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Aim: The long-term stability of coastal ecosystems such as mangroves and salt marshes depends upon the maintenance of soil elevations within the intertidal habitat as sea level changes. We examined the rates and processes of peat formation by man-groves of the Caribbean Region to better understand biological controls on habitat stability. Location: Mangrove-dominated islands on the Caribbean coasts of Belize, Honduras and Panama were selected as study sites. Methods: Biological processes controlling mangrove peat formation were manipulated (in Belize) by the addition of nutrients (nitrogen or phosphorus) to Rhizophora mangle (red mangrove), and the effects on the dynamics of soil elevation were determined over a 3-year period using rod surface elevation tables (RSET) and marker horizons. Peat composition and geological accretion rates were determined at all sites using radiocarbon-dated cores. Results: The addition of nutrients to mangroves caused significant changes in rates of mangrove root accumulation, which influenced both the rate and direction of change in elevation. Areas with low root input lost elevation and those with high rates gained elevation. These findings were consistent with peat analyses at multiple Caribbean sites showing that deposits (up to 10 m in depth) were composed primarily of mangrove root matter. Comparison of radiocarbon-dated cores at the study sites with a sea-level curve for the western Atlantic indicated a tight coupling between peat building in Caribbean mangroves and sea-level rise over the Holocene. Main conclusions: Mangroves common to the Caribbean region have adjusted to changing sea level mainly through subsurface accumulation of refractory mangrove roots. Without root and other organic inputs, submergence of these tidal forests is inevitable due to peat decomposition, physical compaction and eustatic sea-level rise. These findings have relevance for predicting the effects of sea-level rise and biophysical processes on tropical mangrove ecosystems.
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© 2007 The Authors DOI: 10.1111/j.1466-8238.2007.00317.x
Journal compilation © 2007 Blackwell Publishing Ltd
Global Ecology and Biogeography, (Global Ecol. Biogeogr.)
Blackwell Publishing Ltd
Caribbean mangroves adjust to rising sea
level through biotic controls on change in
soil elevation
Karen L. McKee
*, Donald R. Cahoon
and Ilka C. Feller
The long-term stability of coastal ecosystems such as mangroves and salt
marshes depends upon the maintenance of soil elevations within the intertidal habitat
as sea level changes. We examined the rates and processes of peat formation by man-
groves of the Caribbean Region to better understand biological controls on habitat
Mangrove-dominated islands on the Caribbean coasts of Belize, Honduras
and Panama were selected as study sites.
Biological processes controlling mangrove peat formation were manipulated
(in Belize) by the addition of nutrients (nitrogen or phosphorus) to
(red mangrove), and the effects on the dynamics of soil elevation were
determined over a 3-year period using rod surface elevation tables (RSET) and marker
horizons. Peat composition and geological accretion rates were determined at all
sites using radiocarbon-dated cores.
The addition of nutrients to mangroves caused significant changes in rates
of mangrove root accumulation, which influenced both the rate and direction of
change in elevation. Areas with low root input lost elevation and those with high
rates gained elevation. These findings were consistent with peat analyses at multiple
Caribbean sites showing that deposits (up to 10 m in depth) were composed primarily
of mangrove root matter. Comparison of radiocarbon-dated cores at the study sites
with a sea-level curve for the western Atlantic indicated a tight coupling between
peat building in Caribbean mangroves and sea-level rise over the Holocene.
Main conclusions
Mangroves common to the Caribbean region have adjusted to
changing sea level mainly through subsurface accumulation of refractory mangrove
roots. Without root and other organic inputs, submergence of these tidal forests is
inevitable due to peat decomposition, physical compaction and eustatic sea-level
rise. These findings have relevance for predicting the effects of sea-level rise and
biophysical processes on tropical mangrove ecosystems.
Accretion, decomposition, elevation change, habitat stability, Holocene, nutrients,
, subsidence.
*Correspondence: Karen L. McKee, US
Geological Survey, National Wetlands
Research Center, Lafayette, LA 70506, USA.
US Geological Survey, National Wetlands
Research Center, 700 Cajundome Blvd,
Lafayette, LA 70506, USA,
US Geological Survey, Patuxent Wildlife
Research Center, 10300 Baltimore Ave., BARC-
East Bldg 308, Beltsville, MD 20705, USA, and
Smithsonian Institution, Smithsonian
Environmental Research Center, 647 Contees
Wharf Road, Edgewater, MD 21037, USA
Mangrove wetlands offer important ecological and societal
benefits along tropical and subtropical coastlines. Mangroves
provide nursery grounds and refuge for commercially important
marine organisms (Primavera, 1998; Mumby
et al.
, 2004), act
as buffers against hurricanes and tsunamis (Dahdouh-Guebas
et al.
, 2005; Danielsen
et al.
, 2005), and filter terrestrial sediment
and nutrients, minimizing their input into more sensitive systems
such as seagrass beds and coral reefs (Alongi & McKinnon,
2005). Mangroves and other tidal, saline wetlands such as salt
marshes also have the capacity to sequester carbon into soil faster
than in terrestrial ecosystems (Chmura
et al.
, 2003). The habitat
stability of mangroves and other coastal wetlands ultimately
depends on the capacity of the system to maintain soil elevations
relative to sea level. Plants contribute directly to soil formation
K. L. McKee
et al.
© 2007 The Authors
Global Ecology and Biogeography
, Journal compilation © 2007 Blackwell Publishing Ltd
through input of organic matter, but no empirical information
directly links specific biotic processes to elevation change in
coastal systems. This type of information is necessary, however,
to accurately predict the future responses of coastal wetlands to
sea-level rise and their interactions with other factors affecting
biological processes, e.g. nutrients.
Mangroves develop in a wide range of sedimentary environ-
ments, from alluvial habitats with abundant mineral input to
oceanic islands with little or no allochthonous contributions of
sediment. Oceanic mangrove systems dependent upon the bio-
genic processes of vertical land building are common throughout
the Caribbean region and total over 780,000 ha (FAO, 2003).
Caribbean mangrove forests are often underlain by deep peat
deposits formed by the gradual accumulation of organic matter
(Woodroffe, 1983; Parkinson
et al.
, 1994; Cameron & Palmer,
1995; McKee & Faulkner, 2000a; Middleton & McKee, 2001;
Toscano & Macintyre, 2003; Macintyre
et al.
, 2004). These man-
grove forests are fundamentally different from those receiving
mineral sediment (e.g. in alluvial habitats) and thus represent a
model system for the study of biotic contributions to elevation
Although the biogenic nature of these and other peat-forming
systems is generally recognized, the processes controlling the
accumulation of organic matter have not been directly linked to
elevation change. In addition, little information exists on
how peat formation and elevation change may be affected by
eutrophication, a major threat to mangrove systems globally
et al.
, 2001). Recognition of the biotic contributions to
elevation change is essential for the development of models
that can better predict the responses of coastal wetlands to
sea-level rise and climate change. An understanding of the
central role of mangroves in controlling soil elevation is also
critical to the conservation and management of tropical coast-
lines worldwide.
To better understand the rates and processes of mangrove
peat formation, we examined the rates of elevation change and
accumulation of root matter in a mangrove ecosystem in the
Meso-American Barrier Reef of Belize. This ecosystem contains
hundreds of mangrove-dominated islands, and together with
mainland mangroves totals 65,767 ha (FAO, 2003). These
Belizean islands were selected as representative of peat-forming
mangrove systems found throughout the Caribbean (Woodroffe,
1983, 1995; Toscano & Macintyre, 2003; Macintyre
et al.
, 2004)
with some of the deepest (10 m) and oldest deposits of peat
(>7000 years) (Macintyre
et al.
, 1995, 2004; Toscano & Macintyre,
2003). In addition, these islands have experienced less human
interference than many other locations [loss of mangrove area in
Belize is
12 % of the total reported in 1980 compared with a
loss of 21% over the same period for the Americas (FAO, 2003)]. To
compare modern changes in elevation with past peat development,
geological rates of peat accumulation were determined using
radiocarbon dating of peat cores from Belize, Honduras and
Panama. The results were compared with a sea-level curve for the
western Atlantic (Toscano & Macintyre, 2003) as well as with peat
accumulation in other locations throughout the Caribbean region.
We then used the results to predict how peat-forming mangroves
may respond to future scenarios of sea-level rise. Our approach is
applicable to other coastal wetlands, particularly those in which
elevational dynamics are dominated by biological processes.
Study sites
The Meso-American Barrier Reef system extends 220 km from
the southern part of the Yucatan Peninsula to the Bay Islands of
Honduras and contains the longest unbroken reef in the Western
Hemisphere. A manipulative experiment was conducted from
February 2001 to August 2004 at Twin Cays, a 75-ha archipelago
located in the central part of the barrier reef system and about 2
km west of the reef crest (see Figs S1 & S2 in Supplementary
Material for maps and photographs of the study sites). Twin Cays
has been the focus of research by the Smithsonian Institution
since the early 1970s (Rützler & Feller, 1996). These mangrove
islands are far from the mainland, and peat cores contain no
terrigenous sediment (Cameron & Palmer, 1995; McKee &
Faulkner, 2000a; Purdy & Gischler, 2003; Macintyre
et al.
, 2004).
