The vulnerability of Indo-Pacific mangrove forests to sea-level rise

Study Number:

85

Author

Catherine E. Lovelock, Donald R. Cahoon, Daniel A, Friess, Glenn R. Guntenspergen, Ken W. Krauss, Ruth Reef, Kerrylee Rogers, Megan L. Saunders, Frida Sidik, Andrew Swales, Neil Saintilan, Le Xuan Thuyen & Tran Triet

Abstract

Sea-level rise can threaten the long-term sustainability of coastal communities and valuable ecosystems such as coral reefs, salt marshes and mangroves1,2. Mangrove forests have the capacity to keep pace with sea-level rise and to avoid inundation through vertical accretion of sediments, which allows them to maintain wetland soil elevations suitable for plant growth3. The Indo-Pacific region holds most of the world’s mangrove forests4, but sediment delivery in this region is declining, owing to anthropogenic activities such as damming of rivers5. This decline is of particular concern because the Indo-Pacific region is expected to have variable, but high, rates of future sea-level rise6,7. Here we analyse recent trends in mangrove surface elevation changes across the Indo-Pacific region using data from a network of surface elevation table instruments8–10. We find that sediment availability can
enable mangrove forests to maintain rates of soil-surface elevation gain that match or exceed that of sea-level rise, but for 69 per cent of our study sites the current rate of sea-level rise exceeded the soil surface elevation gain. We also present a model based on our field data, which suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070.

Main Results and Conclusions

• Sea-level rise (SLR) can diminish mangrove forests if those forests are not provided enough sediment input to remain above sea-level. Some mangrove forests in areas with low sediment supply could be submerged by 2070.
  • “Although mangrove tree species are able to tolerate inundation by tides, they can die and their former habitat can convert to open water or tidal flats when sea-level rise (SLR) causes the frequency and duration of inundation to exceed species-specific physiological thresholds, resulting in shoreline retreat.” (1)
  • “Throughout the Indo-Pacific region, we found that mangrove soil-surface elevation gains are strongly dependent on rates of accretion of sediment on the soil surface” (1)
  • “sediment supply is important to surface elevation gains and therefore to preventing mangrove-forest loss in the future.” (1)
  • “If the topography allows the mangrove forest to migrate landward, with no anthropogenic barriers (such as infrastructure or flood-defence barriers), then mangroves may delay submergence by ‘back-stepping’ into adjacent habitats. However, barriers to landward expansion of mangrove forests occur throughout the Indo-Pacific region, particularly in sites that have intensive aquaculture, urban development and low-lying agricultural land.” (1)
• Sea-level rise is likely to have a large impact on mangrove forests in the Indo-Pacific region, with some forests projected to be completely submerged in areas where the tidal range is less than four meters. The potential loss of mangroves underscores the need for a strategy that takes into account the impacts of climate change and the accompanying sea-level rise.
  • “[in some of the study sites] we project complete submergence of the forests in 100 years wherever tidal ranges are less than 4 m.” (2)
  • “mangrove forests are likely to persist at sites with high tidal range even with high rates of SLR and low levels of sediment availability” (3)
  • “Our model indicates that outlook for mangrove forests in some locations is poor under relatively low rates of SLR—the Intergovernmental Panel on Climate Change (IPCC) Representative Concentration Pathway 6 (RCP6) scenario.” (3)
  • “In coastal and estuarine systems with reduced upstream sediment inputs due to human modifications, the potential for eco-geomorphic feedbacks that delay the onset of mangrove-forest loss is diminished.” (4)
  • “In Thailand, there has been an 80% reduction of sediment supply in the Chao Phraya River delta, which, in combination with surface subsidence caused by groundwater extraction, has resulted in kilometers of mangrove shoreline retreat.” (4)

Works Cited

1. School of Biological Sciences, The University of Queensland, Brisbane 4072, Australia.
2. Global Change Institute, The University of Queensland, Brisbane 4072, Australia.
3. Patuxent Wildlife Research Center, United States Geological Survey, Maryland 20708, USA
4. Department of Geography, National University of Singapore, 1 Arts Link, Singapore 117570, Singapore.
5. National Wetlands Research Center, United States Geological Survey, Louisiana 70506, USA
6. Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, Downing Place, CambridgeCB23EN, UK.
7. School of Earth and Environmental Science, University of Wollongong, Wollongong 2522, Australia.
8. The Institute for Marine Research and Observation, Ministry of Marine Affairs and Fisheries, Bali 82251, Indonesia.
9. National Institute of Water and Atmospheric Research, Hamilton 3251, New Zealand
10. Department of Environmental Sciences, Macquarie University, Sydney 2109, Australia.
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11. Woodroffe, C.D. Response of tide-dominated mangrove shorelines in Northern Australia to anticipated sea-level rise. Earth Surf. Proc.. Land. 20, 65-85 (1995).
1. Saintilan, N., Wilson, N.C., Rogers, K., Rajkaran, A., Krauss, K.W. Mangrove expansion and salt marsh decline at mangrove poleward limits. Global Change Biol. 20, 147-157 (2014).
1. Giosan, L., Syvitski, J., Constantinescu, S. & Day, J. Climate change: protect the world’s deltas. Nature 516, 31-33 (2014).
12. Kondolf, G.M., Rubin, Z.K. & Minear, J.T. Dams on the Mekong: cumulative sediment starvation. Water Resour. Res. 50, 5158-5169 (2014).