3.4 Sea level rise & biogeography

3.4.1 Ellison et al, 1999

“Origins of mangrove ecosystems and the mangrove biodiversity anomaly” (Ellison, Farnsworth, and Merkt 1999)

Key significance: This study examines two separate hypotheses for the distribution of mangrove diversity globally. In essence, they seek to understand the stark decreases in species diversity in the ACEP region relative to IWP.

Key notes:

Two primary hypotheses for the global distribution of mangrove species diversity exist:

  1. “Center of origin” hypothesis - holds that mangrove taxa originated in the IWP and then dispersed to other parts of the world
  2. “Vicariance hypothesis” - taxa evolved around the Tethys Sea and regional species diversity has results from in situ diversification after continential drift

Research design: The authors consider primarily five lines of evidence to parse whether the Center of Origin versus Vicariance Hypothesis is more likely to explain current patterns of species diversity. The five lines of evidence they use are:

  1. Fossil records - distribution of fossils all around Tethys would support vicariance, whereas fossils in Eastern Tethys (IWP) would support Center of Origin.
  2. Available area:
    • Center of origin - species richness negatively correlated with distance from center of origin rather than available area
    • Vicarance - species richness should be correlated with available area
  3. Distributions of fauna - Would expect to see similarities in diversity anomaly between mangrove tree species and the fauna that inhabit the forests
  4. Geographical nestedness - geographical nestedness may occur through colonization of far-reaching areas by select species, or by extinction of taxa following vicariance.
  5. Nestedness of entire communities - nestedness of mangroves and all gastropods as described above

Key results: Ellison et al conclude that the lines of evidence provides support for the vicariance hypothesis.

In particular, strong species-area relationships were found, evidence in the fossil record, and a unimodality in the distribution of gastropod genera were found to be convincing evidence in support of the vicariance hypothesis.

3.4.2 Mckee & Faulkner, 2000

“Mangrove peat analysis and reconstruction of vegetation history at the Pelican Cays, Belize”

Key contribution: This study examines the historical development of peat formation in the Pelican Cays in Belize. This work is a precursor to (McKee, Cahoon, and Feller 2007) which studied the formation of peat deposits in the Belize cays in depth.

Study design: A transect across Pelican Cays was laid out that spanned Rhizophora fringe, Avicennia basin/woodlands, a treeless area with standing water, and the same pattern to the other coast. Peat cores to 0.5 m and 2 m were taken and soil and peat characteristics were described.

Key results: Upon identifying roots, shoots and leaf fragments within the peat to species type, the authors were able to infer the successional process of the forest. In stands that are currently Avicennia dominated, Rhizophora roots make up the majority of the deep peat, indicating they once dominated that stand. In addition, root biomass was found to make up the majority (~80%) of the peat biomass.

Note: Potentially worth relooking at this as it may be exemplary of considering the “ecological history” of a site and its impact on shaping communities.

3.4.3 Valiela et al, 2001

“Mangrove forests: One of the world’s threatened major tropical environments” (Valiela, Bowen, and York 2001)

Key contribution: Following widespread recognition of the loss of other tropical ecosystems (rain forests and coral reefs), this paper makes the argument that mangroves are under-appreciated but perhaps even more threatened than these other ecosystems.

Study design: They compile data on the global status of mangroves from the literature, and assess to what degree human activities have contributed to their loss.

They use the Global Mangrove Atlas coupled with estimates of status and extent of mangroves across different countries to look at rates of loss for different continents and globally.

They assess the human-based impacts on mangrove extent by correlating lost mangrove extent with GDP and also coastal population density.

Key results: The study estimates that approximately 35% of mangroves have been lost from their “original” extent (i.e., the earliest year in which data could be found; 1920s for some data from Africa and Asia).

Global mean rates of loss were approximately 2% per year, with continent specific rates ranging from 1 to 3.6% (3.6% was for the Americas).

They attribute approximately two-thirds of the loss of mangroves directly to human related activities. The correlation between GDP and loss of mangrove extent is significant, whereas little correlation between loss of mangroves and coastal population density is found.

Furthermore, Valiela et al delve into the importance of mariculture for mangrove habitat losses. Conversion of mangroves to shrimp ponds is the dominant driver of deforestation, and has additional indirect impacts on the environment such as:

  • Reductions in offshore shrimp stocks due to need for juveniles
  • Impacts on offshore fisheries that are used as feed within the shrimp ponds

3.4.4 Krauss et al, 2014

“How mangrove forests adjust to rising sea level” (Krauss et al. 2014)

Key contribution: This review differs from others in that it focuses on the role that vegetation plays in maintaining soil surface elevations in mangroves.

Key notes:

As of 2010, current rate of mean sea level rise is 3.2 +/- 0.4 mm per yr (based on satellite altimetry), though these rates likely vary locally. IPCC projections (AR5) estimate SLR rates of 3 - 5 mm / yr over the next 100 years.

“Relative sea level rise” refers to the rate of sea level rise in conjunction with subsidence or vertical accretion of coastal areas (i.e., consideration of both eustatic SLR as well as changes in the mangrove surface elevation).

