Preservation of in situ, arborescent vegetation and fluvial bar construction in the Burdekin River of north Queensland, Australia

Abstract

Abstract 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.