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David Ward - One of the best experts on this subject based on the ideXlab platform.
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Walter’s two-layer hypothesis revisited: back to the roots!
Oecologia, 2013Co-Authors: David Ward, Kerstin Wiegand, Stephan GetzinAbstract: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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Direct and indirect effects of termites on savanna tree-seedling growth
Plant Ecology, 2013Co-Authors: Kayleigh Muller, David WardAbstract:Termites are considered to be ecosystem engineers because they modify their biophysical environments. We tested the effects of soil-nutrient alteration in termite-enriched soils compared with termite-free open Savannas. We also tested whether non-nutrient alterations (soil disturbance) induced by termites led to changes in tree-seedling growth. Soil samples taken from termite-enriched soils and adjacent open savanna sites in KwaZulu-Natal, South Africa were analyzed for nitrogen, pH, organic carbon and water-holding capacity. Seeds from three dominant tree species, Acacia sieberiana , Celtis africana and Ziziphus mucronata , were grown in soils taken from termite-enriched soils and adjacent Savannas. Overall, organic carbon and nitrogen content were higher in termite-enriched soils than in adjacent Savannas. We found that these differences in nutrients did not directly affect seedling growth rates or final height. However, C. africana had increased growth rates in compacted termite-enriched soils, while A. sieberiana seedlings were taller in non-compacted soils. We conclude that the indirect effects of disturbance by termites may be as important as the direct effects of increased nutrients for growth of savanna trees.
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Patch dynamics integrate mechanisms for savanna tree–grass coexistence
Basic and Applied Ecology, 2009Co-Authors: Katrin M. Meyer, Kerstin Wiegand, David WardAbstract:Abstract Many mechanisms have been suggested to explain the coexistence of woody species and grasses in Savannas. However, evidence from field studies and simulation models has been mixed. Patch dynamics is a potentially unifying mechanism explaining tree–grass coexistence and the natural occurrence of shrub encroachment in arid and semi-arid Savannas. A patch-dynamic savanna consists of a spatial mosaic of patches. Each patch maintains a cyclical succession between dominance of woody species and grasses, and the succession of neighbouring patches is temporally asynchronous. Evidence from empirical field studies supports the patch dynamics view of Savannas. As a basis for future tests of patch dynamics in Savannas, several hypotheses are presented and one is exemplarily examined: at the patch scale, realistically parameterized simulation models have generated cyclical succession between woody and grass dominance. In semi-arid Savannas, cyclical successions are driven by precipitation conditions that lead to mass recruitment of shrubs in favourable years and to simultaneous collapse of shrub cohorts in drought years. The spatiotemporal pattern of precipitation events determines the scale of the savanna vegetation mosaic in space and time. In a patch-dynamic savanna, shrub encroachment is a natural, transient phase corresponding to the shrub-dominated phase during the successional cycle. Hence, the most promising management strategy for encroached areas is a large-scale rotation system of rangelands. In conclusion, patch dynamics is a possible scale-explicit mechanism for the explanation of tree–grass coexistence in Savannas that integrates most of the coexistence mechanisms proposed thus far for Savannas.
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a patch dynamics approach to savanna dynamics and woody plant encroachment insights from an arid savanna
Perspectives in Plant Ecology Evolution and Systematics, 2006Co-Authors: David Saltz, Kerstin Wiegand, David WardAbstract:Abstract The coexistence of woody and grassy plants in Savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many Savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment. We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid Savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid Savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated Savannas, in which rainfall variability is low and fire drives spatial heterogeneity. We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different Savannas.
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A patch-dynamics approach to savanna dynamics and woody plant encroachment – Insights from an arid savanna
Perspectives in Plant Ecology Evolution and Systematics, 2006Co-Authors: Kerstin Wiegand, David Saltz, David WardAbstract:Abstract The coexistence of woody and grassy plants in Savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many Savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment. We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid Savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid Savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated Savannas, in which rainfall variability is low and fire drives spatial heterogeneity. We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different Savannas.
Kerstin Wiegand - One of the best experts on this subject based on the ideXlab platform.
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Walter’s two-layer hypothesis revisited: back to the roots!