The only source of fresh water is rainfall, and the entire system is
intertidal [mean tide range (neap) = 0.2 m]. The vegetation is
dominated by
Rhizophora mangle
(red mangrove), which is the
most common species in the Caribbean region.
Peat cores were collected from Twin Cays and Cat Cay, Belize;
the Bay Islands of Roatán and Guanaja, Honduras; and Isla San
Cristóbal, Panama. Cat Cay is a 15-ha mangrove island in the
Pelican Cays archipelago located 21 km south of Twin Cays
(McKee & Faulkner, 2000a). The Bay Islands of Roatán and Guanaja
lie 30–50 km north of the Caribbean coast of Honduras (Cahoon
et al.
, 2003). Roatán is the largest (133 km
) and contains a 400-ha
tract of mangroves on the eastern extension of the island.
Guanaja is smaller (57 km
) with
300 ha of mangrove forest
prior to Hurricane Mitch. Isla San Cristóbal occurs on the
Caribbean coast of the Republic of Panama within the Bocas
del Toro archipelago (Lovelock
et al.
, 2005). All of these sites
were dominated by
R. mangle
, and the organic matter content of
the soil was 60% or higher, i.e. peat.
Manipulative experiment
We specifically tested the hypothesis that accumulation of
refractory mangrove roots contributes directly to soil volume and
hence elevation. Root dynamics were manipulated experimentally
by the addition of nitrogen (N) or phosphorus (P), and the
effects on elevation change were followed for 3 years. Previous
work showed that mangrove shoot growth varied spatially and was
stimulated by N (fringe), P (interior) or both N and P (transition)
et al.
, 2002; Feller
et al.
, 2003a). Experimental plots were
established at three replicate sites in the Twin Cays archipelago.
At each replicate site, three transects were established from the
shoreline to the island interior across a tree height/productivity
gradient (distance = 30 40 m). Permanent walkways were
installed in 2000 across the entire length of each transect to
minimize soil disturbance. Three vegetation zones relative to
Mangroves adjust to sea-level rise
© 2007 The Authors
Global Ecology and Biogeography
, Journal compilation © 2007 Blackwell Publishing Ltd
the shoreline were designated based on previous work: Fringe
(tree height = 5–7 m; litterfall = 700 g m
), Transition (tree
height = 2–4 m; litterfall = 450 g m
) and Interior (tree
height = 1–1.5 m; litterfall = 280 g m
) (Koltes
et al
1998). Each of the three transects per site was randomly assigned
to a treatment (N-fertilized, P-fertilized and unfertilized
controls) (McKee
et al.
, 2002; Feller
et al.
, 2003a). This approach
was taken to minimize the possibility of cross-contamination of
plots due to tidal movement across zones. Fertilizer in the form
of urea (45:0:0) or superphosphate (0:45:0) was applied below
ground at 6-month intervals from 1997 until the end of the study
in August 2004 (Feller
et al
., 2003a). Individual trees were
targeted, and fertilizer (150 g) contained in dialysis tubing was
placed in two holes (3 cm diameter
30 cm deep) cored into the
substrate beneath the drip line on opposing sides of the tree and
sealed with peat. Control sites were cored and sealed, but no
fertilizer was added. Thus, there were 27 total plots;
= 3 for each
treatment–zone combination. Treatments significantly increased
the bioavailability of porewater NH
+ NO
-N or PO
-P (measured
with resin bags), respectively, from 0.117 or 0.011
g (g resin)
to 0.218 (N-fertilized,
= 5.71,
< 0.01) or 0.030 (P-
= 6.39,
< 0.01)
g (g resin)
(see Table 3 in
et al.
, 2002). Porewater nutrients indicated there was
no migration of fertilizer into adjacent experimental plots (data
not shown).
Elevation change, surface accretion and subsurface
A rod surface elevation table (RSET), which consisted of a
benchmark rod and a portable measuring arm, was used to
determine elevation change over time (Cahoon
et al
., 2002) (see
Fig. 1 & Fig. S2 in Supplementary Material). This technique,
which has been used in multiple studies worldwide, provided a
measure of change in surface elevation relative to the base of a
benchmark rod driven to the point of refusal (see http:// for a detailed description of the tech-
nique and a list of publications based on this method). The RSET
measurement area (
2 m in diameter) was situated adjacent to
each fertilized tree but did not overlap the point of fertilizer
application. RSET installation and measurements were con-
ducted while standing on mangrove prop roots or on portable
platforms. In February 2001, a benchmark rod, in threaded sec-
tions, was driven with a slide hammer in each of 27 experimental
plots; the total depth of the rods to the Pleistocene limestone base
varied from 9–12 m. Baseline measurements of elevation were
conducted 1 month later to allow recovery from rod installation.
Four measurement directions were established, and at each
measurement time, nine fibreglass pins were carefully lowered
through the measurement arm to the soil surface (
= 36 per
RSET and measurement interval). The height of pins relative to
the arm was recorded at 6-month intervals to give the change in
elevation of the soil surface over time. Consequently, movement
of the soil surface as a result of all processes occurring in the
entire soil profile to the Pleistocene limestone was measured for
3 years. One person conducted all elevation measurements to
minimize operator error. In addition, strict protocols, extensive
notes, field markers and photographs were employed to ensure
consistency and to identify problematic data points.
At the same time, the deposition of material (organic and
inorganic) above marker horizons of sand was determined
adjacent to the RSET plots to give surface accretion (Cahoon &
Lynch, 1997). White carbonate sand was collected from a nearby
beach and placed in a marked area (0.1 m
) near each RSET plot
Figure 1 Rod surface elevation tables (RSETs)
were used to measure elevation change across
a tree height/productivity gradient at Twin
Cays, Belize; deposition of material above
marker horizons was simultaneously measured
to give surface accretion. The difference
between elevation change and surface
accretion yielded subsurface subsidence
(negative) or expansion (positive). In-growth
bags were used to measure rates of root
accumulation adjacent to RSETs. Peat cores
were collected and sections radiocarbon dated
to determine geological rates of accretion.
Figure not drawn to scale.
K. L. McKee
et al.
© 2007 The Authors
Global Ecology and Biogeography
, Journal compilation © 2007 Blackwell Publishing Ltd
to a depth of 1 cm. Cores were subsequently collected through
the sand horizon, and the depth of deposited material was measured
with a ruler at four positions and averaged. Differences between
elevation change and surface accretion were used to calculate
shallow subsidence (negative difference) or expansion (positive
difference) (Fig. 1).
Accumulation of mangrove root material
Relative rates of accumulation of root matter were determined
adjacent to each RSET using the implanted mass technique
(Fig. 1). The objective was to measure the relative accumulation
of roots (live and dead) and their potential contribution to
peat formation, rather than absolute rates of root production.
In-growth bags containing a standardized organic substrate were
used in all plots since there was minimal variation in organic
content of mangrove peat across zones (70–76%). Native peat
could not be used because it is composed primarily of mangrove
roots, making separation of in-grown roots impossible. Sphag-
num peat provided a standardized material that is commercially
available and similar to mangrove peat in bulk density (0.12 vs.
0.14 g cm
) and contents of organic matter (95 vs. 74%), total
carbon (47 vs. 42%) and total nitrogen (0.7 vs. 0.8%). Duplicate
in-growth bags (5 cm diameter
30 cm length) made of flexible
mesh material and containing sphagnum peat were installed in
cored holes near each experimental tree. Bags were retrieved at
6-month intervals, and accumulated root mass was separated by
condition (live, dead) and diameter class (fine,
2.5 mm; coarse,
> 2.5 mm), dried in an oven (80
C) and weighed. Annual accumu-
lation of root matter was calculated by summing the mass during
each sampling interval; annual root mortality was estimated
based on the mass of dead roots. Dead roots were identified
based on loss of structural integrity, colour and signs of decom-
position. Live plus dead root biomass was used to estimate total
root matter that accumulated during each interval. The contri-
bution of root biomass to soil volume was calculated using specific
root volume (volume per unit mass) of
R. mangle
fine and coarse
roots. Specific root volume was determined with a 25-ml
pycnometer. This approach allowed relative comparisons of root
matter accumulation across treatments and zones in relation to
concomitant changes in soil surface elevation and accretion.
To independently determine the effects of external nutrients
on decomposition rates, roots of
R. mangle
were collected from a
common (unfertilized) area, separated into size classes, air-dried
to a constant mass, enclosed in mesh bags (1 mm) and buried in
the root zone in each plot. Since the primary objective was to
determine the accumulation of refractory matter, bags were
retrieved after 1 year in the ground and the contents were dried
C) and weighed to calculate the mass remaining.
