Spatiotemporal dynamics of high-temperature tolerance in Australian arid-zone plants

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Many aspects of the Earth’s climate are predicted with high certainty to undergo substantial and rapid changes in the near future, potentially resulting in a plethora of new high stress conditions to which plants must respond to survive. Living in extreme environments, desert plants are expected to be among the most vulnerable. Due to the thermal dependence of photosynthesis, changes in temperature are particularly important for plants. Extreme high-temperature events are becoming more frequent and intense and projected to increase in many regions. General expectations are that species’ vulnerability to increased temperatures varies with latitude, but less is known about how local-scale habitat variation influences thermal tolerance. Variation in the ability to plastically adjust thermal tolerance will undoubtedly influence the distribution of different species and affect community composition. Yet, the extent of variation in thermal acclimatisation in plant species is poorly understood. The overall objective of my PhD research was to provide insight into leaf-level thermal responses of plants under extreme high temperatures in light of a warming climate. Through a series of linked experiments, my research demonstrates how dynamic and varied the heat stress response can be, including cross-species variation of critical thermal limits, heat stress recovery, acclimatisation patterns within and among species over time, and spatial differences relating to native microhabitat. I developed a novel protocol for measuring biologically relevant, species-specific thermal damage thresholds (Chapter 2), which I subsequently used to demonstrate seasonal and spatial effects on species’ thermal responses (Chapters 3 and 4). The latter findings emphasise that a deeper understanding of plant thermal responses requires insight into their capacity to shift their thermal response over time and space. I then showed that species’ innate physiological thermal tolerance aligns in multi-trait space with two alternative leaf-level morphological pathways of thermal protection (Chapter 5). This raises the possibility that other thermal protective processes, e.g., heat shock protein production and increased membrane stability, may also sit along these axes. Lastly, I demonstrated intracanopy variation in leaf-level physiological response, which expands our mechanistic understanding of plant-environment interactions and could benefit models predicting the cost to species of a warming climate (Chapter 6). By revealing these and other key thermal response patterns, this thesis offers a meaningful contribution to the field of plant ecophysiology, and provides information that is crucial for our understanding and management of desert– and potentially many other – ecosystems.
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