Oecologia, 2013Co-Authors: David Ward, Kerstin Wiegand, Stephan GetzinAbstract: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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Patch dynamics integrate mechanisms for savanna tree–grass coexistence
Basic and Applied Ecology, 2009Co-Authors: Katrin M. Meyer, Kerstin Wiegand, David WardAbstract:Abstract Many mechanisms have been suggested to explain the coexistence of woody species and grasses in Savannas. However, evidence from field studies and simulation models has been mixed. Patch dynamics is a potentially unifying mechanism explaining tree–grass coexistence and the natural occurrence of shrub encroachment in arid and semi-arid Savannas. A patch-dynamic savanna consists of a spatial mosaic of patches. Each patch maintains a cyclical succession between dominance of woody species and grasses, and the succession of neighbouring patches is temporally asynchronous. Evidence from empirical field studies supports the patch dynamics view of Savannas. As a basis for future tests of patch dynamics in Savannas, several hypotheses are presented and one is exemplarily examined: at the patch scale, realistically parameterized simulation models have generated cyclical succession between woody and grass dominance. In semi-arid Savannas, cyclical successions are driven by precipitation conditions that lead to mass recruitment of shrubs in favourable years and to simultaneous collapse of shrub cohorts in drought years. The spatiotemporal pattern of precipitation events determines the scale of the savanna vegetation mosaic in space and time. In a patch-dynamic savanna, shrub encroachment is a natural, transient phase corresponding to the shrub-dominated phase during the successional cycle. Hence, the most promising management strategy for encroached areas is a large-scale rotation system of rangelands. In conclusion, patch dynamics is a possible scale-explicit mechanism for the explanation of tree–grass coexistence in Savannas that integrates most of the coexistence mechanisms proposed thus far for Savannas.
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a patch dynamics approach to savanna dynamics and woody plant encroachment insights from an arid savanna
Perspectives in Plant Ecology Evolution and Systematics, 2006Co-Authors: David Saltz, Kerstin Wiegand, David WardAbstract:Abstract The coexistence of woody and grassy plants in Savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many Savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment. We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid Savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid Savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated Savannas, in which rainfall variability is low and fire drives spatial heterogeneity. We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different Savannas.
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A patch-dynamics approach to savanna dynamics and woody plant encroachment – Insights from an arid savanna
Perspectives in Plant Ecology Evolution and Systematics, 2006Co-Authors: Kerstin Wiegand, David Saltz, David WardAbstract:Abstract The coexistence of woody and grassy plants in Savannas has often been attributed to a rooting-niche separation (two-layer hypothesis). Water was assumed to be the limiting resource for both growth forms and grasses were assumed to extract water from the upper soil layer and trees and bushes from the lower layers. Woody plant encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many Savannas and is believed to be the result of overgrazing in ‘two-layer systems’. Recent research has questioned the universality of both the two-layer hypothesis and the hypothesis that overgrazing is the cause of woody plant encroachment. We present an alternative hypothesis explaining both tree–grass coexistence and woody plant encroachment in arid Savannas. We propose that woody plant encroachment is part of a cyclical succession between open savanna and woody dominance and is driven by two factors: rainfall that is highly variable in space and time, and inter-tree competition. In this case, savanna landscapes are composed of many patches (a few hectares in size) in different states of transition between grassy and woody dominance, i.e. we hypothesize that arid Savannas are patch-dynamic systems. We summarize patterns of tree distribution observed in an arid savanna in Namibia and show that these patterns are in agreement with the patch-dynamic savanna hypothesis. We discuss the applicability of this hypothesis to fire-dominated Savannas, in which rainfall variability is low and fire drives spatial heterogeneity. We conclude that field studies are more likely to contribute to a general understanding of tree–grass coexistence and woody plant encroachment if they consider both primary (rain and nutrients) and secondary (fire and grazing) determinants of patch properties across different Savannas.
William J. Bond - One of the best experts on this subject based on the ideXlab platform.
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the savanna grassland treeline why don t savanna trees occur in upland grasslands
Journal of Ecology, 2012Co-Authors: Julia L. Wakeling, Michael D. Cramer, William J. BondAbstract:Summary 1. Treeless grasslands with climates that can support tree growth are common in upland regions around the world. In South Africa, the upland grasslands are adjacent to lowland Savannas in many areas, with an abrupt boundary between them that could be termed a savanna-grassland ‘treeline’. Both systems are dominated by C4 grasses and burn regularly, yet fire-tolerant savanna trees do not survive in the grasslands. The upland grasslands experience lower temperatures throughout the year and frost in winter, compared with the warmer Savannas. 2. We tested whether frost in the dormant season or slow growth in the growing season in conjunction with frequent fires may explain the tree-less state of grasslands. We measured Acacia seedling growth for a year in a transplant experiment at ten sites across an altitudinal gradient (42–1704 m) from Savannas to grasslands. The effect of frost on seedlings was scored during the following winter. 3. Across all species, height (t = −6.04, d.f. = 471, P < 0.001), biomass (t = −4.56, d.f. = 228, P < 0.001) and height increase (t = −3.40, d.f. = 471, P < 0.001) were significantly higher at savanna sites. As the plants were irrigated and initially supplied with nutrients, the main factor affecting growth was likely to be growing season temperature. 4. Saplings that experience slow growing conditions will take longer to reach a height above the flame zone and will therefore have a lower probability of reaching adult tree height and surviving fires. Day length may be the most important cue for the end of the growing season in savanna trees, as growth decreased with shortening day length in February–March while temperatures were still high and plants were not water limited. 5. Synthesis. Savanna trees grew more slowly in cooler upland grassland sites compared with lower elevation warm savanna sites and, under frequent fire regimes, would be prevented from reaching maturity. This may be true globally for similar grasslands where tree growth can occur and could partly explain the lack of trees in grasslands.