Peat composition and geological rates of peat
Six peat cores (up to 10 m in length) were collected from Twin
Cays, and a wood sample was also collected from a dead tree
stump exposed at the peat surface. Cores were also collected from
Cat Cay (two cores), Roatán (three cores), Guanaja (one core)
and Isla San Cristóbal (two cores). Cores were extracted by hand in
50-cm sections with a Russian peat corer, which causes no vertical
compaction of the sample (see Fig. S3 in Supplementary
Material). The corer was carefully washed between extractions
to avoid contamination. Cores were exposed in the field, photo-
graphed, sectioned (10-cm increments) and transported to the
laboratory in water-tight plastic bags where they were kept
refrigerated until analysis. Subsamples from every 10-cm section
from each core were analysed for percentage organic content and
examined under magnification to determine the proportion of
mangrove roots as a component of peat. The percentage organic
and mineral contents of samples were determined by loss-on-
ignition. Subsamples were dried in an oven (70
C) to constant
mass, weighed into porcelain crucibles, ashed in a muffle furnace
for 8 h at 400
C and reweighed after cooling to room temperature
in a desiccator. Radiocarbon dating [radiometric-standard delivery,
radiometric with extended counting or accelerator mass
spectrometer (AMS)] was conducted on selected peat sections,
wood and shell (Beta-Analytic Radiocarbon Dating Laboratory,
Miami, FL, USA). Measured radiocarbon ages were corrected
C ratios and calibrated to calendar dates (Talma &
Vogel, 1993; Stuiver
et al.
, 1998).
One peat core (8.3 m long) collected from an unvegetated
flat in the interior of Twin Cays with stumps of mangroves,
indicating dieback and no modern root production, was examined
in detail to assess peat composition. A subsample of each 10-cm
section was examined under magnification to identify major
components of the peat (based on comparison with modern
material and decomposing material from mesh bags). Seven
selected sections (0.4–8.0 m) were manually sorted under magni-
fication into four components: fine roots (
2.5 mm), coarse root
fragments, wood and leaf fragments (> 200
m) and residual
organic matter (
Statistical analyses
Elevational data (surface elevation relative to the initial baseline)
were averaged (
= 36 pins) for each RSET (= experimental unit)
on each sampling date over a 3-year period. A linear model was
fitted to the data for each RSET separately and the slopes of the
relationship were tested. The same approach was used to analyse
deposition above a marker horizon (surface accretion). The
value was used to assess the linear fit to the data, and a
was used to determine whether the slope was significantly different
from zero, i.e. if there was a significant change in elevation or
accreted material over time. The individual slopes (
= 3) were
used to test for treatment and zone effects as described below.
Elevational data and root accumulation, mortality and
decomposition data were analysed by a simple split-plot analysis
of variance (
) in which fertilizer treatment (Control,
N and P) was the main plot and zone (Fringe, Transition and
Interior) was the subplot (SAS, 2002). Data were transformed
prior to analysis when necessary to meet variance homogeneity
and normality assumptions for
. Relationships among
variables were examined by correlation analysis. A step-wise
Mangroves adjust to sea-level rise
© 2007 The Authors
Global Ecology and Biogeography
, Journal compilation © 2007 Blackwell Publishing Ltd
regression analysis was conducted in which the dependent
variable was elevation change and the independent variables
were vertical change due to surface deposition (accretion), root
input and subsidence (physical compaction plus decomposition).
Modern rates of accretion and elevation change
Few data exist reporting modern rates of elevation change in
mangrove ecosystems, but are clearly important for understand-
ing how such systems may respond to future sea-level rise. At
Twin Cays, accretion and elevation change indicated the direc-
tion and rate of soil surface movement relative to the base of a
benchmark, i.e. to the Pleistocene limestone underlying the
study site (Fig. 2, Table 1). In several cases, slopes were signifi-
cantly different from zero, indicating a gain or loss in elevation
over time (Fig. 2). Fringe control plots with the largest trees
showed an average gain in elevation of 4.1 mm year
and surface
accretion of 1.6 mm year
, indicating subsurface expansion of
2.5 mm year
. In contrast, transition and interior control plots
were subsiding and losing elevation at rates of
1.1 and
3.7 mm
, respectively. These rates of accretion and elevation change
are comparable with those reported for other mangrove forests
(Cahoon & Lynch, 1997; Cahoon
et al.
, 2003, 2006; Krauss
et al.
2003; Whelan
et al.
, 2005).
Nutrient enrichment had different effects on elevation change
depending on the tree zone and the nutrient added (Fig. 2, Table 1).
Compared with controls, fringe plots enriched with either N or P
did not gain elevation and exhibited subsidence rather than
expansion. The addition of nutrients to the transition zone
caused no net change (N) or a gain (P) in elevation. The addi-
tion of N to the interior zone caused greater rates of elevational
loss compared with controls, whereas the addition of P resulted
in a gain in elevation. Peat expansion in P-fertilized interior
plots was visually obvious and resulted in hummocks that were
on average 7 cm higher than in the control plots at the end of the
study. Addition of N or P thus altered vertical land building in
this system, resulting in rates of change in elevation ranging from
7.7 to 8.4 mm year
across the 27 experimental plots.
These results indicate three important points: (1) modern
rates of change in elevation can vary in both direction and
magnitude within the same mangrove ecosystem; (2) change in
elevation varies in conjunction with natural gradients in
mangrove tree productivity; and (3) addition of nutrients can
alter both the direction and rate of change of surface elevation.
Below, we examine the relative role of surface and subsurface
processes to better understand what controls elevation change in
peat-forming mangrove systems.
Surface and subsurface processes controlling
elevation change
Work in marshes has shown that fertilizer-induced increases in
above-ground production and stem density of the marsh grass,
Spartina alterniflora
, more than doubled mineral accretion rates
Figure 2 Elevation change () and surface
accretion () across mangrove zones (Fringe,
Transition, Interior) at Twin Cays, Belize and
in response to nutrient treatments (Control,
Nitrogen, Phosphorus); mean ± 1 SE (n = 3).
Slopes of lines fitted to the data are shown; an
asterisk indicates that the slope is significantly
different from zero.
K. L. McKee
et al.
© 2007 The Authors
Global Ecology and Biogeography
, Journal compilation © 2007 Blackwell Publishing Ltd
et al.
, 2002). Faster accretion of mineral material may be
caused by the baffling effect of plant shoots (marsh) or aerial
roots (mangrove) on water movement and sedimentation
patterns. Spatial variation in above-ground plant structures
has been correlated with sediment accretion and/or elevation
change (Spenceley, 1977; Krauss
et al.
, 2003). Elevation change
in the Twin Cays system, however, was not correlated with
surface accretion (
= 0.10,
> 0.05). Accretion rates were also not
significantly influenced by nutrient treatment or zone (Table 1).
Surface deposition across most of the island consisted of algal/
microbial mats and organic detritus. Carbonate sand accumulated
primarily along leeward shorelines and did not contribute to
surface accretion in experimental plots. Work in south-west
Florida also found minimal influence of surface sedimentation/
erosion processes in a riverine mangrove forest dominated by
R. mangle
et al., 2005). Instead, temporal changes in
groundwater movement appeared to influence mangrove
elevations through shrink–swell of peat deposits (Whelan et al.,
2005). Such considerations may be more important for riverine
mangroves or areas with significant groundwater influence. The
hydrology of the Twin Cays system is dominated by astronomical
tides and has no rivers or groundwater sources of fresh water that
might influence peat swelling (Wright et al., 1991). In any case,
such processes cannot explain the effects of fertilizer treatment
on elevation change at Twin Cays.
Elevation change was significantly correlated with subsurface
change (r = 0.94; P < 0.0001) at Twin Cays, indicating that below-
ground processes were primarily responsible for the observed
variation. Below-ground root accumulation, root mortality and
root decomposition were consequently assessed to determine
whether these biological processes may be influencing soil
elevations here. Accumulation of root biomass varied from 82
(interior) to 525 (fringe) g m2 year1 across control transects
(Table 1) and paralleled above-ground litterfall rates (Koltes
et al., 1998). The addition of P dramatically increased rates of
accumulation of fine and coarse roots in transition and interior
plots, but the addition of N had no effect in any forest zone. Low
root accumulation in interior plots was probably due to flooding
limitations on root growth (McKee, 2001, 2002). Root mortality
was increased by the addition of N, and these losses may explain
greater subsidence in N-fertilized plots (Table 1 and 1 d.f. contrast:
t-ratio = 3.53, P = 0.0123). Few data exist for mangrove forests,
but rates of root accumulation measured in this study are com-
parable with those found in Florida (McKee & Faulkner, 2000b)
and Honduras (Cahoon et al., 2003) using a similar in-growth
method. Although the root in-growth method may over- or
underestimate absolute rates of root production (due to growth
into unoccupied substrate or damage to roots during installation),
it provides an excellent way to compare the relative accumulation
of root matter, especially in relation to change in elevation. This
technique is also minimally destructive, which is essential in
areas where change in elevation is to be measured.
In order for root matter to accumulate, the rate of root
decomposition must be slower than the rate of root production.
Mangrove root decomposition was very slow, with 30 to 60% of
the original mass remaining after 1 year, but was unaffected by
external nutrient treatment (Table 1). Fine roots were particularly
resistant to decay (99% turnover time averaging 10 years), as
Table 1 Elevation change (mm year1), surface accretion (mm year1), subsurface change (mm year1), fine and coarse root accumulation
(g m2 year1), annual root mortality (%) and decomposition (% mass after 1 year) of fine and coarse roots across zones (Z) and in response to
fertilizer treatments (F): nitrogen (N) or phosphorus (P) addition or Control (C) applied to each transect (T). Values are the mean ± SE (n = 3).
anova F-ratios with significance indicated by: *P 0.05; **P 0.01; ***P 0.001; ****P 0.0001; n.s., not significant.
