TDA-IME Project Final Report June, 2013 prospects for and rate of recovery, with potential long-term consequences for ecosystem integrity. Increased precipitation A global increase in average precipitation of up to 25% is predicted, with considerable regional variation (Solomon et al., 2007). Since mangrove trees require fresh water, increased precipitation will generally benefit mangroves, allowing, for example, increased growth and reproduction, and improved propagule survival and settlement. The outcome might therefore be a local expansion of mangrove forest area and increased productivity. Sea level rise Global average (eustatic) sea level is predicted to rise by 0.18 – 0.59 metres by 2100: some predictions suggest a rise of up to 1 metre (Solomon et al., 2007; Nicholls and Cazenave 2010). About 75% of this rise is attributed to thermal expansion of the sea, the remainder being due to the melting of ice. Eustatic sea level rise (SLR) is a global average: the actual sea level rise depends on regional and local factors. These include tectonic movement (the northern Bay of Bengal, for instance, is rising to the west and subsiding to the east); soil subsidence; local sedimentation and erosion; soil expansion through underground root production; soil compaction; and soil shrinking and swelling resulting from variation in sub-surface groundwater flow (Blasco et al., 1996; Cahoon and Hensel, 2006). Although some studies show sedimentation keeping pace with current rates of local SLR, others conclude that the majority of mangrove sites have not been keeping pace (Alongi, 2008; Gilman and Ellison, 2010). Where local shore levels do not keep pace with local SLR, lower-shore mangroves will be inundated more frequently, with adverse effects on plant growth and survival (Ellison and Farnsworth, 1997). Conditions further up the shore may improve with increased tidal inundation. The overall result, therefore, could be contraction of mangrove distribution on the seaward side, and expansion up-shore and inland (López-Medellin et al., 2011). Similarly, mangroves in river estuaries might become distributed more up-river than at present. Upshore migration of mangroves would be possible only where this is permitted by topography, and if expansion is not blocked by human activities such as agriculture, roads, or other infrastructure. It has been estimated that 20 – 70% of intertidal habitat might be lost in ecologically important North American bays, where landward habitat expansion is curtailed by topography and by sea walls, over the next 100 years (Harley et al., 2006). No equivalent estimates are available for Indochina, but coastal roads in most countries, and dykes in Viet Nam and Malaysia especially, represent a significant barrier to the landward migration of mangroves. Uncertainties in prediction: the known unknowns There are many uncertainties in predicting the probable impact of global climate change on mangrove ecosystems: 1) The ecological response to shifting environmental variables may be non-linear Most predictive models assume approximately linear, proportional, response to a shift in environmental variables, such as temperature, over the range in question. This may not be the case: as temperature rises, the ecological response may be greater than linear. 2) Environmental variables may be interactive In general, predictions focus on a single aspect, such as increased temperature or atmospheric CO2 levels. Ecological responses to simultaneous change in several 65
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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