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increased tree densities in south african Savannas 50 years of data suggests co2 as a driver
Global Change Biology, 2012Co-Authors: Robert Buitenwerf, William J. Bond, N Stevens, W.s.w. TrollopeAbstract:For the past century, woody plants have increased in grasslands and Savannas worldwide. Woody encroachment may significantly alter ecosystem functioning including fire regimes, herbivore carrying capacity, biodiversity and carbon storage capacity. Traditionally, increases in woody cover and density have been ascribed to changes in the disturbance regime (fire and herbivores) or rainfall. Increased atmospheric CO2 concentrations may also contribute, by increasing growth rates of trees relative to grasses. This hypothesis is still heavily debated because usually potential CO2 effects are confounded by changes in land use (disturbance regime). Here we analyse changes in woody density in fire experiments at three sites in South African Savannas where the disturbance regime (fire and herbivores) was kept constant for 30 and 50 years. If global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime. Woody density remained constant in a semiarid savanna but tripled in a mesic savanna between the 1970s and 1990s. At the third site, a semiarid savanna near the southern limits of the biome, tree density doubled from the mid 1990s to 2010. Interpretation of the causes is confounded by population recovery after clearing, but aerial photograph analysis on adjacent non-cleared areas showed an accompanying 48% increase in woody cover. Increased CO2 concentrations are consistent with increased woody density while other global drivers (rainfall) remained constant over the duration of the experiments. The absence of a response in one semiarid savanna could be explained by a smaller carbon sink capacity of the dominant species, which would therefore benefit less from increased CO2. Understanding how Savannas and grasslands respond to increased CO2 and identifying the causes of woody encroachment are essential for the successful management of these systems.
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The savanna‐grassland ‘treeline’: why don’t savanna trees occur in upland grasslands?
Journal of Ecology, 2011Co-Authors: Julia L. Wakeling, Michael D. Cramer, William J. BondAbstract:Summary 1. Treeless grasslands with climates that can support tree growth are common in upland regions around the world. In South Africa, the upland grasslands are adjacent to lowland Savannas in many areas, with an abrupt boundary between them that could be termed a savanna-grassland ‘treeline’. Both systems are dominated by C4 grasses and burn regularly, yet fire-tolerant savanna trees do not survive in the grasslands. The upland grasslands experience lower temperatures throughout the year and frost in winter, compared with the warmer Savannas. 2. We tested whether frost in the dormant season or slow growth in the growing season in conjunction with frequent fires may explain the tree-less state of grasslands. We measured Acacia seedling growth for a year in a transplant experiment at ten sites across an altitudinal gradient (42–1704 m) from Savannas to grasslands. The effect of frost on seedlings was scored during the following winter. 3. Across all species, height (t = −6.04, d.f. = 471, P
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When is a ‘forest’ a savanna, and why does it matter?
Global Ecology and Biogeography, 2011Co-Authors: Jayashree Ratnam, William J. Bond, Rod Fensham, William A. Hoffmann, Sally Archibald, Caroline E. R. Lehmann, Michael T. Anderson, Steven I. Higgins, Mahesh SankaranAbstract:Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the high-rainfall end of the tropical savanna biome, where heavily wooded mesic Savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble Savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic Savannas are predominantly mixed tree–C4 grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C4 grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life-history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire-resistant tree species in a C4 grass-dominated understorey, would be better classified as mesic Savannas requiring fire and light to maintain the unique mix of species that characterize them.
