TDA-IME Project Final Report June, 2013 been shown to stimulate below-ground productivity, resulting in increased elevation of the soil surface: if this applied also to mangroves, it would enhance their ability to counter sea level rise (Langley et al., 2009). Ocean acidification An estimated 25 – 30% of total anthropogenic CO2 emission has been absorbed into the surface layers of the world’s oceans. The ocean’s uptake of CO2 lowers pH (raises acidity) and reduces the concentration of carbonate ions. Ocean pH has decreased steadily by 0.02 pH units per decade over the last 30 years, with an overall decrease of 0.1 pH units since the pre-industrial period: bearing in mind the logarithmic nature of the pH scale, this equates to a 30% increase in H+ ion concentration. It is estimated that ocean pH will fall by a further 0.3 – 0.5 pH units by 2100. This represents “larger pH changes over the next several centuries than any inferred from the geological record of the past 300 million years” (Caldeira and Wickett 2003). Lower pH will be accompanied by a decline in carbonate ion concentration (Caldeira and Wickett, 2003; Orr et al., 2005; Doney, 2010). The impact of acidification and reduced carbonate ion concentration is hard to predict because of the complexity of ocean processes. Calcifying organisms such as corals, molluscs, crustaceans and some planktonic species are likely to be the most affected. Molluscs and crustaceans are also important in mangrove ecosystem function (although mangrove crabs tend to be relatively lightly calcified species). Probably more significantly, there are vital ecological linkages between mangroves and coral reefs involving the movement of species and nutrients between these two habitats. Thus, if coral reefs struggle to survive in a lower pH environment, then mangroves will also be affected. Temperature rise Over the last century, global average surface temperature has risen by ~0.74°C: the rise by 2100 may be as much as 1.1 – 6.4°C (Solomon et al., 2007). Photosynthesis, and other variable processes such as leaf production, generally increase with rising temperature. Most mangrove species have temperature optima, which differ between species. In Australia, most species peak between 21 and 28°C: the higher-latitude Avicennia marina has an optimum of 20°C, while the more tropically-distributed Xylocarpus peaks at 28°C (Hutchings and Saenger, 1987). Rising temperature might result in some alteration of species composition. However, as most mangrove trees experience daily temperature fluctuations of an order of magnitude greater that the predicted rise in average temperature, it is unlikely that the effect of increased temperature will be significant. The global distribution of mangroves is defined by temperature, their latitudinal extremes being almost exactly delimited by the position of the 20°C winter isotherm. Thus, a rise in global average temperature might permit a modest geographical expansion of mangroves north and south of the equator. This would probably be limited by intermittent cold events, and also by topography: in the Indian Ocean, for example, the Asian land mass prevents northward expansion. Northward shifts in the distribution and abundance of marine species are already well documented (Walther et al., 2002; Hawkins et al., 2008). Species would respond differentially, hence some alteration in species interaction and ecosystem structure might result. However such effects on mangrove ecosystems are likely to be slight, except possibly at the northern and southern limits of mangrove distribution. Storms An increase in the frequency and severity of tropical storms is predicted (Solomon et al., 2007). Severe storms cause defoliation, tree death, and soil erosion (McCoy et al., 1996; Gilman et al., 2008). Storms remove older and taller trees selectively such that full forest recovery may take considerable time: mangrove ecosystems recover from intermittent events over a period of years. An increase in the severity and frequency of storms would limit the 64
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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