Carbon sequestration in mangrove forests

Abstract

Mangrove forests are highly productive, with carbon production rates equivalent to tropical humid forests. Mangroves allocate proportionally more carbon belowground, and have higher below- to above-ground carbon mass ratios than terrestrial trees. Most mangrove carbon is stored as large pools in soil and dead roots. Mangroves are among the most carbon-rich biomes, containing an average of 937 tC ha-1, facilitating the accumulation of fine particles, and fostering rapid rates of sediment accretion (∼5 mm year -1) and carbon burial (174 gC m-2 year -1). Mangroves account for only approximately 1% (13.5 Gt year -1) of carbon sequestration by the world’s forests, but as coastal habitats they account for 14% of carbon sequestration by the global ocean. If mangrove carbon stocks are disturbed, resultant gas emissions may be very high. Irrespective of uncertainties and the unique nature of implementing REDD+ and Blue Carbon projects, mangroves are prime ecosystems for reforestation and restoration.

Full text

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ISSN 1758-3004
10.4155/CMT.12.20 © 2012 Future Science Ltd
Mangrove forests are the only woody halophytes that live
in salt water along the world’s subtropical and tropical
coastlines. Coincidentally, poverty and dense human
populations flourish along these low-latitude coasts,
partly explaining the high (1–3%) annual de forestation
rates of these tidal forests. Mangroves are true ecotones,
having some components of both marine and terrestrial
biomes, but have also developed a number of unique
structural and functional adaptations, such as viviparous
embryos, physiological mechanisms to tolerate salt and
aerial roots that enable the plants to respire in anoxic,
waterlogged soils [1]. Mangroves are architecturally sim-
ple compared with terrestrial forests, usually harboring
few tree species and lacking an understory of ferns and
scrubs. However, the standing biomass of some mangrove
forests in equatorial regions can be immense, rivaling the
height and weight of many tropical rainforests [1].
Mangroves are ultimately limited by temperature but,
at local and regional scales, variations in pre cipitation,
tides, waves and river flow greatly determine their expanse
and biomass. Attempts have often been made to classify
the sequential changes in forest structure and species dis-
tribution parallel to shore but, in reality, most mangrove
forests represent a continuum of types in relation to
gradients in their physical settings. Variations can be
expressed within a single estuary, where there are usually
upstream–downstream changes in geo morphology, salin-
ity, waves, tides and river flow, with these factors affecting
water circulation by generating mixing and trapping of
coastal water [2]. The development of mangrove forests
occurs where near-horizontal topography coincides with
sea level; a relatively stable period of sea level is, thus, a
prerequisite for the development of old-growth forests
[3]. The response of mangroves to environmental change
is, therefore, often indicative of past changes in coastal
conditions, especially in sea level. Comparing present
patterns in forest species with paleoecological informa-
tion provides considerable insight, not only into how
mangroves responded to past sea level changes, but how
they may respond to climate change in the future.
Human disturbance obscures natural change and
our ability to distinguish one from the another is lim-
ited, as most forests have a history of both natural and
human disturbances, and are often intertwined and
indistinguish able. Mangroves are naturally disturbed by
tsunamis, floods, cyclones, lightning, pests and disease,
Carbon Management (2012) 3(3), 313–322
Carbon sequestration in mangrove forests
Daniel M Alongi*
Mangrove forests are highly productive, with carbon production rates equivalent to tropical humid forests.
Mangroves allocate proportionally more carbon belowground, and have higher below- to above-ground
carbon mass ratios than terrestrial trees. Most mangrove carbon is stored as large pools in soil and dead
roots. Mangroves are among the most carbon-rich biomes, containing an average of 937tCha
-1
, facilitating
the accumulation of ne particles, and fostering rapid rates of sediment accretion (~5 mm year
-1
) and
carbon burial (174gC m
-2
year
-1
). Mangroves account for only approximately 1% (13.5Gtyear
-1
) of carbon
sequestration by the world’s forests, but as coastal habitats they account for 14% of carbon sequestration
by the global ocean. If mangrove carbon stocks are disturbed, resultant gas emissions may be very high.
Irrespective of uncertainties and the unique nature of implementing REDD+ and Blue Carbon projects,
mangroves are prime ecosystems for reforestation and restoration.
Review
*Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland 4810, Australia
E-mail: d.alongi@aims.gov.au

