Mangrove forests are communities of halophytic woody plants distributed along tropical and subtropical riverine and coastal shores. They are exposed to a wide variation in environmental conditions that influence tree structure and productivity. Mangroves provide a range of ecosystem services, including coastal protection from waves, wind and extreme events such as cyclones and tsunamis. The degree of protection they offer to the coast partly depends on their wood density.
The woody tissues of trees perform a range of functions including resistance to breakage and water transport. In addition, wood constitutes a high proportion of forest aboveground biomass. The properties of the woody tissue can be directly related to tree growth and forest productivity, rates of transpiration, vulnerability to water stress, pathogen resistance and rates of decomposition. This thesis explored the wood functions of water uptake (Chapter 2), mechanical strength (Chapter 3) and growth rates (Chapter 2, Chapter 4, Chapter 5 and Chapter 6) across a range of locations.
Firstly, I investigated the extent of freshwater utilization by A. marina across a range of sites in eastern and western Australia by using stable oxygen isotopes. My results suggest that A. marina used a combination of fresh and saline water sources for growth. In addition, stem growth was enhanced by the amount of rainfall, indicating that high levels of mangrove productivity rely on access to freshwater.
Secondly, I studied how the wood structure of mangroves influenced wood density and mechanical strength in South East Queensland mangroves. I found that mechanical strength was correlated with wood density in mangrove branches. Higher mechanical strength and wood density in mangrove branches were explained by reductions in xylem vessel lumen areas, which may have costs through reducing water uptake, carbon gain and growth and with lower pith content (where synthesis can occur, e.g. hormones, enzymes, pigments). In addition, higher mechanical strength and wood density were associated with increases in fibre wall thickness. These associations between wood strength and anatomical characteristics in mangrove branches may indicate trade-offs between mechanical strength and water supply, both of which are linked to tree growth and survival.
Thirdly, in order to understand the anatomical bases of variation in wood density and how wood density varied with tree growth rates, the wood structure of the widespread mangrove species Avicennia marina was investigated. Avicennia marina has an uncommon wood structure in which increments in xylem vessel diameter (and fibre wall thickness) significantly increased wood density and growth rates. My fourth study evaluated climatic influences on the historical growth rates and wood density of A. marina in the arid Exmouth Gulf in Western Australia. By assessing wood density profiles and utilizing radiocarbon dating across stems profiles (from pith to bark), I established that A. marina wood density decreases towards the bark and over time, and that wood density was positively correlated with growth rates, reductions in growth rates were linked to less rainfall availability (positive Pacific Decadal Oscillation index values). Finally, the hypothesis that differences in growth rates and nutrient use among species with increases in nutrient availability result in the dominance of A. marina over Rhizophora stylosa in western Moreton Bay was investigated with an individual based model. This modelling approach indicated that, when nutrients are low, R. stylosa grows faster and dominates, but when nutrients become highly available A. marina grows faster and becomes the dominant species.
Overall, this work contributes to the understanding of wood structure and its associated functions in mangrove trees. This work demonstrated that wood density can be used as a proxy for mechanical strength in mangroves. In addition, the particular wood structure of the mangrove A. marina leads to a positive relationship between wood density, xylem vessel diameter and growth rates. Growth rates, productivity and species dominance of A. marina are enhanced by freshwater sources and nutrient availability.