Search

Abstract: The southern Gran Sabana (SE Venezuela) holds a particular type of neotropical savanna characterized by the local occurrence of morichales (Mauritia palm swamps), in a climate apparently more suitable for rain forests. We present a paleoecological analysis of the last millennia of Lake Chonita (4°39′N–61°0′W, 884m elevation), based on biological and physico-chemical proxies. Savannas dominated the region during the last millennia, but a significant vegetation replacement occurred in recent times. The site was covered by a treeless savanna with nearby rainforests from 3640 to 2180cal yr BP. Water levels were higher than today until about 2800cal yr BP. Forests retreated since about 2180cal yr BP onwards, likely influenced by a higher fire incidence that facilitated a dramatic expansion of morichales. The simultaneous appearance of charcoal particles and Mauritia pollen around 2000cal yr BP supports the potential pyrophilous nature of this palm and the importance of fire for its recent expansion. The whole picture suggests human settlements similar to today – in which fire is an essential element – since around 2000yr ago. Therefore, present-day southern Gran Sabana landscapes seem to have been the result of the synergy between biogeographical, climatic and anthropogenic factors, mostly fire. [Copyright &y& Elsevier]

Abstract: Long-term palaeoecological records are needed to test ecological hypotheses involving time, as short-term observations are of insufficient duration to capture natural variability. In this paper, we review the published palaeoecological evidence for the neotropical Gran Sabana (GS) region, to record the vegetation dynamics and evaluate the potential effects of natural climatic and anthropogenic (notably fire) drivers of change. The time period considered (last 13,000 years) covers major global climate changes and the arrival of humans in the region. The specific points addressed are climate–vegetation equilibrium, reversibility of vegetation changes, the origin of extant biodiversity and endemism patterns and biodiversity conservation in the face of global warming. Vegetation dynamics is reconstructed by pollen analysis and fire incidence is deduced from microscopic charcoal records. Palaeoclimatic inferences are derived from global and regional records using independent physico-chemical evidence to avoid circular reasoning. After analyzing all the long-term records available from both GS uplands and highlands, we conclude that: (1) Upland vegetation (mostly treeless savannas and savanna–forest mosaics, with occasional Mauritia palm swamps) is not in equilibrium with the dominant climates, but largely conditioned by burning practices; (2) a hypothetical natural or “original” vegetation type for these uplands has not been possible to identify due to continuous changes in both climate and human activities during the last 13,000 years; (3) at the time scale studied (millennial), the shift from forest to savanna is abrupt and irreversible due to the existence of tipping points, no matter the cause (natural or anthropogenic); (4) on the contrary, the shift from savanna to palm swamps is reversible at centennial time scales; (5) some of the reconstructed past vegetation types have no modern analogues owing to the individual species response to environmental shifts, leading to variations in community composition; (6) extant biodiversity and endemism patterns are not the result of a long history of topographical isolation, as previously proposed but, rather, the consequence of the action of climatic and palaeogeographic variations; (7) the projected global warming will likely exacerbate the expansion of upland savannas by favouring positive fire-climate feedbacks; (8) in the highlands, extinction by habitat loss will likely affect biodiversity but to a less extent that prognosticated by models based only on present-day climatic features; (9) future highland communities will likely be different to present ones due to the prevalence of individual species responses to global warming; and (10) conservation strategies at individual species level, rather than at community level, are enriched by long-term palaeoecological studies analyzed here. None of these conclusions would have been possible to derive from short-term neoecological observations. [Copyright &y& Elsevier]

Abstract: This paper shows a Holocene paleoecological reconstruction based on a peat bog sequence (El Paují, 4°28′N–61°35′W, 865m elevation) located in the transition zone between the Gran Sabana (SE Venezuela) savannas and the Amazon rainforests. Paleoecological trends are based on the analysis of pollen and pteridophyte spores, algal and animal remains, fungal spores, and charcoal particles. The whole record embraces the last ca. 8000calyr BP, and was subdivided into five pollen zones, representing the following vegetation succession: savanna/rainforest mosaic (8250–7715yr BP), dense rainforests (7715–5040yr BP), savanna/rainforest mosaic (5040–2690yr BP), secondary dry forests (2690–1440yr BP), and peat bog in an open savanna landscape (1440yr BP–present). These vegetation changes have been attributed to the action of climate and/or land use changes, as well as the corresponding synergies between them. Fire has been determinant in the landscape evolution. Based on the reconstructed fire and vegetation shifts, a changing land use pattern could have been recognized. Between the early and the mid Holocene (ca. 8.3–5.0kyr BP), land use practices seem to have been more linked to shifting agriculture in a rainforest landscape – as is usual in Amazon cultures – with medium fire incidence affecting only local forest spots or surrounding savannas. More extensive forest burning was recorded between ca. 5.0 and 2.7kyr BP, followed by land abandonment and the dominance of drier climates between 2.7 and 1.4yr BP. The modern indigenous culture, which prefers open environments and makes extensive use of fire thus preventing forest re-expansion, seem to have established during the last 1500yr. Therefore, a significant cultural replacement has been proposed for the region, leading to the present-day situation. Changing human activities have been instrumental for ecological evolution in this savanna–rainforest transitional region, as well as for the shaping of modern landscapes. [Copyright &y& Elsevier]