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Universität Potsdam (2021)

A palaeoecological approach to savanna dynamics and shrub encroachment in Namibia

Ximena del Carmen Tabares Jimenez

Titre : A palaeoecological approach to savanna dynamics and shrub encroachment in Namibia

Auteur : Ximena del Carmen Tabares Jimenez

Université de soutenance : Universität Potsdam

Grade : Doctor rerum naturalium 2021 (Dr. rer. nat.) 2021

Résumé partiel
The spread of shrubs in Namibian savannas raises questions about the resilience of these ecosystems to global change. This makes it necessary to understand the past dynamics of the vegetation, since there is no consensus on whether shrub encroachment is a new phenomenon, nor on its main drivers. However, a lack of long-term vegetation datasets for the region and the scarcity of suitable palaeoecological archives, makes reconstructing past vegetation and land cover of the savannas a challenge. To help meet this challenge, this study addresses three main research questions : 1) is pollen analysis a suitable tool to reflect the vegetation change associated with shrub encroachment in savanna environments ? 2) Does the current encroached landscape correspond to an alternative stable state of savanna vegetation ? 3) To what extent do pollen-based quantitative vegetation reconstructions reflect changes in past land cover ? The research focuses on north-central Namibia, where despite being the region most affected by shrub invasion, particularly since the 21st century, little is known about the dynamics of this phenomenon. Field-based vegetation data were compared with modern pollen data to assess their correspondence in terms of composition and diversity along precipitation and grazing intensity gradients. In addition, two sediment cores from Lake Otjikoto were analysed to reveal changes in vegetation composition that have occurred in the region over the past 170 years and their possible drivers. For this, a multiproxy approach (fossil pollen, sedimentary ancient DNA (sedaDNA), biomarkers, compound specific carbon (δ13C) and deuterium (δD) isotopes, bulk carbon isotopes (δ13Corg), grain size, geochemical properties) was applied at high taxonomic and temporal resolution. REVEALS modelling of the fossil pollen record from Lake Otjikoto was run to quantitatively reconstruct past vegetation cover. For this, we first made pollen productivity estimates (PPE) of the most relevant savanna taxa in the region using the extended R-value model and two pollen dispersal options (Gaussian plume model and Lagrangian stochastic model). The REVEALS-based vegetation reconstruction was then validated using remote sensing-based regional vegetation data.

Mots clés  : Pollen ; Pollenproduktivitätsschätzungen ; REVEALS ; Savannen-Ökologie ; Zustands-Übergangs-Modelle ; sedimentäre alte DNA


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