Buried Alive or Washed Away; The Challenging Life of Mangroves in the Mekong Delta

Study Number

92

Author

Sergio Fagherazzi, Karin R. Bryan, and William Nardin

Abstract

Mangroves colonize tropical shorelines, protecting coastal communities and providing valuable ecosystem services. Mangroves associated with deltas cope with a very dynamic environment characterized by strong gradients in salinity, deposition triggered by sediment inputs, and erosion caused by waves and currents. Mangroves are adapted to this ever-changing landscape, with different species colonizing different elevations in response to inundation frequency. A series of feedbacks between hydrodynamics, sediment transport, and mangroves was observed in a fringe forest of the Mekong Delta, Vietnam. Sonneratia spp. rapidly encroach upon sandy areas because the stable substrate favors seedling establishment. In contrast, fewer seedlings are present in muddy locations where currents and waves frequently rework the bottom. Along muddy shorelines that are eroding, turbulence increases local scour near roots and trunks, undercutting the trees. Enhanced sediment accumulation due to delta progradation can smother the mangrove roots and lead to forest dieback. We find clear evidence that mangroves affect both hydrodynamics and sediment transport, thus engineering the landscape and enhancing sediment trapping and delta progradation. Sonneratia spp. are replaced by Aegiceras corniculatum, Avicennia marina, and Nypa fruticans when the seabed becomes high enough, indicating that ecological succession is present in a fast prograding deltaic environment. Thus, it is imperative to determine the small-scale feedbacks between mangroves, hydrodynamics, and sediment transport in order to build quantitative ecogeomorphic models of deltaic sedimentation that can be used to explain the distribution of mangrove species, the forest structure, and large-scale dynamics in a tropical deltaic setting.

Main Results and Conclusions

• Mangroves are important to coastal communities
  • “Mangroves enhance the resilience of coastal communities. They stabilize the shoreline, reducing erosion by waves and currents.” (49)
  • “They can also mitigate the devastating effects of tsunamis, thus protecting human dwellings, infrastructure, and agriculture.” (49)
  • “…mangrove forests provide habitat for many fish and other animal species, especially in juvenile stages, which contributes to the world’s biodiversity.” (49)
• The Mekong Delta in Vietnam is a biodiversity hotspot for mangroves. (49)
• This study focused on the mangrove fringe of the island of Cu Lao Dung
  • “Parts of the mangrove forest were converted to shrimp ponds and sugar cane fields, triggering erosion by waves, so more mangroves were planted in the 1980s to protect the shoreline.” (49)
• This study focus on the mangrove species Sonneratia caseolaris as well as other a few other species found in the area as well.
  • “The pioneer species Sonneratia caseolaris was planted at the shore, and other typical Southeast Asian mangrove species have colonized parts of the fringe with high bottom elevations.” (49)
  • “Avicennia marina, Aegericas corniculatum, and Nypa fruticans can be found in the interior parts of the fringe.” (49)
• Mangrove zonation is important to the establishment of mangrove forests.
  • “…zonation is simply the expression of different stages of mangrove forest development. Mature forest starts at low elevation with pioneer species and higher inundation periods; vegetation composition changes several times during accretion until a vegetation cover typical of present bottom elevation is reached.” (50)
• The most critical stage of mangrove growth is seedling establishment.
  • “Conditions surrounding establishment can be so adverse that this early stage can easily become a “bottleneck” to expansion (Friess et al. 2012).” (51)
  • “Recent results have shed light on the feedbacks between seedling establishment, local hydrodynamics, and sediment-transport processes.” (51)
  • “Balke et al. (2011) introduced the concept of window of opportunity—a lull between storms that rework the bottom sediments during which pioneer mangroves can establish.” (51)
• Mangroves are sensitive to erosion both when they are young and when they are already established.
  • “Avicennia alba and Sonneratia alba seedlings (the latter grow in the Mekong Delta fringe) are sensitive to erosion during early establishment; they can only establish if the bottom substrate is stable. Balke et al. (2013) indicate that as little as 1–3 cm of vertical erosion can result in seedling failure. In addition, if a storm that produces strong currents hits the mangroves, the seedlings dislodge (Balke et al., 2011). Once established, mangroves grow and become more resistant to both erosion and currents.” (52)
• Mangroves play a part in the shaping of coastal lands.
  • “…both the shoaling and the scouring effects of vegetation mean that in some places the transition from bare tidal flat to vegetated forest does not always result in a convex or even flat-topped profile. Rather, a linear profile develops, which we observe along Cu Lao Dung, where the scouring effect diminishes gently landward as the tidal wave progressively loses energy. These results unequivocally indicate that mangrove vegetation does alter intertidal hydrodynamics and sediment transport, thus actively engineering the coastal landscape.” (55)
• The conclusion of this study is “…that mangroves do engineer the landscape, favoring sediment deposition and dissipating waves and preferentially colonizing sandy substrates.” (56)