Anatomical variation in twig wood across Australian angiosperms

This thesis explores variation in wood anatomical structure and its potential ecological implications. Woody stems are major plant organs performing vital functions such as mechanical support, water transport, and metabolites storage. The various ways plants perform those functions are determined by wood anatomical structure. Despite this fundamental role of anatomy, surprisingly few studies have rigorously quantified anatomical traits, especially across a broad number of species. Moreover, twig wood has been especially overlooked in favour of main trunks. This thesis quantified anatomical variation across a relatively broad number of species in twig wood and explored the correlations of anatomy with other plant functional traits. The overarching aim was to contribute a quantitative anatomical understanding to our knowledge of plant functions, as they relate to plant ecological strategies. Wood density has been considered a key plant functional trait, yet its anatomical nature is not entirely understood. First, anatomical underpinnings of density variation were quantified across 24 species with densities ranging from 0.37-0.83 g cm-3. Density proved mainly to be driven by the fraction of wood occupied by fibre walls. There was also substantial anatomical variation that was independent from density, representing mainly a trade-off between fibre and parenchyma fractions. The ecological significance of this dimension of variation is not understood. Since fibre-parenchyma variation was wider among lower-density species, anatomical variation in 69 low-medium density species (0.38-0.62 g cm-3) was quantified in order to capture this fibre-parenchyma trade-off more thoroughly. Potential correlates such as plant height, leaf area to sapwood area ratio, and modulus of elasticity were also measured. These other traits proved not to be well correlated with wood anatomy. For example, parenchyma fraction (an index of storage capacity) was expected to correlate with plant height (indexing canopy access to light), but it did not. The fraction of parenchyma (being soft, thin-walled tissue) was expected to covary with mechanical properties, but this expectation was only weakly supported. In addition to these two bodies of new empirical work, literature on anatomical knowledge from across diverse disciplines was synthesized. Other gaps in our current comprehension of wood anatomy and function in angiosperms were highlighted. For example, the function of parenchyma has commonly been described as metabolite transport and storage. However, the available evidence suggests that, next to metabolite storage, parenchyma may play a major role in water storage or vessel refilling.