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effects of four decades of fire manipulation on woody vegetation structure in savanna
Ecology, 2007Co-Authors: Steven I. Higgins, William J. Bond, Edmund C February, Andries Bronn, Douglas I W Eustonbrown, Beukes Enslin, Navashni Govender, Louise Rademan, Sean Oregan, A L F PotgieterAbstract:The amount of carbon stored in Savannas represents a significant uncertainty in global carbon budgets, primarily because fire causes actual biomass to differ from potential biomass. We analyzed the structural response of woody plants to long-term experimental burning in Savannas. The experiment uses a randomized block design to examine fire exclusion and the season and frequency of burn in 192 7-ha experimental plots located in four different savanna ecosystems. Although previous studies would lead us to expect tree density to respond to the fire regime, our results, obtained from four different savanna ecosystems, suggest that the density of woody individuals was unresponsive to fire. The relative dominance of small trees was, however, highly responsive to fire regime. The observed shift in the structure of tree populations has potentially large impacts on the carbon balance. However, the response of tree biomass to fire of the different Savannas studied were different, making it difficult to generalize about the extent to which fire can be used to manipulate carbon sequestration in Savannas. This study provides evidence that Savannas are demographically resilient to fire, but structurally responsive.
Augustine O. Isichei - One of the best experts on this subject based on the ideXlab platform.
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Stocks of Nitrogen in Vegetation and Soil in West African Moist Savannas and Potential Effects of Climate Change and Land Use on These Stocks
Journal of Biogeography, 1995Co-Authors: Augustine O. IsicheiAbstract:Moist Savannas which include undifferentiated moist woodlands and Savannas with abundant Isoberlinia dokal and L tomentosa (also known as Sudanian woodlands and Guinea-Congolian secondary grassland and wooded grassland) extend across West Africa up to the Central African Republic. Nitrogen stocks in the standing woody vegetation, litter and soil as well as the amounts of nitrogen transferred between these stocks and that lost through savanna burning of woodland and open moist savanna types are presented. Most of the total nitrogen is in the soil and then in the woody vegetation, with the least amount in the herbaceous vegetation.
Valentí Rull - One of the best experts on this subject based on the ideXlab platform.
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Holocene global warming and the origin of the Neotropical Gran Sabana in the Venezuelan Guayana
Journal of Biogeography, 2007Co-Authors: Valentí RullAbstract:Aim The assumedly anomalous occurrence of Savannas and forest–savanna mosaics in the Gran Sabana – a neotropical region under a climate more suitable for tropical rain forests – has been attributed to a variety of historical, climatic, and anthropogenic factors. This paper describes a previously undocumented shift in vegetation and climate that occurred during the early Holocene, and evaluates its significance for the understanding of the origin of the Gran Sabana vegetation. Location A treeless savanna locality of the Gran Sabana (4°30′–6°45′ N and 60°34′–62°50′ W), in the Venezuelan Guayana of northern South America, at the headwaters of the Caroni river, one of the major tributaries of the Orinoco river. Methods Pollen and charcoal analysis of a previously dated peat section spanning from about the Pleistocene/Holocene boundary until the present. Results Mesothermic cloud forests dominated by Catostemma (Bombacaceae) occupied the site around the Pleistocene/Holocene boundary. During the early Holocene, a progressive but relatively rapid trend towards savanna vegetation occurred, and eventually the former cloud forests were replaced by a treeless savanna. Some time after the establishment of Savannas, a marked increase in charcoal particles indicates the occurrence of the first local fires. Main conclusions The occurrence of cloud forests at the Pleistocene/Holocene boundary contradicts the historical hypothesis according to which the Gran Sabana is a relict of the hypothetical widespread Savannas that have been assumed to have dominated the region during the last glaciation. The first local fires recorded in the Holocene were on savanna vegetation, which is against the hypothesis of fire as the triggering factor for the establishment of these Savannas. Climate change, in the form of global warming and a persistently drier climate, emerges as the most probable cause for the forest–savanna turnover.
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A palynological record of a secondary succession after fire in the Gran Sabana, Venezuela
Journal of Quaternary Science, 1999Co-Authors: Valentí RullAbstract:Fire has been considered one of the most important factors in the expansion of Savannas in the Gran Sabana region. In Urue, an important fire event that occurred before 1.6 kyr BP led to the replacement of ‘primary’ forests by Savannas and morichales (monospecific communities of the palm Mauritia). In the present work, the secondary succession after fire is reconstructed by palynological analysis of a previously dated clay core, and the results are compared with those from studies based on present-day ecosystems. Charcoal analysis is used to infer fire incidence and surface samples are used as modern analogues. The secondary succession is subdivided into seven seral stages: open secondary forests, helechal or dense fern community, transitional savanna, wet savanna with morichales, treeless savanna, wet savanna with morichales, and treeless savanna. Fires were common at the beginning, but climate constituted the main successional control from the transitional savanna stage onwards. The process is characterised by a continuous impoverishment of taxa, but there was a steady increase in Mauritia, due to its ability to colonise new habitats created by disturbances. The conclusions of this palynological reconstruction show good correspondence with present-day studies on fire ecology. Copyright © 1999 John Wiley & Sons, Ltd.