Root accumulation
Root mortality
Root decomposition
Fine Coarse Fine Coarse
C 3.7 ± 1.0 0.7 ± 0.3 4.4 ± 1.1 43 ± 24 39 ± 24 0.0 ± 0.0 63 ± 5 29 ± 2
N 5.8 ± 1.6 2.2 ± 1.2 8.0 ± 2.7 37 ± 20 45 ± 17 17.2 ± 6.5 68 ± 4 28 ± 4
P 4.8 ± 1.0 3.5 ± 0.1 1.3 ± 0.9 339 ± 44 403 ± 69 5.1 ± 5.1 62 ± 1 24 ± 3
C 1.1 ± 1.5 2.0 ± 1.3 3.1 ± 2.6 189 ± 73 205 ± 109 2.1 ± 2.1 63 ± 4 29 ± 7
N 2.2 ± 1.6 1.8 ± 0.9 0.3 ± 1.1 168 ± 42 132 ± 51 0.8 ± 0.8 63 ± 2 23 ± 1
P 4.4 ± 1.9 1.7 ± 0.3 2.7 ± 2.1 310 ± 52 194 ± 79 0.4 ± 0.4 63 ± 1 38 ± 3
C 4.1 ± 2.2 1.6 ± 0.7 2.4 ± 2.9 197 ± 34 328 ± 132 0.0 ± 0.0 61 ± 1 24 ± 5
N 0.1 ± 1.5 2.1 ± 1.2 2.0 ± 1.0 164 ± 13 316 ± 25 0.5 ± 0.5 65 ± 4 32 ± 6
P 1.6 ± 0.7 2.8 ± 1.0 1.2 ± 0.2 260 ± 51 163 ± 45 4.3 ± 4.3 62 ± 1 25 ± 4
anova source:
F 4.33(n.s.) 2.26(n.s.) 2.32* 12.4*** 2.90(n.s.) 6.23* 1.51(n.s.) 0.67(n.s.)
T [F] 3.76* 0.50(n.s.) 2.47(n.s.) 1.83(n.s.) 0.58(n.s.) 0.32(n.s.) 1.16(n.s.) 0.36(n.s.)
Z 10.1** 0.08(n.s.) 5.50* 4.41* 5.73* 2.14(n.s.) 0.71(n.s.) 0.33(n.s.)
Z × F 10.1*** 0.63(n.s.) 4.93** 3.95* 7.46** 2.28(n.s.) 0.64(n.s.) 0.22(n.s.)
Mangroves adjust to sea-level rise
© 2007 The Authors
Global Ecology and Biogeography, Journal compilation © 2007 Blackwell Publishing Ltd 7
reported previously for Belizean (Middleton & McKee, 2001)
and Australian (van der Valk & Attiwill, 1984) mangroves.
Slow decomposition and lack of response to external nutrient
treatment probably reflected the inability of the microbial
community to decompose refractory root tissues under flooded,
anaerobic conditions (Middleton & McKee, 2001). Change
in elevation was significantly correlated with fine (r = 0.72,
P < 0.0001) and coarse (r = 0.41, P = 0.03) root accumulation,
but was not correlated with root mortality (r = 0.28, P > 0.05) or
root decomposition [r = 0.02 (coarse) or 0.15 (fine), P > 0.05].
Thus, change in elevation of these intertidal islands varied
primarily with inputs of mangrove roots, which resist microbial
decay due to their refractory nature and anaerobic conditions
limiting decomposers.
Vertical change due to root volume inputs varied from 1.2
to 10.8 mm year1 (Fig. 3). Surface deposition above a marker
horizon, in comparison, ranged from 0.7 to 3.5 mm year1 (Fig. 3).
Loss of elevation to decomposition and physical compaction
was calculated by subtraction of the root input from measured
subsurface change. These processes, collectively termed ‘shallow
subsidence’, caused losses ranging from 4.8 to 9.5 mm year1
(Fig. 3). A step-wise regression analysis showed that fine root
input explained 42% of variation in elevation change (P 0.001),
with subsidence, coarse roots and surface accretion explaining an
additional 36, 10 and 2% of variation (P 0.05). Thus, annual
inputs of fine roots contributed substantially to soil volume and
explained a significant amount of the variation in elevation
These findings indicate that modern rates of accretion and
elevation change measured in Belizean mangrove forests were
influenced by the deposition and accumulation of refractory
mangrove roots. They further suggest that mangrove peat
accumulation has been the primary process allowing this island
system and similar peat-forming systems to keep pace with the
rise in sea level over the Holocene. To examine these points further,
we looked at the peat record at several sites in the Caribbean
dominated by mangrove vegetation.
Peat stratigraphy and geological accretion
Cores collected from multiple sites on the Caribbean coasts of
Belize, Honduras and Panama showed that mangrove systems
in these areas accumulate deep deposits of organic material (see
Fig. S4 in Supplementary Material for a graphic log of cores). Peat
thickness varied across study sites: 0.4 m (Guanaja, Honduras),
1.8 m (Roatán, Honduras), 2.8 m (Isla San Cristobal, Panama),
0.9 m (Cat Cay, Belize) and 10.0 m (Twin Cays, Belize). Loss-
on-ignition showed that the organic content of these deposits
averaged 65%. In some cases, deposits of marine sand (from
calcareous algae) were found beneath the peat layer or between
two peat layers. However, continuous deposits of peat composed
of mangrove roots indicated that all of these systems were built
primarily through organic accumulation, rather than physical
processes of mineral sedimentation. The depth range of man-
grove peat found in these sites is comparable to that reported in
other locations throughout the Caribbean region (Ellison, 1993;
Macintyre et al., 1995, 2004; McKee & Faulkner, 2000a; Islebe &
Sanchez, 2002; Gischler, 2003; Toscano & Macintyre, 2003;
Ramcharan, 2004; Whelan et al., 2005). In particular, the peat
deposits at Twin Cays are some of the thickest and oldest on
record in the Western Hemisphere (Macintyre et al., 2004),
which identifies this location as an important peat-forming
system for the Caribbean region. Twin Cays has built up vertically
by the accumulation of mangrove organic matter throughout its
Holocene history, as reported previously (Cameron & Palmer,
1995; Macintyre et al., 2004). The entire profile of an 8.3-m long
core (REU) was composed of mangrove (predominately R. mangle)
fine roots (40 ± 6%), coarse root fragments (9 ± 3%), wood and
leaf fragments (22 ± 5%) and organic particles less than 200 µm
(fine root fragments and microbial mat residue) (27 ± 6%).
Cores from other sites at Twin Cays showed continuous peat
deposits up to 10 m in thickness and were similarly composed of
refractory mangrove roots and other organic particles.
Radiocarbon dating indicated that mangroves established at
Twin Cays between 7000 and 8000 cal bp [cal bp = calendar years
before present (1950); see Table 2 for calibration details) and that
peat accumulation closely followed rates of sea-level rise (SLR)
calculated for the western Atlantic (Fig. 4), based on a combined
coral–mangrove peat record (Toscano & Macintyre, 2003). Man-
groves did not exist at Twin Cays when SLR rates exceeded 5 mm
year1, but appeared when rates slowed to about 3.5 mm year1.
Once established, mangroves accumulated peat at rates of 3 mm
year1 (7600–7200 cal bp), 1.3 mm year1 (7200–5500 cal bp) and
1.0 mm year1 (5500–500 cal bp), showing that peat formation
Figure 3 Variation in surface elevation change () relative to
vertical change attributable to surface accretion, fine and coarse root
production, and shallow subsidence (physical compaction and
decomposition) across mangrove zones and nutrient treatments at
Twin Cays, Belize; mean ± 1 SE (n = 3); SE not plotted on stacked
bars for clarity.
K. L. McKee et al.
© 2007 The Authors
8Global Ecology and Biogeography, Journal compilation © 2007 Blackwell Publishing Ltd
closely tracked sea-level rise. Basal peat dates from Roatán,
Honduras and Isla San Cristóbal, Panama indicated that mangroves
established at those sites within the past 2000–3000 years when
SLR was c. 1 mm year1. Mangroves became established at
sampling sites at Cat Cay, Belize and Guanaja, Honduras within
the past 500 600 years.
The predominance of mangrove root matter throughout most
of these peat cores is consistent with previous work (McKee &
Faulkner, 2000a; Middleton & McKee, 2001) as well as with data
linking change in elevation with accumulation of mangrove
roots (this study). Fossil fine roots of R. mangle were well
preserved and often indistinguishable from modern fine roots
(see Fig. S3 in Supplementary Material), in agreement with
decomposition results indicating their refractory nature (Table 1).
The peat record thus shows a remarkable history of island
development through the slow accumulation of mangrove roots
in concert with changing sea level. Calibrated peat data from
multiple geographical locations further show that peat forma-
tion is an important process in other Caribbean mangrove
systems, allowing them to keep up with SLR (Fig. 4 & Fig. S4 in
Supplementary Material).
