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How Mangrove Forests Adjust to Rising Sea Level

Explore how mangrove forests (a coastal plant community) keep pace with rising sea level. Link to technical paper: http://onlinelibrary.wiley.com/doi/10.1111/nph.12605/full

Karen McKee

on 24 May 2016

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Transcript of How Mangrove Forests Adjust to Rising Sea Level

Physical and biological processes occurring above and below the soil surface influence vertical land development in mangrove ecosystems.
How Mangrove Forests
Adjust to Rising Sea Level

Mangroves are trees and shrubs that grow along tropical coastlines where they are affected by rising sea level.
Mangroves are able to persist where vertical land development balances relative sea-level rise.
Relative Sea-Level Rise =
Increase in
ocean height
Local land
Surface processes
Sediment deposition (+)/Erosion (-)
Sediment trapping by aerial roots (+)
Leaf and wood litter deposition (+)
Benthic algal/microbial mat growth (+)
Microbial decomposition (-)
Tidal flushing (-)
Sub-surface processes
Root production (+)/
decomposition (-)
Scientists use a Surface Elevation Table-Marker
Horizon system to measure vertical movements.
Different types and density of aerial roots affect rates of sedimentation (Krauss et al. 2003).
Prop roots
Plank roots
Leaf and wood litter may accumulate and contribute to vertical accretion where tidal flushing is limited, leaf-eating crabs are absent, and decomposition is slow (Middleton & McKee 2001) (a core showing organic material accreting above a marker horizon (white line)).
Benthic mats may be composed of different types of filamentous algae and microbial communities, which not only contribute organic matter directly to vertical accretion, they also trap and retain inorganic sediment and strengthen soils against shearing and erosion (McKee 2011).
Leaf-eating crabs and snails also influence whether and how fast organic matter accretes and is incorporated into the soil (Middleton and McKee 2001).
What external factors may affect the
ability of mangroves to keep pace
with sea-level rise?
Compared to leaves and wood, roots have a greater potential to contribute to soil volume due to their refractory nature and anaerobic soil environment, which slows decomposition (Middleton & McKee 2001).
Thick organic deposits (peat) can build up over time beneath mangrove forests. This is a half-meter core extracted from a forest in Belize.
Some mangrove systems (Belize)
have accumulated over ten meters
of peat. Fossil roots extracted from
deepest layers were radiocarbon
dated at over 7,000 years before
present (McKee et al. 2007). This peat record shows that this system has kept pace with sea-level rise through gradual accumulation of roots and other plant matter.
Under magnification, this mangrove peat
is seen to be primarily composed of fine roots
(McKee and Faulkner 2000).
Physical compaction (-)
Shrink-swell by water flux (-/+)
Natural disturbances such as hurricanes or tsunamis may damage or kill mangroves (view of a mangrove stand on the Bay Island of Guanaja, Honduras after Hurricane Mitch).
Artificial structures (dikes, seawalls) may block lateral migration of mangroves or alter hydrology and sediment supplies (Lovelock & Ellison 2007).
Changes in atmospheric CO2 concentrations may alter biomass production and/or decomposition rates as well as competitive ability of mangroves (McKee & Rooth 2008).
Climate change (air and sea temperatures, rainfall) may affect production-decomposition processes in mangrove forests (reviewed by McKee et al. 2012)
Such factors may alter the capacity
of a mangrove system to respond to
sea-level rise.
Knee roots
Mangrove mortality can lead to loss of soil integrity as well as reduction in organic matter contributions
to soil volume in peat-forming systems (Cahoon et al. 2003.
Mangrove propagules trapped in front of
the seawall.
Higher CO2 concentrations can stimulate photosynthesis and improve water use efficiency in "C3 species" such as mangroves (reviewed by McKee et al. 2012).
Clearcutting and/or conversion to other uses may affect litter accumulation and turnover (Lee 1989) with consequences for soil building and vertical accretion (view of a shrimp pond in Indonesia).
Upland deforestation can increase sediment
runoff and input to mangrove habitat (satellite view of a mangrove system in New Zealand surrounded by cleared land).
Increased sedimentation is driving mangrove
expansion in some New Zealand estuaries
(Lovelock et al. 2010).
Global surface temperature anomaly (1881-2007) NASA Goddard Institute for Space Studies
Animation: mouse over to see player controls
Compaction, which causes reduction in soil volume, is influenced by particle size (clay, silt, sand) and
organic matter and water contents.
Mining activities can increase inputs of sediment and pollutants (view of a mangrove forest in New Caledonia affected by nickel mining runoff)
(Marchand et al. 2012)).
Clearcutting and excavation of mangrove soils leads to oxidation of organic matter.
300 million cubic meters of mine spoil have been produced in New Caledonia since the late 19th century, with substantial amounts delivered to the coastal zone. Concentrations of nickel, iron, and chromium are 10 to 100 times higher in mangrove soils downstream of effluent (Marchand et al. 2012), with unknown consequences for plant production and other processes important to habitat stability.
Mangroves may receive nutrients from natural (bird rookeries) or anthropogenic (sewage) sources.
Nutrient enrichment of oligotrophic mangroves can alter root accumulation rates, driving elevation gain or loss (McKee et al. 2007).
Read the technical paper for more information:

