COMPARATIVE ECOPHYSIOLOGICAL ANALYSES OF MELALEUCA IRBYANA AND MELALEUCA BRACTEATA – A NARROWLY VERSUS WIDELY DISTRIBUTED CONGENERIC SPECIES Thita Soonthornvipat Submitted in fulfilment of the requirements of the degree of Doctor of Philosophy School of Earth, Environmental and Biological Sciences Science and Engineering Faculty Queensland University of Technology Brisbane, Australia 2018 Keywords Adaptive ability, basal area, biodiversity, canopy, competitive ability, critically endangered species, critically threatened species, crucial abiotic factors, diameter at breast height, dominant, ecological community, ecosystems, edaphic factors, ecological strategies, ecophysiology, extinction, fitness traits, germination characteristics, geographical range, growth performance, habitat fragmentation, habitat’ s specificity, interspecific variation, intraspecific variation, Kaplan-Meier estimate, leaf area index, life history traits, Melaleuca bracteata, Melaleuca irbyana, monoculture, non-dominant, nutrient acquisition, phenotypic plasticity, photosynthetic active radiation, photosynthetic rate, physiological performance, physiology, plant herbivores, Principal Component Analysis, reciprocal seedlings, regeneration programs, regional management plans, relative growth rate, reproductive capability, resin bag, resource acquisition, resource availability, resource use efficiency, restoration, restricted distribution, revegetation programs, seed ecology, seed germination, seedling survival, shoot elongation, soil conditions, stem density, survival traits, tree density, widely distributed, widespread distribution. ii Abstract Melaleuca irbyana R. T. Baker, is a small to medium size tree that forms a unique habitat, in that the forests in which it is found are dense monocultures located mainly in the the South-East Queensland region. Melaleuca irbyana forests were originally restricted in their distribution, but are now listed federally as critically endangered under the Environment Protection and Biodiversity Conservation ( EPBC) Act 1999. Despite considerable conservation efforts to protect remaining M. irbyana forests, it remains under threat of extinction due to increased land clearing for urban expansion, coal seam gas exploration, and the indirect effects of urbanisation, e. g. , eutrophication. The genus Melaleuca is made-up of 290 species. Commonly distributed species include Melaleuca quinquenervia with a distribution ranging from Cape York in Queensland to Botany Bay in New South Wales. Under the EPBC Act 1999, 11 species of Melaleuca are considered endangered within Australia, including M. irbyana. To conserve the remaining M. irbyana populations, there is an urgent need to better understand how this species grows, including reproductive and life history traits, nutrient cycling, and other habitat requirements. In my doctoral research, I compared how M. irbyana and the common and often co-occurring M. bracteata grow across life history stages, from seed germination, seedling survival, and growth, to changes in height, diameter at breast height (DBH), and shoot elongation in adult populations. To understand the reproductive and life history traits of both of these Melaleuca species, I measured seed germination success rates under different controlled environmental conditions in growth cabinets, as well as germination success rates in the field, across four growing seasons. I hypothesised that lower reproductive capabilities may contribute to M. irbyana’ s original narrow distribution and therefore rare status. Melaleuca irbyana displayed germination characteristics that were restricted to a narrower range of temperatures than those of M. bracteata. Melaleuca irbyana showed a lower germination success rate in response to cooler temperatures and lower light conditions when compared to M. bracteata, which displayed high germination rate at all temperatures (15, 25 and 30°C) and photoperiod regimes (0, 10, 12 and 15 hours). Melaleuca irbyana’s germination appears to occur under a narrower range of temperatures than M. bracteata, but overall at higher iii temperatures (e. g. , 30°C), M. irbyana had a high germination success rate than originally expected. I measured seedling survival and growth rates in a reciprocal seedling experiment grown under the canopies of four mature M. irbyana or M. bracteata forests. Overall, I found that M. irbyana showed a lower seedling survival and growth rate than M. bracteata, whether growing under a canopy of M. irbyana or M. bracteata. However, M. irbyana had better survival and growth rates when seedlings were grown under M. bracteata canopies. While, M. bracteata seedlings survived better and had more enhanced growth rates than M. irbyana under both canopies, their survival and