A feasibility study of Melaleuca trees for use in constructed wetlands in subtropical Australia

Three tree species from the genus Melaleuca are being examined for use in constructed wetlands in subtropical SE Queensland, Australia. Growth responses of Melaleuca to secondary treated effluent (100% - approximately 5mgPL⁻¹ and 8mgNL⁻¹), half strength (50%), P enriched (+P) and N enriched (+N) secondary treated effluents were monitored in a 2 year pot trial. A growth index was derived from measurements of height, girth diameter, branch number and new leaf number. Highest rates of growth were achieved in the +N and 100% treatments, and lowest rates in the +P and 50% treatments. Seasonal growth trends were evident. Continuously waterlogged trees had slightly higher growth rates than those subjected to aeration cycles, demonstrating their suitability to wetland environments. An aerated network through the bark extending to the roots may provide a mechanism of root aeration. Biomass and nutrient partitioning were measured in an experimental constructed Melaleuca wetlands receiving pure effluent. Biomass nutrient accumulation rates were comparable to studies of other macrophytes. M. alternifolia stored approximately three times more N and P than M. quinquenervia. However, M. quinquenervia had higher rates of litter fall induced by severe insect damage, increasing the rate of transfer of nutrients to the long term sediment sink. P concentrations in the senescent leaves were highly responsive to external concentrations, and may be used as an indicator of P loading rates in constructed wetlands. Since senescent leaves provide a major pathway of biomass nutrients into the sediment sink, this provides a self regulating P storage mechanism.