This thesis considerably expands our understanding of hydraulic architecture in the genus Eucalyptus. The major finding is that xylem vessels in eucalypts taper at variable rates from the base of the stem to the top of the tree, depending on species and environment. The systematic assessment of changes in the structure of vessel tapering (reduction in xylem vessel diameter per unit length of stem, from the base to the apex) across a gradient of increasing aridity uncovers that the degree of taper is not a function of plant height but for coping with prevalent limitations like water or light. In some tall species from mesic climates (e.g. Eucalyptus regnans F. Muell.) vessel taper may be close to zero for the great majority of the stem, before increasing rapidly within the apical region of the canopy. For other species, such as those from semi-arid environments (e.g. Eucalyptus gracilis F. Muell.), tapering begins much further towards the base of the stem. These findings are highly novel and contradict some major theories (e.g. Metabolic Scaling Theory, MST). Yet, they are entirely in keeping with the general thrust of the ‘cohesion-tension’ theory of water movement in trees. This thesis includes several supporting studies for the above. A glasshouse study suggested thateven at seedling stage, environmental conditions such as temperature and moisture and nutrient availability, play roles in xylem formation (xylogenesis). Despite these suggestions, research in the glasshouse was inconclusive. This was most likely due to the relatively slow response of major biophysical processes, such as the development of structural entities like xylem vessels, when compared to faster responses to environmental conditions of biochemical processes such as those involved in photosynthesis and respiration. A field study of the potential role of water storage in heartwood (i.e. capacitance) in water transport, was hampered by prevailing environmental conditions and uncertainty around the sources of water being used by the study tree. Nonetheless, the data and the knowledge gained by both experiments add to current understanding how functionality of xylem tissues can be maintained under different environmental conditions, including elevated temperatures and water shortage. However, the major body of work in this thesis rests with analysis of xylem vessels at a microscopic scale. This work required significant development of techniques suitable for use with eucalypts that contain some of the hardest wood of all trees. The research also required development of software scripts capable of quantification of properties in large numbers (>150,000) of vessels across a dozen or so species, and multiple field sites. Additionally, the work reported here includes a rigorous assessment of climate across field sites and then use of that to interpret xylem structure. The resultant phase analysis of rates of tapering within trees, is both an Australian and world first. Adopting methodology widely used in other fields of biology, this thesis employs a phase analysis of tapering of xylem vessels to highlight: 1. That the insertion point of vessel taper towards the top of trees differs largely among eucalypt species. 2. That regardless of species and location, vessel diameter at the apex does not differ widely among species – this, at least, accords with MST. 3. That rates of taper within the apical region of canopies where the risk of cavitation is greatest are closely related to environmental conditions, particularly the availability of water and competition for light
