Mangrove forests: Resilience, protection from tsunamis, and responses to global climate change

Study Number:

55

Author:

D. M. Alongi

Abstract:

This review assesses the degree of resilience of mangrove forests to large, infrequent disturbance (tsunamis) and their role in coastal protection, and to chronic disturbance events and the future of mangroves in the face of global (climate) change. From a geological perspective, mangroves come and go at considerable speed with the current distribution of forests a legacy of the Holocene, having undergone almost chronic disturbance as a result of fluctuations in sea-level. Mangroves have demonstrated considerable resilience over timescales commensurate with shoreline evolution. This notion is supported by evidence that soil accretion rates in mangrove forests are currently keeping pace with mean sea-level rise. Further support for their resilience comes from patterns of recovery from natural disturbances (storms, hurricanes) which coupled with key life history traits, suggest pioneer-phase characteristics. Stand composition and forest structure are the result of a complex interplay of physiological tolerances and competitive interactions leading to a mosaic of interrupted or arrested succession sequences, in response to physical/chemical gradients and landform changes. The extent to which some or all of these factors come into play depends on the frequency, intensity, size, and duration of the disturbance. Mangroves may in certain circumstances offer limited protection from tsunamis; some models using realistic forest variables suggest significant reduction in tsunami wave flow pressure for forests at least 100 m in width. The magnitude of energy absorption strongly depends on tree density, stem and root diameter, shore slope, bathymetry, spectral characteristics of incident waves, and tidal stage upon entering the forest. The ultimate disturbance, climate change, may lead to a maximum global loss of 10–15% of mangrove forest, but must be considered of secondary importance compared with current average annual rates of 1–2% deforestation. A large reservoir of below-ground nutrients, rapid rates of nutrient flux and microbial decomposition, complex and highly efficient biotic controls, self design and redundancy of keystone species, and numerous feedbacks, all contribute to mangrove resilience to various types of disturbance.

Main Results and Conclusions:

• Historical trends are not good indicators of whether or not mangroves will be able to keep up with rising sea levels (2-4).
• Mangrove recovery time after a disturbance varies greatly. The factors that affect mangrove recovery include stand composition and structure, physiological tolerance to physical/chemical gradients, and changes in geomorphology, and competitive interactions (5).
• Successful protection by mangroves against catastrophic events depends upon many factors, including “the type of environmental setting and other relevant features and conditions”(6). For example, mangroves offer varying protection against tsunamis based on “…width of forest, slope of forest floor, tree density, tree diameter, proportion of above-ground biomass vested in roots, tree height, soil texture, forest location (open coast vs lagoon), type of adjacent lowland vegetation and cover, presence of foreshore habitats (seagrass meadows, coral reefs, dunes), size and speed of tsunami, distance from tectonic event, and angle of tsunami incursion relative to the coastline”(6).
• The paper uses the 2004 Indonesian tsunami as a case study of mangrove protective capacity (6-7).
• Climate change may have very severe impacts on mangrove habitat. Environmental changes associated with climate change and their relative impacts on mangrove habitat include:
  • “Rise in sea-level: landward progression, increased secondary productivity due to greater nutrient availability from erosion,
  • Rise in atmospheric CO2: advanced flowering, enhanced water-use efficiency, no or little increase in primary production and respiration
  • Rise in air and water temperature: decreased survival in areas of increased aridity, expanded latitudinal ranges, increased net and gross primary production, increase in water vapor pressure deficit, Increased secondary production (especially microbes) and shift in species dominance, changes in phenological patterns of reproduction and growth, and an increase in biodiversity
  • Change in precipitation/storm patterns, frequency and intensity: changes in mangrove species composition and growth due to change in soil water content, salinity, increased primary production due to increase in precipitation/evaporation ratio, changes on faunal biodiversity, increase in gaps and gap recruitment” (Table 2, p. 8: Woodruffe 1990, Aksornkaoe & Paphavasit 1993, Pernetta 1993, UNEP 1994, Semeniuk 1994, Snedaker 1995, Miyagi et al. 1999, Nicholls et al. 1999, Hogarth 2001, Alongi 2002, Schaeffer-Novelli et al. 2002, Done & Jones 2006; Gilman et al. 2006).
• Certain mangrove ecosystems will be more affected than others:

• “Mangroves occupying low-relief islands and/or carbonate settings, where rates of sediment supply and available upland space are ordinarily low, such as on small islands in the Pacific, are most vulnerable. Also most vulnerable are forests where rivers are lacking and/or where the landform is subsiding.
• “The least vulnerable, aside from those occupying macro-tidal estuaries, wet tropical areas and shores adjacent to rivers, are those stands occupying high-relief islands and remote areas where humans are unlikely to block landward migration”(Figure 8, p. 10: Wilkie & Fortuna 2003, Gilman et al. 2006, UNEP-WCMC 2006, Solomon et al. 2007).

Works Cited:

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