Krauss, K.W. et al. 2014. How mangrove forests adjust to rising sea level. New Phytologist (Tansley Review) 202 (1) (http://onlinelibrary.wiley.com/doi/10.1111/nph.12605/full).
Additional Literature Cited

• Cahoon, D.R. et al. 2003. Journal of Ecology 91: 1093–1105.
• Krauss, K.W. et al. 2003. Estuarine, Coastal and Shelf Science 56: 251-259.
• Krauss, K.W. et al. 2010. Ecosystems, 13, 129-143.
• Lee, S.Y. 1989. Estuarine, Coastal and Shelf Science 29: 75-87.
• Lovelock, C.E. and J.C. Ellison. 2007. In: Climate Change and the Great Barrier Reef: A Vulnerability Assessment (pp. 237-269).
• Lovelock, C.E. et al. 2010. Ecosystems 13: 437-451.
• Marchand, C. et al. 2012. Chemical Geology 18: 70-80.
• McKee, K.L. 2011. Estuarine, Coastal and Shelf Science 91: 475–483.
• McKee, K.L. and P.L. Faulkner. 2000. Atoll Research Bulletin 468: 46–58.
• McKee, K.L. et al. 2007. Global Ecology and Biogeography 16: 545–556.
• McKee, K.L. et al. 2012. In: Global change and the function and distribution of wetlands. Springer, 63–96.
• McKee, K.L. and J.E. Rooth. 2008. Global Change Biology 14: 971–984.
• Middleton, B.A. and K.L. McKee. 2001. Journal of Ecology 89: 818–828.
• Sidick F. and C.E. Lovelock. 2013. PLoS ONE 8(6): e66329. doi:10.1371/journal.pone.0066329.

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The Scientist Videographer
How do scientists determine
if mangroves are keeping up
with sea-level rise?
Sediment accretion may vary from less than 1 to over 17 millimeters per year in mangrove forests (Krauss et al. 2010; McKee 2011).
This is a movie; mouse over to see player controls.
Image: Google Earth, DigitalGlobe
The net effect of physical
and biological processes
determines whether a
mangrove forest can
accommodate changes
in sea level.
Conversion of mangrove forests to
aquaculture ponds also contributes
to greenhouse gas emissions with
feedback effects on global warming
and sea-level rise. In Indonesia,
release of CO2 from shrimp ponds
is estimated to be 6 to 14 Tg/year
(Sidick and Lovelock, 2013).
Movie: mouse over to see player controls
Detritivory (-)
How Mangrove Forests Adjust To Rising Sea Level

Visualization by K.L. McKee
Biological (decomposition) and physical (tidal
action) processes may reduce organic contributions to soil volume.
Full transcript