Previous workers have reported peat accretion rates of c. 1.1
mm year1 for mangrove forests (Ellison & Stoddart, 1991; Ellison,
1993; Parkinson et al., 1994; Cahoon & Lynch, 1997). Such rates
were calculated based on radiocarbon dates of basal peat divided
by the depth of the sample. However, these rates represent the
average for the entire peat record and do not indicate variation
during different time periods. Most importantly, geological
accretion rates reflect the net result of peat building interspersed
with intervals of no elevation gain or even loss and consequently
do not reveal precise patterns of change in the peat surface over
time. Modern measurements of elevation change across Twin
Cays showed that rapid vertical development occurs where man-
groves are more productive, but elevation losses occur where
plant growth is low. These modern measurements suggest that
peat development has varied in a similar pattern over the
Holocene as fluctuating environmental conditions affected
mangrove growth. Past disturbance events may have interrupted
peat building in mangrove systems. For example, peat collapse
(11 mm year1) has been documented in a basin mangrove forest
in Honduras that suffered total mortality after Hurricane Mitch
(Cahoon et al., 2003). Although the cause of dieback at one of
the Belize coring sites (REU) is unknown, the 0.4–0.5 m lower
elevation relative to surrounding vegetated areas indicates that
the area has been sinking for some time. This elevation differential
divided by a rate of loss of elevation of 2.1 mm year1 (measured
nearby) suggests that the site has been losing elevation for 190–240
years, which agrees with a modern date for Avicennia germinans
Table 2 Calibration of radiocarbon age of
mangrove wood (exposed tree stump), a shell
and peat samples [relative to mean sea level
(MSL)] to calendar years. Samples are from
Twin Cays (TC) or Cat Cay (CC), Belize;
Roatan (R) or Guanaja (G), Honduras; and
Isla San Cristobal, Panama (PAN). A date (± 1
sigma statistic) or an age range (± 2 sigma
statistic) is given; cal bp = calendar years before
present (dates are referenced to ad 1950).
Sample number
radiocarbon age
(yr bp)
13C/12C ratio
radiocarbon age
(yr bp)
Calendar age
(cal bp)
TC-SSA (0.4 m) 150 ± 50 24.6 160 ± 50 300–0
TC-REU 1-8 (1.3 m) 1320 ± 40 25.9 1410 ± 40 1360–1270
TC-REU 1-32 (3.7 m) 3510 ± 40 23.6 3530 ± 40 3900–3700
TC-REU 1-64 (6.9 m) 6010 ± 40 26.8 5980 ± 40 6900–6710
TC-REU 1-82 (8.7 m) 6630 ± 40 26.5 6610 ± 40 7580–7430
TC3-36B (3.9 m) 4790 ± 80 27.2 4750 ± 80 5620–5310
TC3-64B (6.7 m) 6280 ± 40 26.8 6250 ± 40 7260–7020
TC3-80B (8.3 m) 6820 ±70 26.5 6790 ± 70 7740–7560
TC5-8B (0.6 m) 430 ± 40 27.2 390 ± 40 520–420
TC5-35B (3.3 m) 4020 ± 100 25.3 4010 ± 100 4830–4220
TC5-65B (6.3 m) 5990 ± 40 28.3 5940 ± 40 6860–6670
TC5-100B (9.8 m) 6690 ± 90 27.0 6660 ± 90 7670–7420
TC8-14B (1.6 m) 2500 ± 90 26.0 2480 ± 90 2760–2340
TC8-26B (2.8 m) 2370 ± 60 24.1 2390 ± 60 2720–2330
TC8-85B (8.7 m) 6000 ± 40 27.7 5960 ± 40 6880–6680
TC9-6B (0.5 m) 650 ± 40 26.4 630 ± 40 660–540
TC9-19 B (1.8 m) 2850 ± 40 24.4 2860 ± 40 3080–2870
CC10-7 (0.6 m) 500 ± 40 26.6 470 ± 40 530–500
CLI-1-9,10 (1.0 m) 290 ± 60 22.9 330 ± 60 510–290
CMS-2-10,11 (0.9 m) 460 ± 50 25.6 450 ± 50 540–440
CMI-4-18,20 (1.7 m) 1520 ± 70 24.7 1530 ± 70 1550–1300
PAN-5-29 (3.0 m) 2610 ± 50 26.6 2580 ± 50 2630–2500
PAN8-14 (1.5 m) 700 ± 50 26.5 670 ± 50 680–550
CHI-4-20 (1.9 m) 1170 ± 70 +3.2 1640 ± 70 1310–1050
Mangroves adjust to sea-level rise
© 2007 The Authors
Global Ecology and Biogeography, Journal compilation © 2007 Blackwell Publishing Ltd 9
tree stumps (< 300 cal bp). Revegetation of dieback areas with
mangroves can occur (K.L.M. and I.C.F., personal observation),
leading to new peat accumulation. Consequently, it is likely that
these islands have undergone cycles of peat building and sinking
as mangrove vegetation flourished, died back and recolonized.
These findings link the modern measurements of change in
elevation and associated controls at Twin Cays to geological rates
of accretion in this and other Caribbean mangrove systems.
Thus, there has been a tight coupling between peat building in
Caribbean mangroves and sea-level rise over the Holocene. The
mechanism is likely to be related to feedback processes between
mangrove root accumulation and elevation (presumably through
flooding effects on production–decomposition processes).
Response to sea-level rise
Although mangrove systems at Twin Cays and other Caribbean
locations have persisted for thousands of years, their continued
existence depends on future rates of relative SLR. To avoid
submergence, vertical building of these mangroves must equal
relative SLR (eustatic rise plus local subsidence). Global mean
rates of eustatic SLR, currently estimated at 1.5–2.0 mm year1, are
predicted to increase with global warming over the next century,
with most model scenarios projecting rates of 3–5 mm year1
(IPCC, 2001). Previous work has suggested future submergence
of mangroves in the wider Caribbean region based on historical
(caesium dating) and geological (radiocarbon dating) rates of
peat accretion (Ellison & Stoddart, 1991; Parkinson et al., 1994).
As discussed above, rates of peat accretion may underestimate
the potential of a mangrove system to build vertically, since they
average over the entire record. Modern measurements of eleva-
tion change may overestimate it due to the short-term record.
Another approach is to compare rates of surface plus sub-
surface (root) accretion to estimate potential rates of vertical
building with submergence rates. Since the Caribbean has a
minimal degree of glacial isostatic adjustment and is thought to
be tectonically relatively stable (Toscano & Macintyre, 2003),
shallow subsidence plus eustatic SLR should give a minimum
estimate of submergence. Assuming negligible deep subsidence
and no root input, the submergence rate at Twin Cays is estimated
to be 7–8.5 mm year1 (eustatic SLR = 1.5–2.0 mm year1). Rates
of surface plus subsurface (root) accretion in fringe, transition
and interior zones at Twin Cays were 10.4, 6.3 and 2.0 mm year1.
This comparison shows that transition and interior zones with
low root production could not keep up with current submergence
rates, consistent with observations that interior stands with
stunted trees are flooded most of the time and interior areas
with no vegetation are perennially flooded. However, fringe
mangroves have kept up and could accommodate eustatic SLR
rates of 4 mm year1 if current accretion rates were maintained. If
eustatic rates exceed 5 mm year1 then these mangrove islands
would not be likely to persist, assuming that all other conditions
remain unchanged.
In addition to vertical building, mangrove forests may move
laterally in response to sea-level changes (Snedaker, 1995). The
landward retreat of mangroves in some locations may be limited
because of natural and human barriers, but mangrove islands
may expand and contract with changing conditions (McKee &
Faulkner, 2000a). Lateral movement of these mangrove islands is
evidenced by peat deposits offshore (Macintyre et al., 1995,
2004) as well as by sand lenses in peat cores, indicating past
shorelines. Dates of peat above and below sand lenses at Twin
Cays show that sand was not deposited in a storm event, but was
instead accumulated over thousands of years. For example, a
transgression of lagoonal sand occurred 5000–6000 cal bp on the
eastern island of Twin Cays; mangroves reoccupied these sites
about 2000 cal bp and peat building resumed (see core TC-7,
Fig. S4 in Supplementary Material).
The importance of plant production–decomposition processes
in peat-forming systems is widely recognized and has been
extensively studied. Early work described the role of mangroves
in soil accretion and noted the contribution of mangrove roots to
peat formation (e.g. Bird, 1971; Woodroffe, 1983; Parkinson
et al., 1994; Cahoon & Lynch, 1997; McKee & Faulkner, 2000a).
The indirect effects of plants on sedimentation in mangrove and
marsh systems have also been demonstrated in a few cases
Figure 4 Time–depth plot of mangrove peat samples in relation to
a sea-level curve for the western Atlantic. Data are from Twin Cays
and Cat Cay, Belize; the Bay Islands of Roatán and Guanaja,
Honduras; and Isla San Cristóbal, Panama (1) (this study);
Lighthouse Reef, Turneffe Atoll and Glover’s Reef, Belize (2)
(Gischler, 2003); Quintana Roo, Mexico (3) (Islebe & Sanchez,
2002); and Trinidad (4) (Ramcharan, 2004). 14C dates were
calibrated to calendar years [cal bp = calendar years before present
(1950)] as reported in the referenced sources. The sea-level curve is
based on a separate set of calibrated mangrove peat and coral
(Acropora palmata) dates from multiple sites (Antigua, Bahamas,
Belize, Florida, Jamaica, Martinique, Panama, Puerto Rico, St Croix)
(Table 4 in Toscano & Macintyre, 2003). Rates of SLR from three
periods are indicated on the plot and show an initially rapid rate of
5.2 mm year1 (10,600–7700 cal bp), decreasing to 1.47 mm year1
(7700–2000 cal bp) and 0.93 mm year1 (2000–400 cal bp).
