Mangroves and geomorphological processes

 

Professor Colin David Woodroffe

School of Earth and Environmental Sciences,
University of Wollongong,
Wollongong, NSW 2522,
Tel :  (02) 42213359
Fax : (02) 42214250
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Mangroves occur on low-energy tropical and subtropical shorelines where there is suitable substrate in the upper intertidal zone. Their distribution and shoreline dynamics are therefore a function of coastal geomorphology. Although there are important ecological interactions that influence species distribution, longer-term changes can be viewed across a range of time and space scales. At the smallest scale, the ‘instantaneous’ scale at which physiological and sediment processes operate, mangroves can occur across a broad range of environmental gradients on the shoreline. There can be significant disruptions to mangrove ecosystems as a result of unusual or extreme events, such as individual storm events, and the ‘event’ time scale exerts a particular influence on mangrove ecosystems because the trees have life histories of decades, preserving evidence of such events. By contrast, the longest time scale, termed the ‘geological’ time scale, is concerned with thousands to millions of years. This is the time scale over which coastal plains have prograded, deltas have been built out at river mouths, and mangrove ecosystems have responded to altered boundary conditions, such as changes in sea level. The response of mangroves at this time scale, and the evolutionary history of associated landforms, can be inferred from the stratigraphy and age of sedimentary sequences. Coastal managers, involved with planning need to consider the ‘engineering’ time scale of several decades. This is the scale at which planning decisions need to be made, anticipating behaviour of the shoreline, but it is perhaps the most difficult to understand because it falls between the knowledge based on observations of processes and events over the past few decades, and the long-term behaviour recorded in the sedimentary record. In this review, the geomorphological behaviour of a variety of coastal systems will be examined, with examples from around Australia, and elsewhere in the southeast Asia and Oceania region. The idea of ‘environmental settings’ within which mangroves occur, and the predominant processes in each of these settings will be discussed as a basis for considering what the likely impacts of changes in climate, particularly sea-level rise, may be on mangrove ecosystems.

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