Mangrove Generations

It was a thrill last week to attend a meeting of four generations of folk interested in mangrove science. The gathering took place at University of Wollongong and involved my former PhD student, Professor Neil Saintilan, his former PhD student Kerrylee Rogers now Associate Professor, and her postgraduate students, Kirtie Lal and Kristian Kumbier. Colin Woodroffe was also present highlighting the breadth of interest and range of work undertaken over the past 50 plus years by this group in mangrove ecology and links to geomorphology.

I was invited to give a brief overview of my mangrove studies—well it may not have been that brief! But it did give me the opportunity to delve into a large 35mm slide collection and dig out 50 slides from past field work and site visits covering the period 1963 to 1985. I transferred the slides to PowerPoint as days have long gone from using slide tray projection. However, there was interest in these figures as they indicated an approach to studying mangrove habitats from a geomorphic perspective.  This meant mapping, drilling, sampling, collecting and using transects to define eco-geomorph relationships. Two papers in the Journal of Ecology (1967, 55, 301-343; and 1975, 63, 203-232) demonstrated these relationships in Tabasco, Mexico and Ord River region, respectively. The work contested standard mangrove succession theory based on earlier work in Florida and Malaya. Field studies by Goodlett and Zimmermann in terrestrial ecosystems complemented the approach of relating landform-soil conditions to plant dynamics. I later collaborated with Zimmermann on this approach which we called “physiographic plant geography” (Progress in Physical Geography, 1982, 6, 45-59).

Neil has developed an outstanding international reputation in wetlands research. He is now based at Macquarie University with his own new team of students. His collaboration with Don Cahoon of the US Geological Survey and others is a great example of pushing the boundaries of wetlands science globally. It involved amongst other things the application of the Surface Elevation Table as a tool for assessing wetland vulnerability including to sea level rise. Kerrylee has been part of this work and was recently the lead author in a paper in Nature (2019, 567, 7746, p.91) (also involving Neil and Colin, and a Post-doctoral Fellow at Macquarie, Jeff Kelleway). This paper brought together several strands of their work over the past 20 years, showing how sea-level rise influences the “blue carbon” accumulating in coastal regions.

It was interesting to hear from the two of Kerrylee’s PhD students at Wollongong. It was like old times asking lots of questions (hard to contain myself!). Kirtie is nearly finished in her study of dynamics of mangroves on sandy surfaces in the Shoalhaven using detailed field measurements. Kris is building on his modelling skills and looking at comparative changes to mangrove systems to sea level rise in examples of macro and micro tidal estuaries. He was very interested in observations I made in the Ord area in 1971 of die-back on the landward side of the mangrove fringe that borders the bare high tidal mud flats. Here Rhizophora forest formed an abrupt inner margin, or wall. This pattern has been seen elsewhere in northern Australia suggesting less frequency of inundation required to maintain that species. I hope Kris can throw further light on this die-back phenomenon.

I was delighted to again talk mangroves with Colin Woodroffe. He had worked on mangrove communities in the West Indies many years ago while at Cambridge.  His various travels and positions led him to Darwin in the early 80s. He was the leading field scientist of an ANU team involving John Chappell and Marilyn Ball in a study of the geomorphology and ecology of the South Alligator River in Northern Territory. I brought along Karl Shaw and a drill rig. It was an amazing adventure and the amount of data collected and papers published was substantial. One paper in particular needs mention. In 1985 we published in Nature (317, 711-713) on the “Big Swamp” highlighting how widespread were mangrove swamps in the mid-Holocene soon after sea level reached its present position. Drilling and C14 dating from South Alligator, King Sound and Ord areas were brought together to help show that at this time mangroves were far more extensive than at present in these monsoonal, high tidal regions. Accretionary processes had progressively displaced the swamps confining them to channel margins. It is a hypothesis that has many implications and one can only anticipate that these generations of mangrove scientists will explore them further.

Bruce Thom

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2019, for correspondence about this blog post please email austcoastsoc@gmail.com

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