Coastal ecosystems reducing risks from natural hazards and climate change

The response of mangrove soil surface elevation to sea level rise

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Executive summary:

Image of front page of sea level rise reportCoastal ecosystems such as mangroves can reduce risk to people and infrastructure from wave damage and flooding. The continued provision of these coastal defence services by mangroves is dependent on their capacity to adapt to projected rates of sea level rise. This report explores the capacity of mangrove soil surfaces to increase in elevation in response to local rises in sea level.

Historical evidence suggests that mangrove surface elevations have kept pace with sea level rise over thousands of years in some places, such as Twin Cays, Belize. Rates of surface elevation increase ranged between 1 mm/yr and 10 mm/yr in different locations and settings. Key controls on this include external sediment inputs and the growth of subsurface roots.

Recent evidence based on measurements using the Surface-Elevation Table – Marker Horizon methodology (from studies published between 2006 and 2011) suggest that mangrove surfaces are rising at similar rates to sea level in a number of locations. However, surface elevation change measurements are available for a relatively small number of sites, and most records span short time periods. Longer term mangrove surface elevation datasets are needed from more locations, and these need to be analysed relative to sea level changes over the same periods of measurement.

Six sets of processes are known to influence surface elevation change in mangroves: sedimentation/resuspension; accretion/erosion; faunal processes (e.g. burrowing of crabs); growth/decomposition of roots; shrinkage/swelling of soils in the presence/absence of water; and compaction/compression/rebound of soils over time and under the weight of soil/water above. A variety of factors affect the rates of these processes, including the supply of external sediment, the types of benthic mats that bind surface sediments together, vegetation characteristics such as tree density and aerial root structure, nutrient availability to sub-surface roots, storm impacts, and several hydrological factors such as river levels, rainfall and groundwater pressure. The sum of these processes results in surface elevation change.

The number and complexity of processes involved in surface elevation change create significant challenges to the modelling and prediction of future elevation change in the face of sea level rise. It is likely that negative feedbacks exist between sea level change and surface elevation change, but evidence for these feedbacks is currently lacking. Such feedbacks might enable mangrove soil surfaces to maintain their surface elevation with respect to local sea level over the longer term. Threshold rates of sea level rise are also likely to exist, beyond which mangrove surfaces are no longer able to keep up. An improved understanding of the different processes and feedbacks involved in surface elevation change will increase our ability to predict the response of surface elevation to sea level rise, and to manage mangrove areas in ways that enhance their ability to keep pace with sea level rise.

Monitoring and management of mangrove areas is recommended to ensure continued provision of coastal defence services into the future. In particular, sediment inputs need to be maintained, mangroves should be protected from degradation, and space should be allowed for mangroves to colonise landward areas. In many areas, short term anthropogenic losses of mangroves represent a greater threat to the provision of coastal defence services by mangroves than the longer term effects of sea level rise.