DEGEO - Departamento de Geologia

URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8

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2020 - 20223

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Autor: search.filters.author.Ledru, Marie-PierreTem arquivos: Sim

Resultados da Pesquisa

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    Estimated degradation of the Caatinga based on modern pollen rain deposited in reservoirs.
    (2022) Ledru, Marie-Pierre; Cassino, Raquel Franco; Gomes, Vaneicia dos Santos; Sfair, Julia Caram; Araújo, Francisca Soares de
    The vegetation cover is the result of many biological and abiotic interactions. To identify the different factors that cause changes is crucial when defining future sustainable development and protection of natural resources. In the Brazilian semiarid region, the vegetation cover has been subjected to drastic deforestation and land use at centennial-scale that has led to desertification. Pollen analysis is an efficient tool to reconstruct the different processes of degradation of the vegetation cover over time. We built a referential data set for the vegetation cover using 48 pollen surface samples collected in the reservoirs of the Ceará. We used satellite images for comparison with the pollen signatures and defined an alteration score to express the correlation between terrestrial pollen and anthropic cover. Our results showed our surface samples to be generally representative of the vegetation cover and of the general degradation of the landscape. Our study areas can be considered as degraded as the initial categories “preserved” or “intermediary” are not reflected in the pollen assemblages, in agreement with results of botanical surveys and the historical background. The on-going process of desertification is climate-independent and was initiated many decades or centuries ago by intensive land use for agriculture and grazing.
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    Quantitative reconstruction of vegetation cover from modern pollen rain in the Cerrado biome of Brazil.
    (2021) Cassino, Raquel Franco; Ledru, Marie-Pierre
    Fossil pollen assemblages are an important tool to reconstruct vegetation variability at centennial and millennial- scales. However, in the Cerrado biome of Brazil, the influence of the local vegetation, the heterogeneity of the landscape and the accuracy of pollen identification leads to a wide margin of error in interpretation. In this study, we present a method of quantitative reconstruction of past vegetation cover (forest, savanna, and grassland) that accounts for these different aspects. Our method is based on analysis of 47 surface samples collected in the southern Cerrado biome (east-central Brazil). First, we defined criteria that can be used to identify the local ecosystem (lake, marsh, palm swamp, or gallery forest) represented by the pollen assemblages. The pollen as- semblages were then compared to data extracted from vegetation maps, resulting in the assessment of three pollen score equations related to each of the vegetation types (forest, savanna, and grassland). Based on these scores, we developed linear regression models that can be used to reconstruct the proportion of each type of land cover from pollen assemblages. We compared the results obtained for three different areas (respectively with 300 m, 500 m, and 1 km radius) to evaluate the size of the pollen source area for the Cerrado sites. We apply our models to two fossil pollen records to illustrate their use and application. Our method for the quantitative reconstruction of past land cover in the Cerrado biome is a new tool to investigate climate drivers and the long- term effects of fire in this Neotropical savanna.
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    Vegetation and fire variability in the central Cerrados (Brazil) during the Pleistocene-Holocene transition was influenced by oscillations in the SASM boundary belt.
    (2020) Cassino, Raquel Franco; Ledru, Marie-Pierre; Santos, Rudney de Almeida; Favier, Charly
    This study investigates historical fire regimes and arboreal cover variability in the Brazilian Cerrados, a large Neotropical savanna ecosystem, during the Pleistocene-Holocene transition and early Holocene, and then tests whether the observed variation is linked to South American Summer Monsoon (SASM) variability. We present high-resolution pollen, XRF and charcoal records from Lagoa Feia, located in central Cerrados, and assess how they compare with other pollen records from the surrounding region to investigate regional trends of vegetation and fire regime variability. Our results show that the Pleistocene-Holocene transition was marked by a wet episode, which included moist forest expansion and rising lake levels, that correlates well with increased monsoon activity in a large area of central South America during the same period. This wet episode chronologically coincides with the Younger Dryas cold episode in the northern hemisphere. Our data revealed moisture declines in central Cerrados after 11.2 kyr BP, along with centennial-scale fluctuations from dry to wet conditions throughout the early Holocene period until around 6 kyr BP. These dry/wet oscillations are associated with weakened SASM activity and repeated shifts of its belt position during this time range. Fire activity increased in central Cerrados just after the onset of drier conditions during the early Holocene, and likely contributed to decreased arboreal cover at that time. A trend of increasing moisture in the region was observed after 6 kyr BP. Our study reveals how centennial and millennial-scale changes in monsoon activity influenced arboreal cover, diversity and fire regimes in the central Cerrados.