mean Annual Rainfall (MAR) gradient for a typical savanna. Cyclic fire recurrence is reproduced using a predator prey approach in which fire is the "predator" and vegetation is the "prey". For the first time, fire frequency is not defined a priori but rather arises from the composition of vegetation, which determines fuel availability and water limitation. Soil aridity affects fuel production and fuel composition, thus indirectly affecting the ecosystem vulnerability to fire and fire frequency. The model demonstrates that two distinct eco-hydrological states correspond to different fire frequencies: (i) at low MAR, grass is abundant and the impact of fire on the environment is enhanced by the large fuel availability, (ii) at higher MAR, tree density progressively increases and provides less fuel for fire, leading to more frequent and less destructive fires, and (iii) the threshold MAR that determines the transition between the two states and the fire frequency at high MAR are affected by the vulnerability of trees to grass fire. The eco-hydrology-driven predator-prey model originally predicts that the transition between dry and wet savanna is characterized by a shift in wildfire frequency driven by major differences in soil moisture available for plants and savanna structure. The shift and the role of fire in conserving savanna ecosystems could not have been predicted if fire was considered as an external forcing rather than an intrinsic property of the ecosystem.
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Affiliation(s)
- Nadia Ursino
- Department ICEA, Universita di Padova, via Loredan 20, I 35131 Padova, Italy.
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60
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van Nes EH, Hirota M, Holmgren M, Scheffer M. Tipping points in tropical tree cover: linking theory to data. GLOBAL CHANGE BIOLOGY 2014; 20:1016-21. [PMID: 24106057 DOI: 10.1111/gcb.12398] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/27/2013] [Revised: 08/29/2013] [Accepted: 09/11/2013] [Indexed: 05/26/2023]
Abstract
It has recently been found that the frequency distribution of remotely sensed tree cover in the tropics has three distinct modes, which seem to correspond to forest, savanna, and treeless states. This pattern has been suggested to imply that these states represent alternative attractors, and that the response of these systems to climate change would be characterized by critical transitions and hysteresis. Here, we show how this inference is contingent upon mechanisms at play. We present a simple dynamical model that can generate three alternative tree cover states (forest, savanna, and a treeless state), based on known mechanisms, and use this model to simulate patterns of tree cover under different scenarios. We use these synthetic data to show that the hysteresis inferred from remotely sensed tree cover patterns will be inflated by spatial heterogeneity of environmental conditions. On the other hand, we show that the hysteresis inferred from satellite data may actually underestimate real hysteresis in response to climate change if there exists a positive feedback between regional tree cover and precipitation. Our results also indicate that such positive feedback between vegetation and climate should cause direct shifts between forest and a treeless state (rather than through an intermediate savanna state) to become more likely. Finally, we show how directionality of historical change in conditions may bias the observed relationship between tree cover and environmental conditions.
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Affiliation(s)
- Egbert H van Nes
- Department of Aquatic Ecology and Water Quality Management, Wageningen University, P.O. Box 47, Wageningen, NL-6700 AA, The Netherlands
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61
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Schertzer E, Staver AC, Levin SA. Implications of the spatial dynamics of fire spread for the bistability of savanna and forest. J Math Biol 2014; 70:329-41. [PMID: 24570348 DOI: 10.1007/s00285-014-0757-z] [Citation(s) in RCA: 41] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2013] [Indexed: 11/29/2022]
Abstract
The role of fire in expanding the global distribution of savanna is well recognized. Empirical observations and modeling suggest that fire spread has a threshold response to fuel-layer continuity, which sets up a positive feedback that maintains savanna-forest bistability. However, modeling has so far failed to examine fire spread as a spatial process that interacts with vegetation. Here, we use simple, well-supported assumptions about fire spread as an infection process and its effects on trees to ask whether spatial dynamics qualitatively change the potential for savanna-forest bistability. We show that the spatial effects of fire spread are the fundamental reason that bistability is possible: because fire spread is an infection process, it exhibits a threshold response to fuel continuity followed by a rapid increase in fire size. Other ecological processes affecting fire spread may also contribute including temporal variability in demography or fire spread. Finally, including the potential for spatial aggregation increases the potential both for savanna-forest bistability and for savanna and forest to coexist in a landscape mosaic.
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Affiliation(s)
- E Schertzer
- UPMC Univ. Paris 06, Laboratoire de Probabilités et Modèles Aléatoires, CNRS UMR 7599, Paris, France,
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62
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Moncrieff GR, Scheiter S, Bond WJ, Higgins SI. Increasing atmospheric CO2 overrides the historical legacy of multiple stable biome states in Africa. THE NEW PHYTOLOGIST 2014; 201:908-915. [PMID: 24400901 DOI: 10.1111/nph.12551] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/20/2013] [Accepted: 09/15/2013] [Indexed: 06/03/2023]
Abstract
The dominant vegetation over much of the global land surface is not predetermined by contemporary climate, but also influenced by past environmental conditions. This confounds attempts to predict current and future biome distributions, because even a perfect model would project multiple possible biomes without knowledge of the historical vegetation state. Here we compare the distribution of tree- and grass-dominated biomes across Africa simulated using a dynamic global vegetation model (DGVM). We explicitly evaluate where and under what conditions multiple stable biome states are possible for current and projected future climates. Our simulation results show that multiple stable biomes states are possible for vast areas of tropical and subtropical Africa under current conditions. Widespread loss of the potential for multiple stable biomes states is projected in the 21st Century, driven by increasing atmospheric CO2 . Many sites where currently both tree-dominated and grass-dominated biomes are possible become deterministically tree-dominated. Regions with multiple stable biome states are widespread and require consideration when attempting to predict future vegetation changes. Testing for behaviour characteristic of systems with multiple stable equilibria, such as hysteresis and dependence on historical conditions, and the resulting uncertainty in simulated vegetation, will lead to improved projections of global change impacts.
