Come with me on a journey through xylem and space

The continued and inevitable rise in global temperatures and likely increase in extreme drought events, pose an immediate and intensifying threat to tree survival due to plant water stress. The global increase in drought-induced forest dieback, and the irreplaceable value of forests, creates an urgent need for research to uncover the mechanistic drivers of tree death. Water travels through plants via a complex, interconnected network of tube-like cells called xylem. Loss of water from the leaves creates a negative pressure or ‚ÄövÑv=tension‚ÄövÑv¥ within the xylem network, providing the driving force for water transport in plants. It is likely that the failure of this process during acute dehydration causes tree damage and death in drought conditions. While the ‚ÄövÑv=cohesion-tension‚ÄövÑv¥ theory describing plant water transport was conceived over 100 years ago, we are only just beginning to understand the process and consequences of how this process fails during drought. Drought conditions lead to increasing tension on the water columns in the xylem. As soil water deficit makes it harder for plants to extract water via their roots, the increasing tension can cause the water columns in the xylem to snap. This is called ‚ÄövÑv=cavitation‚ÄövÑv¥ which induces the formation of air bubbles (embolisms), which block water flow. With continued drying, cavitation is believed to lead to complete blockage of the water transport system and plant death. Xylem cavitation has been correlated with the wide-spread and increasing forest mortality occurring across the world. However, due to the complexities associated with detecting xylem cavitation, we still know very little about how embolism propagates through trees, nor do we know whether it drives tree damage and death in drought events. The difficulties associated with measuring cavitation arise due to the tension or negative pressure in the plant water transport system. As the water columns in the xylem exist in a delicate metastable state, attempts to observe this system are prone to observer effects, whereby the act of observation can alter the system‚ÄövÑv¥s function. As many techniques used to measure xylem cavitation in trees use excised samples, they can be subject to observer effects and other measurement-related artefacts. Inlight of these issues, research over the last 10 years has revealed that some traditional techniques used to quantify xylem cavitation may, under certain circumstances, yield inaccurate results. The recent development of non-invasive visual techniques to monitor xylem cavitation, not only provides a solution to observer effects, but also allows embolism to be monitored in intact plants for the first time. By providing a window into the plant water transport system, we can begin to understand how xylem cavitation progresses through intact trees, how much it varies within trees, and whether it is the cause of tissue damage and death. In this thesis I use visual techniques to investigate xylem cavitation in trees at different spatial scales. I use a combination of the x-ray tomography (microCT) technique and the Optical Vulnerability Technique (OVT, which utilises optical cameras and light) to investigate embolism formation; across tree canopies (Chapter 2), among branches (Chapter 3) and within individual stems (Chapter 4). In Chapter 1 I introduce how terrestrial photosynthesis inherently places plants at risk of xylem cavitation, then describe the difficulties associated with quantifying cavitation with reference to the strengths and limitations of existing techniques. I then discuss the innovation provided by visual techniques, which allow us to accurately quantify embolism and drought-induced damage in-vivo. Finally, I outline the aim of this thesis and specific aims of each chapter. In Chapter 2 I use the Optical Vulnerability Technique (OVT) to track the pattern and timing of drought-induced embolism through the canopies of intact trees. I reveal wide variation in the vulnerability of branch tips within canopies of native Australian conifer, Callitris rhomboidea. Through simultaneous measurements of tissue health, I also find that the branch tips are highly resistant to tissue death, providing evidence for a causal relationship between cavitation (specifically, runaway cavitation) and tissue damage. I investigate the possible causes of the extensive variation found in the vulnerability of C. rhomboidea branchlets (Chapter 2) in Chapter 3. Firstly, I test whether stem diameter may influence this variation and find that vulnerability is highly variable in smaller diameter stems (< 2 mm). Next, I test the hypothesis that differences in growth rate at the time of branchlet extension (reflected in different internode lengths at distal branch tips) lead to differences in cavitation vulnerability. Through optical monitoring of embolism in distal branchlet internodes of varying lengths, I reveal a correlation between internode length and xylem vulnerability. This provides evidence suggesting that a trade-off between growth rate and vulnerability to xylem cavitation may be driving drought resistance in C. rhomboidea canopies. I also measured a number of anatomical traits related to tissue allocation and xylem anatomy to investigate possible explanations for the differences observed in vulnerability. In Chapter 4, I combine two non-invasive imaging techniques to track embolism spread between individual xylem conduits in the stems of three angiosperm trees. I find that embolism spreads predominantly by events in single conduits within these stems. Through anatomical measurements, I find evidence suggesting that the degree of xylem network connectivity is linked to the pattern of embolism spread. In the final chapter (Chapter 5) I discuss how these findings contribute to an integrative, holistic understanding of how xylem cavitation leads to damage and death in trees. I also discuss how future research could utilise visual techniques to answer some of the key questions surrounding tree death under drought conditions. The findings from this thesis will aid and inform accurate prediction of tree damage and death. Combined with existing knowledge, these findings will ultimately allow us to quantify the vulnerability of trees to drought-induced death in forests across the world.