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Gallery and floodplain forests in monsoonal northern Australia are mostly sclerophyllous and dominated by five closely related species of Melaleuca (Myrtaceae) amongst which niche differentiation is unclear. We present a floristic and environmental analysis of ‘the flooded forest’ using data from 340 plots distributed across 450 000 km2 of the Top End of the Northern Territory. Melaleuca argentea was confined to streams and occurred on sandier substrates, whereas M. cajuputi mostly occurred in the near-coastal lowlands on clay soils. The greater basal area of M. cajuputi suggests an association with productive sites. Melaleuca dealbata, M. viridiflora and M. leucadendra occurred on a wide range of soils. More deeply floodprone sites were occupied by M. argentea and M. leucadendra along streams and by M. leucadendra and M. cajuputi on floodplains and in swamps. A general deficiency but occasional abundance of Melaleuca seedlings suggests that regeneration is episodic. Seedlings were more frequent in recently burnt areas and especially where fires had been severe. We propose that Melaleuca forests occur where disturbance by fire and/or floodwater is too great for rain forest to persist, rendering them the wetland analogue to the eucalypts that dominate well-drained portions of the north Australian environment.

Vegetation-induced obstacle marks are described from Dalrymple Bend in the Burdekin River of Queensland, Australia, and their preservation potential is discussed. The bend is divided into a sand- and gravel-covered lower bar, and a vegetated upper bar. Obstacle marks, comprising an erosional scour and a depositional sediment tail, are recognised on the lower bar and the lower margin of the upper bar. Two types of obstacle marks are dominant in the lower part of the lower bar; sediment tail-dominated obstacle marks associated with inclined trees of Melaleuca argentea (Silver-leaved Paperbark), and comparatively small obstacle marks around stranded driftwood. Obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel. At the lower margin of the upper bar, scours and gravelly sediment tails are recognised around mature tall trees. In these cases, the scour is >4 m in width, and covered with alternating beds of fine sand and mud. Among the obstacle marks described, sediment tail-dominated obstacle marks around groves of M. argentea have the highest preservation potential. Obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential. Obstacle marks around stranded driftwood have the lowest preservation potential. Obstacle marks around grass clumps survive more than 1 year, but their long-term preservation potential is nonetheless low. Implications for the stratigraphic record are also discussed.

The presence of terpinen-4-ol in the steam distillates from Melaleuca argentea, M. dissitiflora and M. linariifolia can be linked to high levels of (+)-sabinene and cis- and trans-sabinene hydrate in the leaf material. Incorporation of 18O into the terpinen-4-ol from steam distillates performed in H2 18O confirms the generation of the terpinen-4-yl carbocation intermediate from (+)-sabinene and cis- and trans-sabinene hydrate. This same mechanism is also operating during leaf aging.

The leaf oil components of six Melaleuca species were analyzed. M. symphyocarpa had a higher content of 1,8-cineole and M. bracteata contained large amounts of phenyl propanoids, while the other species had oils which contained complex mixture of mono- and sesquiterpenes. The leaf oil of M. bracteata exhibited the strongest miticidal activity against European house-dust mites (Dermatophagoides pteronyssinus) . It also had the strongest germination and growth-inhibition activity against radish seeds.

In sub-humid parts of north Queensland, NE Australia, certain types of trees are well adapted to living in river bed habitats. The bed of the tropical, variable-discharge, upper Burdekin River hosts a community dominated by the paperbark Melaleuca argentea. Trees grow preferentially in flow-parallel, linear groves, and engineer their own environment by deflecting currents, building sand and gravel bars and stabising banks. This is the first study to document in-channel bar development resulting from vegetation growth, rather than the reverse which has been inferred by previous workers. In the Burdekin River study site, individual Melaleuca range from seedlings to mature trees over 100 years old. These trees survive regular, partial to total submergence and impact damage during wet season runoff events (often reaching over 20,000 m3 s−1 at peak discharge) partly by adopting structural and growth modifications. These modifications include a reclined, downstream-trailing habit, multiple-stemmed form, modified crown with weeping foliage, development of thick, spongy bark, root regeneration and group strategies, notably development of flow-parallel, linear groves. Following death, in situ remains of trees are preserved within the mainly coarse sand to gravel channel fill, either as reclined stems/trunks stripped of branches and foliage or as more upright trunks snapped at a height of typically 1–2 m above base, both with roots. The morphological adapations and styles of preservations of in situ vegetation within the Burdekin River are considered distinctive of variable-discharge rivers. and may be useful in the identification of facies formed in such environments in the rock record, particularly when associated with bar development.

We examined sources of water and daily and seasonal water use patterns in two riparian tree species occupying contrasting niches within riparian zones throughout the wet-dry tropics of northern Australia: Corymbia bella Hill and Johnson is found along the top of the levee banks and Melaleuca argentea W. Fitzg. is restricted to riversides. Patterns of tree water use (sap flow) and leaf water potential were examined in four trees of each species at three locations along the Daly River in the Northern Territory. Predawn leaf water potential was higher than -0.5 MPa throughout the dry season in both species, but was lower at the end of the dry season than at the beginning of the dry season. Contrary to expectations, predawn leaf water potential was lower in M. argentea trees along the river than in C. bella trees along the levees. In contrast, mid-day leaf water potential was lower in the C. bella trees than in M. argentea trees. There were no seasonal differences in tree water use in either species. Daily water use was lower in M. argentea trees than in C. bella trees. Whole-tree hydraulic conductance, estimated from the slope of the relationship between leaf water potential and sap flow, did not differ between species. Xylem deuterium concentrations indicated that M. argentea trees along the riverbank were principally reliant on river water or shallow groundwater, whereas C. bella trees along the levee were reliant solely on soil water reserves. This study demonstrated strong gradients of tree water use within tropical riparian communities, with implications for estimating riparian water use requirements and for the management of groundwater resources. © 2006 Heron Publishing.