Carbon Management (2012) 3(3)
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and become more susceptible when
human stressors such as pollutants
are introduced. However, mangroves
often exhibit considerable resilience
to disturbance, undergoing perpet-
ual change in ecosystem develop-
ment commensurate with the evo-
lution of the environmental settings
they inhabit, and are, thus, mosaics
of successional stages arrested or
interrupted over time and space by
natural ecological responses in rela-
tion to disturbances both large and
small [4].
Mangrove forests are a valuable
ecological and economic resource,
providing essential services such as
food and fuel resources; nursery
grounds for fish, mammals and
other semi-terrestrial and aquatic fauna; depocenters
for sediment, carbon and other elements; and, in some
instances, offering some protection from coastal erosion
due to tsunamis and intense tropical storms [1]. Despite
their uses to humans, approximately 50% of the world’s
mangrove forests have disappeared over the past 50 years
[5], ironically reflecting their importance as a valuable
economic resource. Major causes for this destruction
have been urban development, aquaculture, mining, and
overexploitation of timber, fish, crustaceans and shell-
fish. The average monetary value of mangroves has been
estimated as second only to the value of estuaries and
seagrass meadows, and greater than the economic value
of coral reefs, continental shelves and the open sea [6].
Of greater eventual value is the role of mangroves in
storing carbon to help ameliorate the impact of climate
change. There is a growing consensus that it will be
impossible to achieve significant cuts in GHG emis-
sions without passive and active means to capture and
store CO
2
[7]. The role of carbon storage in mangroves
has often been overlooked and either underestimated
or overestimated [1], and it is the purpose of this review
to critically assess the role of mangroves in carbon
sequestration
and its global significance.
Carbon production
Mangroves are usually highly productive forests and, as
a significant fraction of their soil carbon is plant-derived
[8], it is crucial to assess rates of net primary productivity
of mangroves and associated plants, especially benthic
microalgae. Measurement of primary production in
mangrove forests is limited by methodological short-
comings, but the best estimates suggest that mangrove
carbon production is more rapid than other estuarine
and marine primary producers [9]. Rates of mangrove net
primary production (NPP) based on different methods
range from 0.5 to 112.1 t dry weight (DW) ha
-1
year
-1
but most methods either significantly overestimate (the
light attenuation method) or underestimate (litterfall)
the true rates of production.
The most reasonable means at present to assess NPP of
forests is to measure aboveground biomass accumulation
plus litterfall, and there are quite a number of such mea-
surements for both mangroves and tropical terrestrial
forests. For mangroves, the mean rate of above ground
NPP is 11.1 t DW ha
-1
year
-1
with a median value of
8.1 t DW ha
-1
year
-1
; for tropical terrestrial forests, the
mean rate of aboveground NPP is 11.9 t DW ha
-1
year
-1
with a median value of 11.4 t DW ha
-1
year
-1
; for both
mangroves and terrestrial forests, NPP declines with
increasing latitude [1]. Considering the differences within
and between both forest groups in biomass, height, age
and species, the rates are very close and clearly imply
that rates of NPP are equivalent between mangroves and
other forests.
Like other forests, mangroves vary in size and age and,
therefore, vary in rates of production and in the bal-
ance between carbon production and respiration. The
few studies that have measured mangrove tree growth
over time or in stands of known age have observed stand
dynamics similar to other forests, identifying stages of
early rapid growth during colonization and early estab-
lishment, followed by a slow decline in growth rate into
maturity and senescence [1,10,11]. The stable-state matu-
rity phase can be prolonged in some mangrove stands
and may represent an alternate succession state in which
the clock for the climax stage is reset by successive dis-
turbances [10]. The relationship between mangrove forest
age and photosynthetic production [11] suggests prolonga-
tion or arrested progression when forests are disturbed;
Rhizophora apiculata forests in southeast Asia show log-
phase photosynthetic rates until approximately 20 years,
after which photosynthesis levels off but does not signifi-
cantly decline for nearly a century [1]. These data imply
that mangroves might indeed constitute a carbon sink
for up to a century if left relatively undisturbed.
Other primary producers inhabit mangrove forests
and their rates of NPP can be significant, especially in
comparatively open canopies and on tidal banks where
sufficient light penetrates to the forest floor [1]. Various
autotrophic and mixotrophic microbes and microalgae,
as well as macroalgae, live on the soil surface and as
epiphytes on tree parts, especially aerial roots and decom-
posing wood. The quantitative contribution of these
smaller autotrophs is dwarfed by tree production, but
belies their importance as food and refugia for consum-
ers. However, some evidence suggests that they can play
an important role in soil carbon and nitrogen cycling,
especially when found as intact mats [12].
Key terms
Mangroves: Trees and associated
plants, microbes and animals that live at
the interface between land and sea.
These tidal ecosystems have both
semi-terrestrial and marine
components.
Coastal: Land, water and aquatic
habitats that reside where the
continents meet the ocean. These
habitats are usually only a few
kilometers in width but are highly
dynamic and interactive with respect to
energy and material flow between land
and sea.
Carbon sequestration: Term used to
describe the acquisition and storage of
carbon. Refers most often in relation to
the ability of ecosystems to reduce the
impact of increasing CO
2
concentrations
in the atmosphere.

