School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemLandscape-scale disturbance history and the dynamics of the conifer taxa of Bidoup–Nuiba National Park, Vietnam( 2023-03)Tropical forests are complex and poorly understood ecosystems. In recent decades large-scale, long-term forest dynamics plots have been providing important insights into basic demographic processes such as recruitment, growth, and mortality. However, this information offers little insight into long-term forest dynamics and the role of infrequent disturbances that may occur at the scale of decades to centuries. Understanding how species-rich tropical forests will respond to global change requires understanding these long-term dynamics. Dendroecology, the study of tree rings, has provided the foundation for understanding forest dynamics in temperate forests in the northern hemisphere and parts of the southern hemisphere. It has been little applied in the tropics where most tree species do not have annual growth rings. The overarching aim of this study was to better understand the ecology and dynamics of Vietnam’s Central Highlands, and, in particular, the rich conifer taxa that are found there. To address this, my thesis involves the application of dendroecological approaches to: (1) reconstruct historical fire regimes in the Central Highlands of Vietnam; (2) understand the historical dynamics of Pinus kesiya forests and (3) the spatial and temporal patterns of recruitment and growth of the endemic conifer taxa in mixed conifer-angiosperm forests, and (4) identify angiosperm species with significant potential for future tree-ring based studies. A detailed reconstruction of how disturbances have varied across the Central Highlands landscape over recent centuries will provide important insights into the response of these forests to past disturbances and climate variability, as well as potential future trajectories of development under a changing climate. My thesis presents one of the most comprehensive reconstructions of fire in tropical forests using tree rings of Pinus kesiya in the Bidoup Nuiba National Park (BNNP) in the Central Highlands of Vietnam. A tree ring-based fire reconstruction showed that fire occurrence increased after 1900 and was highly correlated with climate indices (ENSO and PDSI) during the period 1900 – 1960. However, after 1964 the relationship between climate and fire disappeared due to the overwhelming pressure of human ignition sources, which have effectively eliminated climate as a factor limiting fire in these landscapes. While climate change is seen as a potential driver of changing fire regimes in many parts of the world, the role of local human populations may be equally or more important in shaping them. A detailed history of fire in these forests provides a baseline against which recent and future changes can be assessed. Dendrochronological analysis also indicated that extreme droughts, fire, and canopy disturbance have been associated with tree regeneration, growth, and canopy recruitment for centuries. The age distributions of Pinus dalatensis, Keteleeria evelyniana and Fokienia hodginsii show a distinct pulse of recruitment during the 1750s and during the period 1800-1850 that consistently coincided with extreme regional droughts. Periods of sudden and sustained growth release amongst these endemic conifers were associated with regional mega-droughts in the late 18th and 19th centuries. Results from superposed epoch analysis between Palmer Drought Severity Index and discrete tree release events indicated that extreme droughts and abrupt changes in moisture availability may have contributed to increased canopy disturbance rates and tree-level mortality. The effects of drought on tree growth were typically recorded in the tree rings 1-5 years after the drought event. This indicated that extreme drought conditions may drive large-scale canopy disturbances in BNNP. My results also confirmed that growth releases occurred within ~5 years of recorded fire events. This relationship between growth releases and fire events is consistent over 250 years of tree-ring records (1770-2020). Finally, my thesis demonstrated that a number of the angiosperm tree species occur in BNNP have significant potential for tree-ring studies. This is the first study to systematically examine the potential of a large number of angiosperm species for dendrochronological study in Vietnam. Demographic information derived from tree-rings of angiosperms could provide well-dated, long-term data that complements the tree-ring record from the conifers at BNNP. This would further advance our understanding of forest dynamics and the impacts of climate change in these species-rich tropical landscapes.