K. L. McKee et al.
© 2007 The Authors
10 Global Ecology and Biogeography, Journal compilation © 2007 Blackwell Publishing Ltd
(Bird, 1971; Spenceley, 1977; Morris et al., 2002; Krauss et al.,
2003), presumably through the physical effects of vegetation on
the trapping of mineral sediment. However, to our knowledge
our work is the first to experimentally demonstrate direct biotic
contributions to elevation change in a coastal ecosystem and
to identify the biological process involved. We further linked
modern elevation change due to root accumulation to past peat
formation and the response of Caribbean mangrove systems to
rising sea levels over the Holocene. These data provide direct
evidence that mangroves are an integral and active component
of the coastal landscape and thus represent a natural defence
against submergence and wetland loss due to sea-level rise. Natural
or anthropogenic disturbances that alter root dynamics may
consequently impair the ability of such systems to accommodate
sea-level rise.
The Caribbean region is an important reservoir of mangrove
peat, with deposits up to 10 m thick. Belizean mangrove islands,
including the Twin Cays archipelago, have formed some of the
thickest peat deposits reported to date. Such peat-forming man-
grove systems are particularly vulnerable to human interference.
Their management consequently requires different strategies
from those used in other sedimentary settings. Our work shows that
removal of mangrove vegetation would stop soil accretion, while
decomposition, physical compaction and erosion processes con-
tinue, ultimately leading to submergence and land loss. Other types
of mangroves, e.g. riverine systems, may be better buffered against
SLR due to high rates of mineral accretion. However, mangrove
roots are important even in those settings to stabilize deposited
sediment and prevent erosion (McKee & McGinnis, 2002).
Mangrove ecosystems are additionally subject to nutrient
overloading, which has numerous and complex effects on
structure and function (McKee et al., 2002; Feller et al., 2003a,b;
Lovelock et al., 2004). A change in the nutrient regime of peat-
forming mangroves may alter the balance among biotic processes
controlling the accumulation of organic matter and soil
elevations relative to sea level, as demonstrated at Twin Cays.
Even though nutrients temporarily enhanced peat formation
by interior mangroves, eutrophication may have unforeseen
consequences for the functioning and long-term stability of these
biogenic systems. Given these insights, the challenge now is to
incorporate this information into coastal management plans to
minimize loss of ecosystem services, which mangroves and other
coastal wetlands provide.
This work was supported by the National Science Foundation’s
Biocomplexity in the Environment Program (DEB-9981483)
and by the Smithsonian Institution’s Caribbean Coral Reef
Ecosystems Program (this is CCRE contribution number 787,
supported in part by the Hunterdon Oceanographic Research
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T. Martinez, T. McGinnis and E. Travis for field assistance, and
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© 2007 The Authors
12 Global Ecology and Biogeography, Journal compilation © 2007 Blackwell Publishing Ltd
The following supplementary material is available for this article:
Figure S1 Maps of the study areas in Belize, Honduras and
Figure S2 Photographs of the study site at Twin Cays, Belize.
Figure S3 Photographs of a peat core and modern and fossil
mangrove roots.
Figure S4 Graphic summary of dated peat cores collected in
Belize, Honduras and Panama.
This material is available as part of the online article from:
(This link will take you to the article abstract).
Please note: Blackwell Publishing is not responsible for the
content or functionality of any supplementary materials supplied
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be directed to the corresponding author for the article.
Editor: David Currie
Karen McKee is an ecologist at the National Wetlands
Research Center, US Geological Survey. Her main
research area is in the field of wetland plant ecology,
particularly salt marsh and mangrove communities.
Her current research focuses on global change factors,
including sea-level rise, elevated CO2 and disturbance
events such as hurricanes and climate extremes.
Don Cahoon is an ecologist at the Patuxent Wildlife
Research Center, US Geological Survey. His main
research area is in the field of coastal wetland plant
ecology. His current research focuses on soil elevation
dynamics of salt marsh and mangrove communities in
response to global change factors, including sea-level rise
and disturbance events such as hurricanes.
Candy Feller is an ecologist at the Smithsonian
Environmental Research Center. Her research area is
animal–plant interactions in forested ecosystems. Her
current research focuses on biocomplexity, animal–plant
interactions, community structure and nutrient dynamics
in mangrove ecosystems.
... Similarly, we expected there would be a positive relationship between surface elevation change and subsurface change because processes such as root growth, organic matter accumulation, decomposition, and compaction are known to contribute to soil elevation change in coastal wetlands. Root growth and the accumulation of organic matter are especially important for building soil elevation in coastal wetlands located in carbonate settings with little mineral sediment inputs, such as the Greater Everglades region of south Florida (Breithaupt et al. 2017;McKee 2011;McKee et al. 2007). ...
... Rates of subsurface change were less than zero (i.e., the rate of surface elevation change was less than the rate of vertical accretion) at the majority of sites (35 out of 51 sites, or 69%; Table 1; Fig. 5c), indicating that net elevation change at these sites was greatly influenced by shallow subsidence via soil compaction, groundwater soil shrinkage, and/or organic matter decomposition . Rates of subsurface change were positive at 16 sites (i.e., the rate of surface elevation change was greater than the rate of vertical accretion), indicating that net elevation change at these sites was driven by root growth or groundwater soil swelling (Whelan et al. 2005;Cahoon et al. 2006;McKee 2011;McKee et al. 2007), although the rates of surface elevation change and vertical accretion differed by less than 2 mm year −1 at 11 of these sites. ...
... Indeed, our results show that there is a positive relationship between surface elevation change and subsurface change, indicating that belowground processes such as physical expansion/contraction of the sediment, groundwater flow, root production/ turnover, or gas efflux contributed to 57% of the variation in surface elevation change (Fig. 8b). Other studies conducted in coastal wetlands located in carbonate settings with minimal terrigenous mineral sediment inputs from upriver ecosystems, such as the Greater Everglades, have also demonstrated that surface elevation change is largely controlled by subsurface root production (Breithaupt et al. 2017;Cahoon and Lynch 1997;McKee 2011;McKee et al. 2007). Our results also show that there is a negative relationship between vertical accretion and subsurface change (Fig. 8c). ...
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Coastal wetlands adapt to rising seas via feedbacks that build soil elevation, which lead to wetland stability. However, accelerated rates of sea-level rise can exceed soil elevation gain, leading to wetland instability and loss. Thus, there is a pressing need to better understand regional and landscape variability in rates of wetland soil elevation change. Here, we conducted a regional synthesis of surface elevation change data from mangrove forests and coastal marshes in the iconic Greater Everglades region of south Florida (USA). We integrated data from 51 sites in which a total of 122 surface elevation table-marker horizon (SET-MH) stations were installed. Several of these sites have been periodically monitored since the 1990s and are among the oldest SET-MH datasets in the world. Rates of surface elevation change ranged from −9.8 to 15.2 mm year−1, indicating some wetlands are keeping pace with sea-level rise while others are at risk of submergence and conversion to open water. Vertical accretion rates ranged from 0.6 to 12.9 mm year−1, and subsurface change rates ranged from −13.5 to 8.6 mm year−1. Rates of surface elevation change were positively related to subsurface change but not vertical accretion. There were no significant relationships between rates of surface elevation change and elevation (NAVD 88) or rates of sea-level rise. Site-specific examples indicate that hurricanes, plant productivity, hydrologic exchange, and proximity to sediment and nutrient inputs are critical but confounding drivers of surface elevation change dynamics in the Greater Everglades region. Collectively, our results reinforce the value of long-term SET-MH data that incorporate spatial variability for advancing understanding of surface elevation change dynamics in coastal wetlands.
... In contrast to northern Belize, where the limestone of the barrier reef lagoon is near or at the ground surface (Dunn and Mazzullo 1993), the southern barrier reef lagoon where the Paynes Creek sites are located is much deeper, with a Holocene sediment history yielding as much as 11 m (36 ft.) of mangrove peat on the limestone bedrock (Macintyre et al. 1995;McKee and Faulkner 2000;McKee et al. 2007;McKillop 2002). A test probe was carried out between sites 14 (K'ak' Naab') and 15 In 2008, a column of sediment was excavated from the seafloor between Sites 14 and Site 15 (Figure 1). ...
... The lowest level for organic material is level 1 (0-10 cm), which is 50% organic material. The organic content is high and consistent with other peat sediment deposits on the cays of Belize (McKee and Faulkner 2000;McKee et al. 2007;McKillop 2002). ...
... Due to the anoxic and salty conditions in mangrove substrates, root remnants and litterfall accumulate in the sediments making them one of the most carbon-rich ecosystems on Earth (Adame et al., 2021;Donato et al., 2011;McKee et al., 2007). Because of the importance of atmospheric carbon sequestration by mangrove ecosystems and its long-term trapping in the flooded substrate either as peat or as amorphous organic matter, many studies have devoted efforts to estimate the amount of carbon trapped in mangrove sediments as a key input in the calculation of their ecosystem services and their relevance for the growing market for carbon emissions mitigation. ...