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Carbon allocation & ecosystem storage
Critical to our ability to estimate the role of mangroves
in coastal and global carbon cycling is an accurate under-
standing of where carbon fixed by the trees is allocated.
Like other woody plants, mangroves construct new foli-
age, reproductive organs, stem, branches and root tissues
and maintain existing tissue, as well as creating storage
reserves and providing chemical defense. Approximately
half of all CO
2
assimilated by mangroves is returned to
the atmosphere via above- and below-ground respiration
[1,11]. This is only a crude estimate owing to the lack
of empirical data and the difficulty of measuring root
processes and respiration of woody parts. The propor-
tional allocation of fixed carbon within trees varies with
many factors, such as light intensity, species composi-
tion, nutrient and water availability, salinity, tides, waves,
temperature and climate [11].
The greatest unknown with regard to carbon alloca-
tion is root production, which is difficult to measure,
especially in waterlogged soils. The few studies that have
measured root growth in situ estimated rates ranging from
18 to 1145 g DW m
-2
year
-1
with most estimates between
300 and 380 g DW m
-2
year
-1
[1]. These estimates are at
the lower end of the range of values measured in tropical
terrestrial forests [13]. However, most measurements were
made in mangrove fringe stands, so it is likely that the
growth and production of mangrove roots is similar to
their terrestrial counterparts. A recent ana lysis of carbon
allocation suggests that mangroves allocate proportion-
ally more carbon belowground than terrestrial trees [14].
Carbon inventories from a number of mangrove eco-
systems show that both above- and below-ground biomass
increases, and that the ratio of below- to above-ground
biomass decreases with increasing stand age (Table1).
These data show that belowground carbon biomass is,
on average (mean = 1.3), equivalent to carbon allocated
aboveground; other studies have indicated that more car-
bon biomass is allocated belowground [15–18] supporting
the notion that mangroves store a disproportionate frac-
tion of fixed carbon underground. Further, the amount
of soil carbon increases with forest age (see Figure 5.1 in
Alongi [1]).
Complete inventories of ecosystem components show
that carbon fixed within the forest, as well as carbon
imported from adjacent terrestrial and marine waters,
are stored as large pools of soil carbon [19,20]. Analysis of
carbon in Rhizophora stylosa and Avicennia marina in arid
coastal areas of Western Australia [19] and in R. apiculata
forests in southern Thailand [20] showed that although
most carbon was unassociated with roots, the majority
(75–95%) of tree carbon belowground was vested in
dead, rather than live, roots. The Thai study also showed
that the soil and dead root carbon pools increased in size
with increasing stand age [20].
A recent assessment of carbon stored in various forest
domains found that in comparison with boreal, temper-
ate and tropical upland forests, mangroves throughout
the Indo-Pacific are among the most carbon -rich forests
in the tropics containing, on average, 1023 tC ha
-1
, most
of which is stored in soils >30 cm deep [21]. Adding pub-
lished and unpublished data by authors from southern
China, Vietnam, Indonesia, arid Western Australia,
Queensland, Thailand and Malaysia (Table1) to the data
