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.
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ItemDynamics of a flammable species in a forest landscape: A case study on forest wiregrass Tetrarrhena juncea R.Br.( 2022)Species abundance often determines the extent of influence of a species to ecosystem function and processes. Typically, the abundance of a species is constrained by environmental factors within its habitat. However, there are instances where native species becomes prolific and the shift in abundance greatly impacts the ecosystem. Such is the case when a flammable species becomes prolific within its range and alters the flammability of the ecosystem. This is a concern with climate change, as conditions might be tipped in favour of such species. Hence, it is crucial to understand the drivers of abundance to understand how native species can be released from environmental constraints of abundance to become prolific within their own range, and to predict the potential effect of changing environmental conditions on their abundance. Thus, the overarching aim of this thesis was to understand how a flammable native species can become prolific within its own range. This is achieved using a case study species – forest wiregrass Tetrarrhena juncea R.Br. (hereafter wiregrass) – an understorey native species that is of high importance to flammability in the eucalypts forests of south-eastern Australia and grows prolifically under certain conditions. The overarching aim of the thesis was addressed using a mix of research methods to identify the key drivers of wiregrass distribution and abundance. Firstly, a database of the current distribution for wiregrass were analysed using species distribution modelling to identify highly suitable habitat for wiregrass (Chapter 2). Temperature seasonality, precipitation of the driest month, rainfall seasonality, annual mean temperature, the minimum temperature of the coldest month and soil pH were strongly associated with the suitable habitat of wiregrass. The high importance of climatic factors indicates the distribution of wiregrass may be sensitive to climate change. Highly suitable habitats do not necessarily harbor abundant wiregrass because site-specific factors can also control abundance. Hence, Chapter 3 sought to identify the factors most important to wiregrass abundance in the highly suitable habitat of Mountain Ash-dominated forest. Wiregrass cover was assessed in a field survey across a chrono-sequence of 126 sites with contrasting disturbance histories. Canopy cover and net solar radiation were the most important predictors of wiregrass abundance, with wiregrass cover highest in recently disturbed areas with sparse canopy cover, high light levels, and low precipitation. The final component of the thesis used a glasshouse experiment to quantify causal links between resource availability and wiregrass abundance. Wiregrass growth was more sensitive to water availability than light, whereas biomass allocation and leaf morphology were more sensitive to light availability. Collectively, the results showed that, where wiregrass is present (distribution), three key conditions will greatly favour its prolific growth (abundance): (i) non-limiting water resource; (ii) reduced canopy cover and increased light; and (iii) recent disturbance. These key results strongly suggest wiregrass can become prolific when resources are increased, and the vegetation community is substantially disturbed. Under such conditions, increased wiregrass abundance could create a window of increased flammability for the forest ecosystem. Since climate change can alter resource availability and disturbance regime, shifts in wiregrass abundance are likely to occur under future climate scenarios.