... Others use a larger, 6-cm-diameter, open-faced corer designed for swampy substrates (Donato et al., 2011). Other researchers use the Russian peat corer, which takes semi-cylindrical cores 5 cm in diameter (McKee et al., 2007) while, finally, some have used a 10-cm-diameter core if they need a large sample for other analyses in addition to C content (Smoak et al., 2013). The soil probe and the open-faced corer cut through the sediment, roots, and peat as they are driven down into the substrate, while the Russian peat corer is driven down empty to the desired depth and closed by rotating the corer to enclose a sample. ...
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Carbon accumulation in coastal wetlands is normally assessed by extracting a sediment core and estimating its carbon content and bulk density. Because carbon content and bulk density are functionally related, the latter can be estimated gravimetrically from a section of the core or, alternatively, from the carbon content in the sample using the mixing model equation from soil science. Using sediment samples from La Paz Bay, Mexico, we analyzed the effect that the choice of corer and the method used to estimate bulk density could have on the final estimates of carbon storage in the sediments. We validated the results using a larger dataset of tropical mangroves, and then by Monte Carlo simulation. The choice of corer did not have sizable influence on the final estimates of carbon density. The main factor in selecting a corer is the operational difficulties that each corer may have in different types of sediments. Because of the multiplication of errors in a product of two variables subject to random sampling error, when using gravimetric estimates of bulk density, the dispersion of the data points in the estimation of total carbon density rises rapidly as the amount of carbon in the sediment increases. In contrast, the estimation of total carbon density using only the carbon fraction as a predictor is very precise, especially in sediments rich in organic matter. This method, however, depends critically on the accurate estimation of the two parameters of the mixing model: the bulk density of pure peat and the bulk density of pure mineral sediment. The estimation of carbon densities in peaty sediments can be very imprecise when using gravimetric bulk densities. Estimating carbon density in peaty sediments using only the estimate of organic fraction can be much more precise, provided the model parameters are estimated with accuracy. These results open the door for simplified and precise estimates of carbon dynamics in mangroves and coastal wetlands.
... Moreover, the addition of N fertilizer can indirectly affect the structure and diversity of microbial communities by changing plant communities and the physical and chemical properties of the soil (Zhao et al., 2015;Zeng et al., 2016;Hu et al., 2017), resulting in changes to riparian ecosystem function (Tian et al., 2015;Ling et al., 2016). As an example, an appropriate amount of N fertilizer can significantly increase plant growth and affect the biogeochemical process of the soil carbon cycle (Craig et al., 2021), leading to changes in the soil microbial community composition and structure by increasing the input of leaf and root litter to the soil (Lovelock et al., 2004;McKee et al., 2007;Dangremond et al., 2020). These changes in microbial community structure were accompanied by changes in the activity of enzymes involved in C, N, and P cycling (Craig et al., 2021;Sun et al., 2022b). ...
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Introduction Climate change and anthropogenic activities are the greatest threats to floodplain ecosystems. A growing body of literature shows that floodplain ecosystems have experienced increased chemical fertilizer and pesticide loads, which will disturb the above and belowground ecosystems. However, we lack knowledge regarding the effects of such human activities on the vegetation and soil microbiomes in these ecosystems. Methods In the present study, plant functional traits and Illumina Mi-Seq sequencing were to assess the impact of nitrogen fertilizer and glyphosate addition on the structure and function of the vegetation and soil microbiomes (bacteria, fungi, and protists) in a floodplain ecosystem, and to assess the influence of seasonal variation. Results We identified distinct response mechanisms of plant and microbial communities to the addition of nitrogen fertilizer and glyphosate, and seasonal variation. Nitrogen fertilizer and glyphosate significantly affected plant diversity, aboveground and underground biomass, and C and N content and significantly changed the leaf area and plant stature of dominant plants. However, the addition of nitrogen fertilizer and glyphosate did not significantly affect the diversity and structure of bacterial, fungal, and protist communities. The application of nitrogen fertilizer could improve the negative effects of glyphosate on the functional traits of plant communities. The seasonal variation of floodplain has significantly changed the soil’s physical, chemical, and biological properties. Our results showed that compared with that in summer, the soil ecosystem multifunctionality of the floodplain ecosystem in autumn was significantly lower. Seasonal variation had a significant effect on plant diversity and functional traits. Moreover, seasonal variation significantly affected the community compositions, diversity, and structure of bacteria, fungi, and protists. Seasonal variation had a stronger impact on fungal community assembly than on that of bacteria and protists. In summer, the assembly of the fungal community was dominated by a deterministic process, while in autumn, it is dominated by a stochastic process. In addition, the negative association among bacteria, fungi, and protists has been strengthened in autumn and formed a more robust network to cope with external changes. Discussion These results extended our understanding of the ecological patterns of soil microbiomes in floodplain ecosystems and provided support for enhancing the ecological barrier function and the service potential of floodplain ecosystems.
... The mean total root production in Algodones (159.3 g·m −2 ·year −1 ) and Guásimas (142.1 g·m −2 ·year −1 ) was similar to the values reported by Torres et al. (2021) for three lagoons in the southern Gulf of California (214 g·m −2 ·year −1 ) and McKee et al. (2007) in Twin Cays, Belize (220.8 g·m −2 ·year −1 ). However, a high root production (420.1 g·m −2 ·year −1 ) was recorded at Lobos (Site 5), influenced by the presence of C. erectus with high root production and biomass; these values are similar to those reported by Adame et al. (2014) for Celestún Lagoon (167. ...
Blue carbon ecosystems are recognized as natural climate solutions due to their capacity to store carbon (C) in the vegetation and soil. Thus, the extent of such role by mangrove ecosystems has raised interest in the scientific community. Mangrove carbon stock above ground biomass is higher in humid tropical areas than in dry subtropical ones. However, a lack of information persists when it comes to the belowground C. This research aimed to determine the carbon stock in relation to total mangrove biomass in three semi-arid coastal lagoons in the Gulf of California, Mexico. Different methods were used, tree structure and pneumatophore measurements, litter traps, root and production cores; along with measurements of physicochemical parameters in sediment in surface and interstitial water. Mean carbon in the total mangrove biomass was 54.1 MgCorg·ha −1 , with a maximum of 102.1 ± 14.2 MgCorg·ha −1 at the Lobos site. In arid areas, mangroves are characterized by larger amounts of belowground biomass associated with environmental factors such as pH, redox potential, and temperature, leading to a high C org content belowground. Root production are high, enhancing the efficiency of belowground C org storage related to the life cycle of roots (turnover). The presence of Conocarpus erectus (L), a new record of the distribution limit of the species in the Gulf of California (Lobos), attained the highest values of structural development, litter-based primary productivity, and belowground root production and biomass. Therefore, this species is recommended for restoration (reforestation), given its high primary productivity that stabilizes the coastline and increases carbon storage dynamics.
... Accretion rates we report here largely result from plant-generated processes. Accretion in other wetlands, such as mangroves, also depend on accumulation of plant-originated organic matter (McKee et al., 2007;Blum et al., 2021), but organogenic wetlands (such as Great Sippewissett Marsh) are usually at a disadvantage relative to SLR. ...
Vegetative cover was mapped annually, 1976-2022, in experimental plots in Great Sippewissett Marsh, Cape Cod, USA, chronically fertilized at different doses, and subject to changes in sea level and other climate-related variables. Dominant species within areas of higher elevation in the plots followed different decadal trajectories: rise in sea level diminished cover of Spartina patens; higher N supplies increased cover of Distichlis spicata. The opportunistic growth response of D. spicata to high N supplies unexpectedly fostered increased platform accretion, a feature that persisted for succeeding decades and led to further changes in vegetation: D. spicata functioned as an effective ecosystem engineer with long-term ecological consequences. Shrubs usually found in upper marsh margins expanded into areas where D. spicata had stimulated accretion, then shaded and excluded D. spicata, but subsequently lost cover as sea level rise continued. Increased N supply converted stands of Spartina alterniflora, the dominant low marsh species, from short to taller ecophenotypes; sea level rise had minor effects on S. alterniflora, but during 2019-2022 appeared to reach a tipping point that fostered taller S. alterniflora and bare space even in un-fertilized control plots, and in Great Sippewissett Marsh in general. Model results anticipate that-in spite of potential accretion enhanced by vegetation and ecosystem engineer effects-there will be loss of high marsh, transient increases of low marsh, followed by loss of low marsh, and eventual conversion to shallow open water by the end of the century. Dire local projections match those of the plurality of recent reports from salt marshes around the world. Proposed management strategies may only delay unfortunate outcomes rather than maintain wetlands. Concerted reductions of warming from greenhouse gases, and lower N loads seem necessary to address the coming crises in wetlands-and many other environmental threats.