Surface elevation changes are a result of both biotic and abiotic processes. Abiotic processes include:

  1. Inorganic sedimentation
  2. Groundwater fluxes (anthropogenic depletion of groundwater reservoirs may be significant)
  3. Deep land movements

Whereas biological processes include:

  1. Plant litter and woody debris deposition
  2. Root accumulation
  3. Sediment trapping by vegetative structures
  4. Algal mat development on soil surface

Other processes include acute deposition from storm events, or feedback processes between vegetation systems and degree of inundation.

Aerial roots and sedimentation - Different functional root types will influence sedimentation rates differently, and through different processes. Pneumatophores are believed to retain deposited soil better than other types, whereas stilt roots may result in greater deposition of sediment.

Litter and woody debris accumulation - In absence of tidal flushing as well as crabs and other organisms responsible for turnover of organic matter, accumulation of OM may be significant.

Benthic mat formation - Intact algal mats may bind sediments and resist erosion.

Subsurface root accumulation - May be the most important biological contributor to surface elevation change in some settings (carbonate settings or other settings with limited mineral sediment input). Can be significant enough to affect the direction and rate of change in surface elevation.

  • Refractory nature of root tissues and existence within anaerobic environment are prone to slow rates of decomposition
  • Rates of surface elevation change were correlated with both fine and coarse root production
  • “Specific root length” is a measure of the length of root per unit mass; mangroves have more dense roots than tropical species, which is likely due to anoxic conditions

Influence of roots on surface elevation is bidirectional; death of roots can result in collapse of roots under weight of soil

Factors affecting root contributions to soil accretion:

In general, the influence or roots on SEC is a relative function of production and decomposition of roots.

  • Salinity - response of root growth to salinity is species dependent, but root growth only slightly reduced with increasing salinity; increases in root to shoot ratio may actually be a result of increased aboveground biomass rather than increased root biomass (for example through nutrient release)
  • Nutrients - in general do not see an increase in root production, but rather increases in biomass production aboveground; in some settings may even see decrease in elevation due to release of nutrient-limited microbial communities and thus decomposition of belowground organic matter
  • Flooding - effect of flooding on root-production is hard to identify, and may occur in both directions depending on setting (increasing root-production in basin type settings, decreasing root production in others)
  • Soil texture - high bulk density decreases root growth due to more difficult conditions of growing
  • Disturbance - may be biotic or abiotic:
    • Largely hypothesized that organisms may influence root production, though the mechanisms and degree to which this occurs is unknown
    • Acute deposition of sediments from storm events may kill trees, or may provide much needed substrate for subsiding forests; root zone expansion following deposition of sediment may be significant and may further increase surface elevations
    • Lightning may kill trees, causing root collapse and alter microhabitat hydrology (Sherman, 2000)

Climatic and environmental feedbacks:

  1. Precipitation – seasonal or storm based variability in rainfall can cause swelling or subsidence of peat layers or water table
  2. Elevated CO2 - likely that increases in CO2 concentrations will improve biomass growth, though most of this is likely to occur in the aboveground systems
  3. Feedbacks related to SLR - growth rates may increase sharply for a short period of time, but eventually fall off under prolonged inundation

3.4.5 Gilman, 2008

“Threats to mangroves from climate change and adaptation options: A review” (Gilman et al. 2008)

Key significance: The paper reviews the dominant threats to mangroves from climate change related impacts. In addition, they provide several suggestions for measures that can be taken to potentially bolster mangrove resilience to climate change related pressures.

Key notes: The key threats to mangroves from climate change as identified by the paper are:

Sea level rise - relative sea level rise is a key threat, and is relatively difficult to measure as tidal gauges may not accurately reflect sea level conditions within the actual forest of interest.

Mangroves are vulnerable when they are unable to increase surface elevation at a rate greater than sea level rise, which is due to both surface accretion as well as belowground processes.

Considerable feedbacks exist in which increasing surface elevation may decrease hydroperiod and thus sediment accretion, or alternatively subsidence may promote hydroperiod and increased sediment accretion.

Three general response of mangroves to changing sea-levels are:

  1. Stable site-specific relative sea-level - sea-level not changing relative to mangrove and thus mangrove is “stable”
  2. Site-specific relative sea subsidence - mangrove margins migrate seawards
  3. Site-specific relative sea rising - mangroves retreat landward; may expand into landward areas

Other factors beyond sea level rise are also important for determining resilience of mangroves to SLR. Four main factors are identified:

  1. Rate of SLR relative to changes in mangrove surface elevation
  2. Species composition - different species have different rates of change in sediment elevation
  3. Physiographic setting - slope of land upslope and ability of mangroves to migrate landwards
  4. Cumulative effects of stressors - affect ability to persist and also colonize new areas

Extreme high water events - The impacts of high water events is relatively poorly understood at present, but the impacts may be significant or may induce extensive stress in mangroves.

Storms - Winds, precipitation and storm surge heights are expected to increase with climate change. More intense storms may damage or defoliate mangroves, but may also cause erosion, peat collapse, or otherwise adversely impact edaphic conditions.