growth rates were lower under M. irbyana canopies. Reduction in seedling survival and growth rates under M. irbyana mature trees suggests that these endangered habitats may inhibit recruitment, possibly due to dense canopies, litter, or specific soil conditions (such as high alluvial clay levels in the soil). Adult tree densities in M. irbyana forests may have negative effects on seedlings’ survival and growth because of reduced light availability. Low survival rates and slower growth rates of seedlings in natural environments of M. irbyana may contribute to explaining its original restricted distribution. Ecophysiological traits such as aspects of rate of shoot growth, plant height and tree DBH of mature trees were measured to produce useful information on the performance of adult M. irbyana versus M. bracteata, as possible indicators of the competitive ability of these species. Melaleuca irbyana presented high growth performance in the adult stage, refuting our hypothesis that rare species would present lower performances at every stage of their life cycle. Measurements, although recorded cover three growing seasons, were taken during drought periods, which provides some evidence that M. irbyana may access groundwater, which may explain its higher growth rates during this period than M. bracteata, which is not thought to be groundwater dependent. I compared the soil properties and nutrients levels in remnant mature forests of M. irbyana and M. bracteata in terms of different habitat conditions. Soil nutrient analyses showed similar levels of nutrients in soils collected from both M. irbyana and M. bracteata forests. I found, however, that M. irbyana sites showed higher inorganic nitrogen levels, and that M. bracteata forests had higher levels of sand in the soil, which suggests that M. irbyana forests are more nutrient-rich that M. bracteata. I also found higher levels of some metals such as aluminium in M. irbyana soils, which can be toxic to plants at high concentrations. iv The next steps with these analyses would be to focus on understanding how the two species acquire nutrients from the soil. Understanding resource acquisition and use in these tree species would involve comparing nutrient levels in plant tissue (e.g., leaves, young shoots, roots, and old wood) in M. irbyana and M. bracteata possibly under controlled conditions in the glasshouse. My research findings will assist with the management of remaining populations of M. irbyana by providing much needed information on basic survival and growth characteristics to assist with future revegetation projects. My research results can also contribute more broadly to understanding the traits and environmental conditions that limit some plant species’ range, while enabling the widespread distribution of other species. In this research, I found that M. irbyana had more specialised germination requirements than M. bracteata under controlled conditions in a growth cabinet, but under field conditions, both species showed low germination successalthough M. irbyana in every test had a lower germination success rate. These findings suggest that when growing seedlings for restoration projects, M. irbyana will germinate successfully under warmer conditions where light is available, but direct seeding projects are not likely to be a viable option for restoration of either species, and that natural recruitment under canopies of either species may be limited. I also found that seedling survival and growth rates of M. irbyana are higher in areas with higher light availability, which indicates that the most suitable site for restoration of this species is not under dense canopies, but in open areas or areas with sparse tree cover. A key result of comparing the growth and habitat-specific conditions of remnant populations of M. irbyana and M. bracteata is that mature populations of M. irbyana had a higher growth rate in terms of height and diameter at breast height (DBH) than M. bracteata. This finding suggests that once M. irbyana reaches the adult stage of its life cycle it possibly has fewer limitations on its growth, explaining why and how it can form dense monocultural plantations. I also found some differences in soil characteristics between M. irbyana and M. bracteata sites, which may explain recruitment of M. irbyana at some localities and not others. These differences in soil may also be explained by M. irbyana adult monocultures altering the environment to suit their own successful growth and development, which may hinder the recruitment of other species in its canopy. Collectively, my findings show that it is probable that M. irbyana originally had a limited distribution because of specialised requirements in the early stages of its life cycle, but adult populations