Long-Term Impacts of Mid-Holocene Drier Climatic Conditions on Bolivian Tropical Dry Forests
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Long-term impacts of mid-Holocene drier climatic conditions on Bolivian tropical dry forests Doctor of Philosophy School of Archaeology, Geography and Environmental Science Heather Plumpton October 2019 1 Declaration I confirm that this is my own work and the use of all material from other sources has been properly and fully acknowledged. Signed Heather Plumpton 2 THESIS ABSTRACT Dry forests are the most threatened tropical biome in South America, with only 10% of their original cover remaining. Chapter 1 reviews literature on tropical dry forests and palaeoecology to demonstrate why the impact of the drier climatic conditions predicted for much of South America in the coming century on these forests is uncertain. This thesis uses the mid-Holocene (6000 years BP) drier climatic period to provide insight into the long-term ecological responses of tropical dry forest to drier conditions. One of the reasons for uncertainty over the impacts of the mid-Holocene drier climate on these forests is the lack of large lakes from which to draw palaeoecological records. The large lakes that do exist within the dry forest biome in Bolivia are situated in complex catchments with hydrological connections to tropical wetlands. In this thesis we therefore first develop our understanding of a relatively new palaeoecological proxy – phytoliths – to assist in our interpretation of the mid-Holocene palaeo-vegetation record. Chapter 2 demonstrates the palaeoecological potential of phytoliths from lake sediments, including the first empirical evidence that phytoliths from lakes most strongly represent local vegetation. Chapter 3 builds on these findings to investigate the sensitivity of phytoliths to vegetation changes within key Bolivian ecosystems, and compares it to pollen, showing that phytoliths are most sensitive to changes within ecosystems with strong herbaceous components such as forest understories. Chapter 4 uses the results of the previous chapters to investigate the response of the Bolivian dry forest to mid-Holocene drier climatic conditions, demonstrating resilience of the dry forest as a biome, with a shift in composition towards more drought-tolerant taxa. However, savannah encroachment did occur at the ecotone during the mid-Holocene, with fire identified as 3 potentially reinforcing the ecotone shift. The vulnerability of tropical dry forests to future climate change will therefore depend on complex interactions between climate, fire and human land-use. 4 ACKNOWLEDGMENTS Firstly, I would like to thank my primary supervisor, Frank Mayle, for introducing me to palaeo-ecology and the wonderful country of Bolivia in our fieldwork. His sustained support, patience and encouragement throughout this PhD have been invaluable. I would also like to thank my second supervisor, Bronwen Whitney, for her positive encouragement, inspiring me to think ‘outside the box’ in the last year of my PhD, and for introducing me to Newcastle beaches. I would also like to thank my third supervisor Jose Iriarte for introducing me to phytoliths and training me in their preparation and analysis. My particular thanks go to his PhD students at the time, Jenny Watling and Lautaro Hilbert, and post-doc Sheehan Bestal, for welcoming me into their friendly department at the University of Exeter and taking the time to teach me and answer my questions. My thanks also go to John Carson and Macarena Cardenas who trained me in tropical pollen and charcoal analysis and became friends as well as inspirational mentors. I am grateful for the whole Tropical Palaeoecology Research group for their support and friendship, particularly Richard Smith with whom I started my PhD journey. I thank the University of Reading for funding this PhD project and NERC for providing funding for the radiocarbon dates analysed at the NERC Radiocarbon facility, UK. I am thankful for the support of many wonderful friends, who have helped me through the inevitable ups and downs of PhD research. Thank you to the GES PhD students for being a constant source of support, encouragement, understanding, technical assistance and inspiration. I couldn’t have done my PhD without you all. In particular I am hugely grateful to 5 Harriet Robson, Aliki McDonald and Cat Pschenyckyj who have been a backbone of support for the last 4 years. My thanks also go to my friends from Cambridge whose love and encouragement has been invaluable to me over the last 4 years. My endless thanks go to Stephen Pates for his unwavering support and love, for inspiring me, for his ability to keep me calm and his willingness to share his expertise on figure re-sizing in moments of panic. I am also grateful to my family for their determined belief in me. My particular thanks go to my parents, for welcoming me back when I asked to moved home for the last three months of PhD write-up and supporting me throughout. 6 CONTENTS Thesis abstract .................................................................................................................................. 3 Acknowledgments ............................................................................................................................ 5 List of Figures .................................................................................................................................. 13 List of Tables ................................................................................................................................... 16 1. Chapter 1: General Introduction ........................................................................................... 18 1.1 Bolivian tropical dry forests .............................................................................................. 18 1.1.1 Threats to tropical dry forest .................................................................................... 20 1.2 Mid-Holocene drier climatic period in tropical South America ........................................ 24 1.3 Palaeoecological context .................................................................................................. 28 1.3.1 Ecotone sensitivity to mid-Holocene drier climate ................................................... 28 1.3.2 Dry forest sensitivity to mid-Holocene drier climate ................................................ 31 1.4 Phytoliths as a palaeoecological proxy ............................................................................. 32 1.4.1 Taxonomy .................................................................................................................. 34 1.4.2 Spatial scale ............................................................................................................... 37 1.5 Thesis aims ........................................................................................................................ 39 1.6 References ........................................................................................................................ 41 7 2 Chapter 2: Palaeoecological potential of phytoliths from lake sediment records from the tropical lowlands of Bolivia ................................................................................................................... 82 2.1 Preface .............................................................................................................................. 82 2.2 Abstract ............................................................................................................................. 85 2.2.1 Keywords ................................................................................................................... 85 2.3 Introduction ...................................................................................................................... 86 2.4 Study area and site selection ............................................................................................ 90 2.4.1 Chiquitania-Pantanal. ................................................................................................ 94 2.4.2 Beni basin. ................................................................................................................. 97 2.4.3 Terra firme humid evergreen forest. ........................................................................ 99 2.5 Methods .......................................................................................................................... 101 2.5.1 Use of previously published datasets ..................................................................... 101 2.5.2 New data analysis - laboratory methods – phytolith processing and identification 102 2.5.3 Numerical analysis .................................................................................................. 105 2.5.4 Spatial analyses - GIS ............................................................................................... 106 2.6 Results ............................................................................................................................. 106 2.6.1 Key trends in phytolith and pollen assemblages from each region ........................ 106 2.6.2 PCA results .............................................................................................................. 113 2.6.3 RDA results .............................................................................................................. 119 2.7 Discussion ........................................................................................................................ 122 8 2.7.1 Differentiating ecosystems