Precipitation - Projections are for more precipitation, but in a spatially heterogeneous manner. Changes in precipitation (total as well as seasonal amounts) may impact vegetation growth, soil conditions, and also ability of the mangrove to migrate landwards or seawards.

Temperature - The main effects of temperature on mangroves are:

  1. Changing species composition
  2. Changing phenological patterns
  3. Increasing mangrove productivity where temperature doesn’t exceed an upper threshold
  4. Expanding mangrove ranges to northern latitudes

Atmospheric CO2 - Likely to improve productivity of trees in low-salinity environments, but may not have substantial impact on those in high-salinity environments.

Recommendations for improving resilience - The author provide a list of recommendations for improving mangrove resilience to the impacts of sea level rise:

  1. “No regret” reductions of stress – Simply better management techniques that reduce anthropogenic stresses on existing mangrove forests
  2. Manage activites that directly influence sediment elevation – Not over-depleting groundwater reservoirs or cutting off landward supplies of sediment to the forest will improve its ability to respond to SLR
  3. Managed retreat – Manage landscapes such that mangroves have a region in which they can retreat towards
  4. System of protected area networks – Ensuring representation of mangrove types within protected area networks or replication of identical communities to improve resilience to risk (i.e., “portfolios” of communities)
  5. Mangrove restoration – Restoring impacted areas through reforestation or afforestation
  6. Regional monitoring network – Using standardized techniques to monitor and examine key criteria (e.g., SLR and surface elevation change a la Sasmito 2016)
  7. Education and outreach

3.4.6 Sasmito et al 2016

“Can mangroves keep pace with contemporary sea level rise? A global data review” (Sasmito et al. 2016)

Key contribution: This study systematically reviews data on surface elevation change (SEC) and surface accretion rate (SAR) in mangroves to examine their ability to keep pace with projections of sea level rise globally.

Of critical importance is:

  1. Surface accretion rate (SAR) – process of sediment accumulation on the surface of the forest floor
  2. Surface elevation change (SEC) – a function of biotic and abiotic processes, such as growth of roots or compaction or subsidence due to OM decomposition
  3. Sub-surface change (SSC) – the difference between SAR and SEC

The data review examines studies that use Rod Surface Elevation Table and Marker Horizon methods to examine elevation change.

Key results Only 19 studies with data from some 74 sites were found through the literature review. Running different models of the influence of geomorphological setting and management type (restored, plantation, natural, etc.) on SAR, SEC and SSC found them to be influential.

In particular, basin mangroves may be better able to adapt to SLR than fringe mangroves or those on islands with shallow tidal ranges (and thus little sedimentation).

The study nods to the importance of belowground processes in conjunction with geomorphological setting and management practices for SEC.

Extrapolation of SAR and SEC rates with projected SLR shows that mangroves may be able to keep pace with low (RCP 2.6) emissions trajectories, but are unlikely to cope with high emissions pathways (RCP 8.5) regardless of geomorphological setting or management regime.

Additionally, there is more systematic monitoring of SEC using RSET-MH in Indonesia, the Philippines, Singapore, SW Thailand and Vietnam underway. This data will better inform the ability of mangroves to respond to sea level rise.

  • Additional thought – how can this be worked into my research trajectory?

Relevance for research: Interesting in that they call for standardized global monitoring of surface elevation data. Could potentially work into remote sensing of geomorphology in mangroves.

References

Ellison, Aaron M, Elizabeth J Farnsworth, and Rachel E Merkt. 1999. “Origins of Mangrove Ecosystems and the Mangrove Biodiversity Anomaly.” Global Ecology and Biogeography 8 (2): 95–115. doi:10.1046/j.1466-822X.1999.00126.x.

McKee, Karen L, Donald R Cahoon, and Ilka C Feller. 2007. “Caribbean Mangroves Adjust to Rising Sea Level Through Biotic Controls on Change in Soil Elevation.” Global Ecology and Biogeography 16 (5): 545–56. doi:10.1111/j.1466-8238.2007.00317.x.

Valiela, Ivan, Jennifer L Bowen, and Joanna K York. 2001. “Mangrove Forests: One of the World’s Threatened Major Tropical Environments.” Bioscience 51 (10): 807–15. doi:10.1641/0006-3568(2001)051[0807:MFOOTW]2.0.CO;2.

Krauss, Ken W, Karen L McKee, Catherine E Lovelock, Donald R Cahoon, Neil Saintilan, Ruth Reef, and Luzhen Chen. 2014. “How Mangrove Forests Adjust to Rising Sea Level.” New Phytologist 202 (1): 19–34. doi:10.1111/nph.12605.

Gilman, Eric L, Joanna Ellison, Norman C Duke, and Colin Field. 2008. “Threats to Mangroves from Climate Change and Adaptation Options: A Review.” Aquatic Botany 89 (2): 237–50. doi:10.1016/j.aquabot.2007.12.009.

Sasmito, Sigit D, Daniel Murdiyarso, Daniel A Friess, and Sofyan Kurnianto. 2016. “Can Mangroves Keep Pace with Contemporary Sea Level Rise? A Global Data Review.” Wetlands Ecology and Management 24 (2): 263–78. doi:10.1007/s11273-015-9466-7.