An accelerating rate of sea level rise (SLR) is causing huge inundation pressure on coastal wetlands worldwide. Vegetation of coastal wetlands plays a key role in stabilizing the coast and accreting sediment in order to mitigate the negative impact of SLR. The ability to accrete sediment is influenced by individual species traits; however, there are insufficient information and indicators to identify differences in the adaptability of various coastal vegetations to SLR at a regional or global scale. Here, the potential adaptation of 27 plant populations in coastal wetlands subject to SLR was evaluated using a compiled global dataset and a marsh equilibrium model. Sediment accretion efficiency differed among plant populations, but most coastal marsh populations and a few mangrove populations had relatively high accretion rates; habitats with high accretion rates will have a better potential to deal with the threat of SLR. These results showed that latitude and efficiency shared a nonlinear relationship, and plant stem density and root structure were among the important factors that influenced the efficiency. Fibrous root plant populations had a greater sediment accretion efficiency than tap root plant populations, and perennial populations had a greater sediment accretion efficiency than annual plant populations. These findings can provide key parameters relating to the sediment accretion efficiency of hydrological and geomorphic models on a global scale. This study offers some novel insights into the dynamic changes in coastal wetlands following SLR that will be particularly useful in devising appropriate strategies for the protection and management of coastal wetlands.
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Recent Researches on Mangroves
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In order to improve the quality of natural seawater to meet the needs of aquaculture production, a large-scale subsurface flow constructed wetland was constructed and operated stably for 2 years to study its purification effect on natural seawater. Studies had shown that the system could maintain high purification effect on natural seawater during aquaculture, and could effectively remove harmful algae such as Cyanobacteria, especially in warm months. The pyrosequencing of 16S ribosomal DNA revealed that Proteobacteria , Nitrospirae , chloroflexi were the main and key bacterial phyla in the system. Members of these key gates were regarded as the important roles in resisting water purification. In summary, the subsurface flow wetland system was suitable for large-scale seawater aquaculture plants, which could effectively improve seawater quality conditions and reduced the density of harmful algae cells.
Coastal areas around the globe are threatened with sea-level rise (SLR). Many of these areas support coastal forests, and determining the threat is complicated by wind patterns and sea floor and shoreline variability, beyond simple elevational concerns. Lichens can be used as indicators of SLR at site specific locations. Developing vegetative indicators of SLR could assist land managers, municipalities, and homeowners in evaluating the risk of building homes, roads, and other structures in some locations. The absence of salt sensitive lichen species can be an indicator of SLR potential. We base this on lichen distribution patterns within two Florida state parks – one a barrier island, the other on the nearby mainland. Rapidly colonizing lichen taxa with asexual reproductive structures are poor indicators of SLR, because of their ability to quickly recolonize following storm events and other disturbances. Knowledge of lichen distribution patterns in combination with reproductive traits help facilitate the development of lichen indicator guidelines. We have learned that Coccocarpia palmicola, Dirinaria picta, and Heterodermia albicans do not appear to be salt-tolerant but have abundant asexual propagules and recolonize rapidly. Sites lacking salt sensitive lichens such as Bulbothrix laevigatula, Cryptothecia rubrocincta, Leptogium cyanescens and Parmotrema rampoddense could indicate areas that are at high risk to SLR at a fine scale. Such areas should be avoided for the construction of homes, roads, and other structures. Lichens make good indicator species and could help guide management of coastal areas in Florida and much of the U.S. southeast Coastal Plain.
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Mangrove forests are one of the world's most threatened tropical ecosystems with global loss exceeding 35% (ref. 1). Juvenile coral reef fish often inhabit mangroves, but the importance of these nurseries to reef fish population dynamics has not been quantified. Indeed, mangroves might be expected to have negligible influence on reef fish communities: juvenile fish can inhabit alternative habitats and fish populations may be regulated by other limiting factors such as larval supply or fishing. Here we show that mangroves are unexpectedly important, serving as an intermediate nursery habitat that may increase the survivorship of young fish. Mangroves in the Caribbean strongly influence the community structure of fish on neighbouring coral reefs. In addition, the biomass of several commercially important species is more than doubled when adult habitat is connected to mangroves. The largest herbivorous fish in the Atlantic, Scarus guacamaia, has a functional dependency on mangroves and has suffered local extinction after mangrove removal. Current rates of mangrove deforestation are likely to have severe deleterious consequences for the ecosystem function, fisheries productivity and resilience of reefs. Conservation efforts should protect connected corridors of mangroves, seagrass beds and coral reefs.
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Mangrove islands in Belize are characterized by a unique switching from nitrogen (N) to phosphorus (P) limitation to tree growth from shoreline to interior. Fertilization has previously shown that Rhizophora mangle (red mangrove) fringe trees (5-6 m tall) growing along the shoreline are N limited; dwarf trees (less than or equal to 1.5 m tall) in the forest interior are P limited; and transition trees (2-4 m tall) are co-limited by both N and P. Growth patterns paralleled a landward decrease in soil flushing by tides and an increase in bioavailable N, but P availability remained consistently low across the gradient. Stable isotopic composition was measured in R. mangle leaves to aid in explaining this nutrient switching pattern and growth variation. Along control transects, leaf delta(15)N decreased from +0.10parts per thousand (fringe) to -5.38parts per thousand (dwarf). The delta(15)N of N-fertilized trees also varied spatially, but the values were consistently more negative (by similar to3parts per thousand) compared to control trees. Spatial variation in 815 N values disappeared when the trees were fertilized with P and values averaged +0.12parts per thousand, similar to that in control fringe trees. Neither variation in source inputs nor microbial fractionation could fully account for the observed patterns in delta(15)N. The results instead suggest that the lower delta(15)N values in transition and dwarf control trees were due to plant fractionation as a consequence of slower growth and lower N demand. P fertilization increased N demand and decreased fractionation. Although leaf delta(13)C was unaffected by fertilization, values increased from fringe (-28.6parts per thousand) to transition (-27.9parts per thousand) to dwarf (-26.4parts per thousand) zones, indicating spatial variation in environmental stresses affecting stomatal conductance or carboxylation. The results thus suggest an interaction of external supply, internal demand, and plant ability to acquire nutrients under different hydro-edaphic conditions that vary across this tree-height gradient. The findings not only aid in understanding mangrove discrimination of nitrogen and carbon isotopes, but also have implications for identifying nutrient loading and other stress conditions in coastal systems dominated by mangroves.
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Mangrove forests are characterized by distinctive tree-height gradients that reflect complex spatial, within-stand differences in environmental factors, including nutrient dynamics, salinity, and tidal inundation, across narrow gradients. To determine patterns of nutrient limitation and the effects of nutrient availability on plant growth and within-stand nutrient dynamics, we used a factorial experiment with three nutrient treatment levels (control, N, P) and three zones along a tree-height gradient (fringe, transition, dwarf) on offshore islands in Belize. Transects were laid out perpendicular to the shoreline across a mangrove forest from a fringe stand along the seaward edge, through a stand of intermediate height, into a dwarf stand in the interior of the island. At three sites, three trees were fertilized per zone for 2 yr. Although there was spatial variability in response, growth by R. mangle was generally nitrogen (N) -limited in the fringe zone; phosphorus (P) -limited in the dwarf zone; and, N- and/or P-limited in the transition zone. Phosphorus-resorption efficiency decreased in all three zones, and N-resorption efficiency increased in the dwarf zone in response to P enrichment. The addition of N had no effect on either P or N resorption efficiencies. Belowground decomposition was increased by P enrichment in all zones, whereas N enrichment had no effect. This study demonstrated that essential nutrients are not uniformly distributed within mangrove ecosystems; that soil fertility can switch from conditions of N to P limitation across narrow ecotonal gradients; and, that not all ecological processes respond similarly to, or are limited by, the same nutrient.
Low islands will be particularly vulnerable to the loss of mangrove ecosystems during the rises of relative sea-level projected for the next 50 yr. Mangrove ecosystems in these locations could keep up with a sea-level rise of up to 8-9 cm/100 yr, but at rates of over 12 cm/100 yr could not persist, due to low rates of sediment accumulation. Other factors contributing to mangrove persistence are the primary production rate of forests, shoreline erosion due to deeper and more turbulent water, and the frequency and intensity of tropical storms. -from Authors
Mangrove swamps develop only where coastal physiography and energy conditions are favourable. They are most extensive where there is a low shore gradient, and occupy a broader belt on shorelines which have a large tidal range (Walsh, 1974). Sheltered habitats are essential for mangrove development, and on coasts which are exposed, mangroves are localised in the lee of other coastal landforms (Davies, 1972).
There are extensive mangrove forests along the tropical shorelines of the world, and stratigraphic investigations of the sediments underlying both the mangrove forests themselves and associated wetlands indicate that there have been considerable changes in the extent of these wetlands in the past. The development and the pattern of regeneration of mangrove forests varies from one geomorphological setting to another. The exact response will depend on shoreline topography, sources of sediment, rates of sediment supply, and the rate of sea-level change. Stratigraphy and chronology of Holocene deposits on mangrove shorelines allow reconstruction of the way that mangrove forests have responded to past conditions, and thus provide some insight into how they may respond in the future. Different responses can be expected in: (i) river-dominated deltas where sediment is supplied from terrestrial sources; (ii) macrotidal estuaries, where tidal resuspension of sediment can rework estuarine sediments landwards: and (iii) mangrove forests in carbonate areas, where dynamics are largely a function of the evolution of carbonate landforms. Climatic setting is also an important constraint on swamp development. Planning for the utilisation of mangrove forests needs to take these varying responses in the different settings into account.