| Summary: | Essential players within the planet's water cycle, plants are themselves sensitive to ongoing global change. Future shifts in water availability are predicted to change the face of the earth's forests. In the last decades there have been many advances in our understanding of water relations of plants and their response to environmental conditions. However, most of these studies are based on relatively few North-American and European tree species, with a notable underrepresentation of southern-hemisphere or tropical tree species. This kind of knowledge is needed, not only to reach a representative understanding of eco-physiological diversity of plant function, but also to establish the full spectrum of forest responses to changing environmental conditions. In my project, I study the water relations of the grey mangrove, Avicennia marina (Forssk.) Vierh., in the temperate mangrove forests of New Zealand. Mangroves are enigmatic species, which live in the harsh conditions imposed by brackish water and periodically flooded soils. Although they have elicited much scientific curiosity throughout history, much of our knowledge of mangrove water-related physiology is based on manipulative experiments with seedlings. Over the course of two years, I employed a diverse array of ecophysiological techniques to closely monitor different aspects of adult A. marina water relations in the stem and crown. When investigating the seasonal and monthly courses of water-induced stem radial changes with a variety of stem cycle analysis techniques, I found that these were highly heterogeneous even within the same individual. Despite this heterogeneity however, it was still possible to correlate the amplitude of stem radius changes with environmental conditions. Presence or absence of precipitation events was an important driver of stem swelling periods, whilst atmospheric water availability indicators, such as VPD explained stem shrinkage periods. I found that Avicennia marina shows unusual daytime refilling of stem elastic water storage tissues, which deviates from most terrestrial trees and is hypothesized to be due to endogenous osmotic adjustment. This theory is reinforced by my finding that light sum, a proxy for photosynthesis, is a prominent driver of stem swelling amplitudes. In order to understand the "peristaltic" depletion of internally stored water within the tree stem, I studied the seasonal changes in the timing and time-lags of peak stem swelling at different tiers. I discovered an annual switch in the direction of the peristaltic water depletion wave along the stem, potentially related to seasonal changes crown photosynthesis and influence of non-structural carbohydrate dynamics on stem radial change caused by the onset of the growing season. Daytime stem swelling was studied further by performing measurements directly on xylem tissue, revealing the inner-bark turgor driven signal behind whole-stem daytime swelling. However, due to A. marina's unique wood structure, consisting of multiple phloem-xylem layers, the use of this methodology yielded heterogeneous and highly variable results. Lastly, I found that leaf turgor pressure-probes proved a reliable source of information on leaf turgor dynamics. The study of leaf turgor confirmed the importance of fresh water inputs for leaf hydration. My results also suggested that osmoregulation behind daytime stem swelling momentarily overrides the water demands of transpiring leaves, causing delayed recovery of leaf turgor in the evenings. The time-lags between stem water storage mobilization and leaf turgor recovery followed changes in atmospheric water demand, and also showed differences in the sensitivity of the upper and lower stem to leaf water demands. My work is a significant contribution not only to the understanding of mangrove ecophysiology in particular, but also adds to our increasingly complex map of plant physiological diversity which goes far beyond the established paradigms of plant-water relations.
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