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Abstract

An assessment was made of American Samoa mangroves' vulnerability and
predicted changes in position from sea-level rise. The study also evaluated
capacity in the Pacific Islands region to assess mangrove vulnerability to
climate change and institute adaptation measures. Of the outcomes from
climate change, relative sea-level rise may be the greatest threat to
mangroves. By 2100, mangrove losses from relative sea-level rise could be
as high as 47 percent in American Samoa and 22 percent when extrapolating
regionally, causing about a quarter of total predicted annual regional losses.
American Samoa mangrove vulnerability to sea-level rise was
determined and future position was predicted through analyses of sea-level
trends and projections, mangrove spatial change analysis, reconstruction and
monitoring of sedimentation rates, and determination of potential migration
areas. These analyses provided three categories of requisite information: (i)
Observed and projected rate of change in sea-level relative to the mangrove
sediment surface, determined from trends in relative sea-level through
analysis of sea-level data from a local tide gauge and observations of trends
in the elevation of mangrove sediment surfaces; (ii) observed and projected
trends in mangrove seaward margin positions; and (iii) physiographic settings
(slope of land upslope and location of obstacles along the landward margin).
Results indicate that American Samoa mangroves are not likely
keeping pace with rising sea-level, both surface and subsurface process
controls on sediment elevation are important factors, and a large proportion
(16, 23 and 68 percent) of the landward margins of the three mangrove study
sites are obstructed from natural landward migration with sea-level rise.
Based on observed trends in sediment surface elevations and movement of
two mangroves' seaward margins, these sites have likely not been keeping
pace with relative sea-level rise, with an elevation deficit of about 2 mm eat
both sites. An embayment mangrove experienced sea-level rise relative to
the mangrove sediment surface of 2.22 (± 2.22 95% Cl) mm a-1 and a basin
mangrove experienced 1.97 (± 0.32 95% Cl) mm a -1 . At these sites, a highly
significant positive correlation between the change in position of the seaward
margins and change in relative sea-level suggests that rising sea-level
relative to the mangrove surface caused the observed landward migration.
Shoreline movement was not significantly correlated with changing sea-level
at a third site, where development activities have likely been dominant factors
determining changes in mangrove position; vulnerability, based on
observations of trends in sediment surface elevation, was not determined for
this third site.
This study was the first to employ broad spatial coverage and a large
number of sampling locations (330 sampling locations) to observe trends in
the elevation of two mangroves' sediment surfaces, a necessary sampling
design to adequately characterize mangrove sites, based upon previously
documented high spatial variability in trends in mangroves' surface elevation.
Both surface and subsurface processes exhibited large controls on sediment
elevation, highlighting the need to monitor the full soil profile to accurately
measure trends in mangrove surface elevation. Highly significant different
mean changes in sediment surface elevation occurred for mangroves in
different geomorphic settings (a difference of 3.4 [± 1.3 SE] mm a -1 , N =
1412, P<0.007), supporting the hypothesis that mangroves in an
estuarine/drowned river valley composite geomorphic setting are more
resistant to relative sea-level rise than embayment mangroves. Mean
landward migration of the mangroves' seaward margins was 12 to 37 times
the relative sea-level rise rate. This is the first documentation of significantly
different mean sediment surface elevation change for mangroves in different
geomorphic settings, and the first documentation of the relationship between
the rate of seaward mangrove margin erosion and relative sea-level rise rate,
information needed to develop reliable predictive elevation models for
mangrove ecosystems. Changes in extreme high water levels and frequency
were found to not pose an increasing threat to American Samoa mangroves
beyond the effects from rising mean sea-level. This site-specific assessment
supports the hypothesis that, in this region, which experiences large El Nino
Southern Oscillation-related steric changes lasting several months to years,
extreme high waters are likely to be related to mean sea-levels.
This was the first comprehensive assessment to determine both (i)
whether the mangrove site's threshold for resistance to sea-level rise has
been exceeded, and (ii) the site's capacity to naturally migrate landward in
response to rising sea-level. This was the first study to select research
methods suitable for employment in Pacific Small Island Developing States,
considering both cost and staff abilities.
Results support instituting adaptation measures in American Samoa to
reduce obstacles to landward mangrove migration with sea-level rise and to
manage activities within catchments that affect mangrove elevation.
Regionally, there is extremely low capacity to assess mangrove vulnerability
to climate change and to institute adaptation measures. Regional adaptation
priorities include coastal planning that facilitates mangrove migration with
sea-level rise, better management of non-climate stressors, and identification
of climate change impacts on mangroves through regional standardized
monitoring.

Item Type:	Thesis - PhD
Authors/Creators:	Gilman, E
Keywords:	Mangrove plants, Mangrove restoration, Coastal ecology, Climatic changes
Copyright Holders:	The Author
Additional Information:	Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references. Ch. 1. Introduction -- Ch. 2. Study area -- Ch. 3. Methods: predicting American Samoa mangrove responses to regional relative sea-level rise -- Ch. 4. Results: American Samoa mangrove vulnerability and responses to relative sea-level rise -- Ch. 5. Discussion - predicting American Samoa mangrove responses to regional relative sea-level rise -- Ch. 6. Capacity of Pacific Island countries and territories to assess vulnerability and adapt to mangrove responses to climate change -- Ch. 7. Adaptation options for mangrove responses to climate change -- Ch. 8. Conclusions
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