Threats to mangroves from climate change and adaptation options: A review
Year Published:
2008
Study Number:

28

Author:

E. L. Gilman, J. Ellison, N. C. Duke & C. Field

Abstract:

Mangrove ecosystems are threatened by climate change. We review the state of knowledge of mangrove vulnerability and responses to predicted climate change and consider adaptation options. Based on available evidence, of all the climate change outcomes, relative sea-level rise may be the greatest threat to mangroves. Most mangrove sediment surface elevations are not keeping pace with sea-level rise, although longer-term studies from a larger number of regions are needed. Rising sea-level will have the greatest impact on mangroves experiencing net lowering in sediment elevation, where there is limited area for landward migration. The Pacific Islands mangroves have been demonstrated to be at high risk of substantial reductions. There is less certainty over other climate change outcomes and mangrove responses. More research is needed on assessment methods and standard indicators of change in response to effects from climate change, while regional monitoring networks are needed to observe these responses to enable educated adaptation. Adaptation measures can offset anticipated mangrove losses and improve resistance and resilience to climate change. Coastal planning can adapt to facilitate mangrove migration with sea-level rise. Management of activities within the catchment that affect long-term trends in the mangrove sediment elevation, better management of other stressors on mangroves, rehabilitation of degraded mangrove areas, and increases in systems of strategically designed protected area networks that include mangroves and functionally linked ecosystems through representation, replication and refugia, are additional adaptation options.

Main Results and Conclusions:
  • There are many concerns about climate change and the associated effect on mangrove habitat: “Climate change components that affect mangroves include changes in sea-level, high water events, storminess, precipitation, temperature, atmospheric CO2 concentration, ocean circulation patterns, health of functionally linked neighboring ecosystems, as well as human responses to climate change”(238). These are each discussed in detail within the paper.
    • Sea-level rise: The biggest concern connecting sea-level rise to mangrove ecosystems is the rate of change in elevation of mangrove sediment compared to the rate of sea level rise: “Mangrove systems do not keep pace with changing sea-level when the rate of change in elevation of the mangrove sediment surface is exceeded by the rate of change in relative sea-level”(239).
      • The following influences the elevation of mangroves’ sediment surface: sediment accretion and erosion, biotic contributions, belowground primary production, autocompaction, and fluctuations in the water table levels and pole water storage (239). These points are discussed in greater detail in the paper.
      • There are three different mangrove responses to sea-level trends (240)
        • Stable: mangroves generally do not change position when sea level remains stable
        • Site-specific relative sea-level falling: mangroves will gravitate towards the sea and sometimes laterally if sea levels fall
        • Site-specific relative sea-level rising: mangroves will retreat landward as sea levels rise. Mangroves nearest the sea margin die back due to stress caused by the rising tide while new growth occurs at the landward fringe.
    • Higher water events: “Increased levels and frequency of extreme high water events may affect the position and health of mangroves in some of the same ways that storms have been observed to effect mangroves, including through altered sediment elevation and sulfide soil toxicity, however, the state of knowledge of ecosystem effects from changes in extreme waters is poor”(241).
    • Storminess: “The increased intensity and frequency of storms has the potential to increase damage to mangroves through defoliation and tree mortality. In addition to causing tree mortality, stress, and sulfide soil toxicity, storms can alter mangrove sediment elevation through soil erosion, soil deposition, peat collapse, and soil compression (Smith et al., 1994;Woodroffe and Grime, 1999; Baldwin et al., 2001; Sherman et al., 2001; Woodroffe, 2002; Cahoon et al., 2003, 2006; Cahoon and Hensel, 2006; Piou et al., 2006)”(241).
    • Precipitation: Based primarily on links observed between mangrove habitat condition and rainfall trends (Field, 1995; Duke et al., 1998), decreased rainfall and increased evaporation will increase salinity, decreasing net primary productivity, growth and seedling survival, altering competition between mangrove species, decreasing the diversity of mangrove zones (extinction), causing a notable reduction in mangrove area due to the conversion of upper tidal zones to hypersaline flats”(242).
    • Temperature: “Increased surface temperature is expected to affect mangroves by (Field, 1995; Ellison, 2000):
      • Changing species composition (extinction);
      • Changing phenological patterns (e.g., timing of flowering and fruiting);
      • Increasing mangrove productivity where temperature does not exceed an upper threshold
      • expanding mangrove ranges to higher latitudes where range is limited by temperature, but is not limited by other factors, including a supply of propagules and suitable physiographic conditions”(242).
  • Atmospheric CO2 concentrations: “A direct effect of elevated atmospheric CO2 levels may be increased productivity of some mangrove species (Field, 1995; Ball et al., 1997; Komiyama et al., 2008)… The greatest effect may be under low salinity conditions. Elevated CO2 conditions may enhance the growth of mangroves when carbon gain is limited by evaporative demand at the leaves but not when it is limited by salinity at the roots. There is no evidence that elevated CO2 will increase the range of salinities in which mangrove species can grow. The conclusion is that whatever growth enhancement may occur at salinities near the limits of tolerance of a species, it is unlikely to have a significant effect on ecological patterns (Ball et al., 1997)”.
  • Ocean circulation patterns: “Changes to ocean surface circulation patterns may affect mangrove propagule dispersal and the genetic structure of mangrove populations, with concomitant effects on mangrove community structure (Duke et al., 1998; Benzie, 1999; Lovelock and Ellison, 2007)”(244-245).
  • Health of functionally linked neighboring ecosystems: “Mangroves are functionally linked to neighboring coastal ecosystems, including seagrass beds, coral reefs, and upland habitat, although the functional links are not fully understood (Mumby et al., 2004). Degradation of adjacent coastal ecosystems from climate change and other sources of stress may reduce mangrove health”(245).
  • Human responses: “…we can expect an increase in the construction of seawalls and other coastal erosion control structures (coastal development) adjacent to mangrove landward margins as the threat to development from rising sea-levels and concomitant coastal erosion becomes increasingly apparent”(245).
  • Conclusions regarding overall impact of climate change on mangrove habitat are as follows:
    • Projections: “…relative sea-level rise could be a substantial cause of future reductions in regional mangrove area, contributing about 10–20% of total estimated losses”(245).
    • “Rising sea-level will have the greatest impact on mangroves experiencing net lowering in sediment elevation, that are in a physiographic setting that provides limited area for landward migration due to obstacles or steep gradients”(246).
    • “Rise in temperature and the direct effects of increased CO2 levels are likely to increase mangrove productivity, change the timing of flowering and fruiting, and expand the ranges of mangrove species into higher latitudes”(246).
    • “Changes in precipitation and subsequent changes in aridity may affect the distribution of mangroves”(246).
    • Combination of climate change effects will most likely cause more severe impacts on mangrove ecosystems (246). 
       
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