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Affiliation(s)
- Glenn R Moncrieff
- Institute for Physical Geography, Goethe University Frankfurt am Main, Altenhöferallee 1, 60438, Frankfurt am Main, Germany
| | - Simon Scheiter
- Biodiversity and Climate Research Centre, Senckenberg Research Institute and Natural History Museum, Senckenberganlage 25, 60325, Frankfurt am Main, Germany
| | - William J Bond
- Department of Biological Sciences, University of Cape Town, Private Bag, Rondebosch, 7701, Cape Town, South Africa
| | - Steven I Higgins
- Department of Botany, University of Otago, PO Box 56, Dunedin, 9054, New Zealand
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63
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de L. Dantas V, Batalha MA, Pausas JG. Fire drives functional thresholds on the savanna–forest transition. Ecology 2013; 94:2454-63. [DOI: 10.1890/12-1629.1] [Citation(s) in RCA: 130] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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64
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Støen OG, Okullo P, Eid T, Moe SR. Termites facilitate and ungulates limit savanna tree regeneration. Oecologia 2012; 172:1085-93. [PMID: 23269321 DOI: 10.1007/s00442-012-2561-z] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2011] [Accepted: 11/30/2012] [Indexed: 10/27/2022]
Abstract
Both large herbivores and termites are key functional groups in savanna ecosystems, and in many savanna areas, large termite mounds (termitaria) are associated with distinct woody clusters. Studies on the effect of large mammals on tree regeneration are few, and the results are conflicting. Large herbivores have been found to be important seedling predators in some areas, but facilitate tree regeneration by outcompeting small mammals and reducing grass cover in other areas. Through the use of the experimental fencing of termite mounds and adjacent savanna areas in this study, we investigated how termites and large herbivores influence tree regeneration. Termite mounds had a higher number of seedlings, more species richness, more alpha diversity (OD) and lower evenness (E) than savanna plots. Large herbivores did not significantly affect overall seedling density, species richness, OD or E. Beta diversity was higher in savanna areas than on termitaria, and beta diversity decreased in savanna areas when herbivores were excluded. Herbivore exclusion increased the density of the 12 (40%) most common seedling species, representing 79% of all seedlings, and fenced plots had relatively taller seedlings than open plots. Thus, termites were the main determinants of tree regeneration in our study area, but large mammals regulated the most common species. Although our study confirms previous work suggesting that large herbivores affect tree regeneration, we found that termites were an even more important determinant. Termite impacts on tree regeneration deserve increased attention by savanna ecologists.
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Affiliation(s)
- Ole-Gunnar Støen
- Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, 1432, Ås, Norway,
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65
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Ward D, Wiegand K, Getzin S. Walter's two-layer hypothesis revisited: back to the roots! Oecologia 2012; 172:617-30. [PMID: 23266712 PMCID: PMC3679411 DOI: 10.1007/s00442-012-2538-y] [Citation(s) in RCA: 74] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2012] [Accepted: 11/16/2012] [Indexed: 11/26/2022]
Abstract
Walter (Jahrb Wiss Bot 87:750–860, 1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has been a prominent part of the savanna literature since first proposed. We review the literature on Walter’s hypothesis and reconsider his original intentions. Walter intended this hypothesis to be restricted to dry savannas. In his opinion, mesic and humid savannas were controlled by biotic factors and disturbances. We surveyed the global savanna literature for records of vertical niche partitioning by grasses and trees. We find that, within the scope of Walter’s original intentions, this hypothesis works remarkably well, and in some cases is appropriate for deserts as well as for dry temperate systems and even some mesic savannas.
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Affiliation(s)
- David Ward
- School of Life Sciences, University of KwaZulu-Natal, Scottsville, South Africa.
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66
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Abstract
Although the boreal region is warming twice as fast as the global average, the way in which the vast boreal forests and tundras may respond is poorly understood. Using satellite data, we reveal marked alternative modes in the frequency distributions of boreal tree cover. At the northern end and at the dry continental southern extremes, treeless tundra and steppe, respectively, are the only possible states. However, over a broad intermediate temperature range, these treeless states coexist with boreal forest (∼75% tree cover) and with two more open woodland states (∼20% and ∼45% tree cover). Intermediate tree covers (e.g., ∼10%, ∼30%, and ∼60% tree cover) between these distinct states are relatively rare, suggesting that they may represent unstable states where the system dwells only transiently. Mechanisms for such instabilities remain to be unraveled, but our results have important implications for the anticipated response of these ecosystems to climatic change. The data reveal that boreal forest shows no gradual decline in tree cover toward its limits. Instead, our analysis suggests that it becomes less resilient in the sense that it may more easily shift into a sparse woodland or treeless state. Similarly, the relative scarcity of the intermediate ∼10% tree cover suggests that tundra may shift relatively abruptly to a more abundant tree cover. If our inferences are correct, climate change may invoke massive nonlinear shifts in boreal biomes.
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