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Australian Academy of Science, Becker House, Canberra. Friday 16 December 1994
Professor Peter Saenger
I have been asked to look at the relationship of mangroves to sea-level change. In Australia we are particularly well suited in that regard because we have some of the best preserved, most extensive and species-rich mangroves in the world. In that sense I think it is fitting that we look at those mangrove shorelines.
The other comment to make is that what I am about to say is my own responsibility. However, I was involved with the LOICZ group that looked at the mangrove suggestions in terms of what research needs to be done. There is a draft report of that out, so I have the benefit of those deliberations to draw on.
In Australia, sealevel was at its lowest level around 18,000 years BP then rose to a maximum about 3,500 years BP before reaching its present level about 1780 years BP (Flood and Frankel 1989; Woodroffe 1990; 1993). At the same time, the climate became moister and milder with rising sealevels with consequent changes in coastal vegetation was established or re-established within the past 10,000 or so years. In other words, the existing coastal vegetation does not comprise in situ ancient ecosystems but, rather, the outcome of recent dynamic coastal changes.
If we could read and interpret the outcomes of these past changes correctly, predictions concerning coastal vegetation response to rising sealevels would be greatly facilitated.
So, what do we need to know to correctly interpret recent changes and how can these be extrapolated to various scenarios of sealevel rise?
Australia has a coastline length of 30,266 km of which approximately 6089 km or 20% represents a mangrove shoreline.
Based on the total Australian mangrove area of 11,617 km2 (Galloway et al. 1984), it should be noted that 70.5% of Australian mangrove areas occur in estuaries while 29.5% occur on open shorelines (Bucher and Saenger 1984) - a point to which I will return later.
Because of the geographical significance of mangroves and related communities, these vegetation types need to be our primary focus. Hence, the title of my presentation Mangrove Coastlines & Sealevel Change.
Many of these mangrove shorelines are conspicuously zoned parallel to the shoreline.
These zones are not related directly to tide levels as such but to ecophysiological conditions determined by tide levels (Snedaker 1982). These relate to things such as gradients of sediment salinity, degree of anoxia which is usually induced by waterlogging, and a number of other gradients that occur across these fairly uniform shorelines.
For individual species, similar relationships to tidally induced ecophysiological conditions can be recognised.
Both the optima and boundaries of individual species of mangroves are determined largely by the innate characteristics of each species which determine its tolerance particularly to salinity and waterlogging-induced anoxia (Overhead 1).
So what implications do these findings have in relation to rising sealevels?
While there is broad agreement that mangroves (and their pollen and sediments) are accurate indicators of palaeo sealevel (Woodroffe 1988a; 1988b; Ellison 1989; 1994; Umitsu 1991; Woodroffe and Grindrod 1991; Woodroffe et al. 1985), predicted effects of rising sealevels on modern mangrove systems appear more variable.
Rising sealevels on extant mangroves have been predicted to result in complete ecosystem collapse (Elison and Stoddart 1991), to mangrove regression (Ellison 1993), and to selective gains and losses for different situations (Woodroffe 1990; Snedaker 1993; Pernetta 1993). This slide summarises various sea level effects (Overhead 2).
To increase our predictive ability concerning mangrove shorelines under various sealevel rise scenarios, we need a number of research initiatives to address each of these areas.
One major research initiative needs to concern the optimal and boundary conditions of individual (?key) mangrove species at least in relation to salinity and waterlogging-induced anoxia. I would suggest that changes in boundaries (which reflect extreme conditions) to detect very early sealevel responses. In any case, such investigations are likely to shed light on species compositional changes, changes in zonation etc.
A second major research initiative needs to be concerned with sediment stability - both on open shorelines and within estuaries where the major mangrove areas occur. This needs to include hydrodynamic changes that might occur with changing sealevels, particularly in estuaries, and how these changes could affect mangrove ecosystems (e.g. 'Big Swamp' changes - Woodroffe et al. 1985).
At the third level, research initiatives are needed into the effects of air temperature increases, raised CO2 concentrations and increases in storm frequency and intensity not only on mature vegetation but also on seedling establishment and performance.