set of Donato et al. [21] to diversify the geographical,
subtropical and arid-zone forest domains, we obtain a
revised mean whole-ecosystem carbon storage estimate
of 937 tC ha
-1
(Figure1), which still indicates that man-
groves are among the world’s most carbon-rich forests.
It is possible, of course, this statement may not hold true
globally, especially when data is obtained from Central
and South America and Africa, and from more forests in
the arid tropics and subtropics where fringing mangroves
and mangroves growing on hard and/or substrates of
limited depth are common. Nevertheless, throughout the
equatorial regions (e.g., the wet tropics of southeast Asia)
it is true that mature mangrove stands attain highest car-
bon mass compared with other carbon-rich ecosystems,
such as tropical rainforests.
What does inarguably appear to be a global pattern
among mangrove forests is that their belowground pools
of root and soil carbon are large, having a higher below-
to above-ground carbon mass ratio than any other woody
vegetation [22].
With the bulk of belowground carbon stored in dead
roots and soil rather than in live roots, mangroves have
a tendency to accumulate carbon relatively quickly.
Belowground roots may only represent approximately
10–15% of total tree biomass, but the allocation of fixed
carbon to replace sloughed root hairs and fine roots is
considerably greater [23,24]. Moreover, carbon concen-
trations in dead roots are greater than in live roots,
suggesting that dead roots store proportionally more
carbon [19,20].
Vertical profiles of live versus dead root matter in a
number of mangroves show that most living roots are
shallow, within the upper 0–40 cm of soil [1]. Most fine
roots are dead, probably the net result of rapid root turn-
over coupled with slow rates of root decomposition [23].
Rates of belowground decomposition of fine and coarse
mangrove roots are indeed slow, with most rates ranging
from 0.07 to 0.17% root mass lost per day; only roots of
A. marina decompose more quickly at rates varying from
0.09 to 0.34% root mass lost per day [1]. Roots decom-
pose at equivalent rates regardless of intertidal elevation,
but coarse roots decompose less quickly than fine roots.
These slow decay rates explain the formation of peat in
many mangrove forests as inputs must exceed decay rates
in order for peat to accumulate [23–25].

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Why do mangrove forests have such large amounts of
carbon vested belowground compared with terrestrial
forests? The presence of a large pool of dead roots can
serve as a nutrient conserving mechanism, and even large
dead roots may serve this purpose. For instance, old root
channels have been found in mangroves in central Belize
with a proliferation of living roots among the decaying
roots, taking paths of least resistance and recovering
nutrients released from decomposing roots [25]. A large
pool of belowground live and dead root biomass mixed
with rich soils may reflect their numerous physiological
and morphological adaptations to life in a harsh, saline
waterlogged environment. Salt negatively affects water
use and under such conditions it may be advantageous
for mangrove trees to invest more fixed carbon in grow-
ing very expensive root systems that turnover rapidly in
order to maximize water gain. Large reservoirs beneath
the forest floor may also help to stabilize the trees and
the entire ecosystem from the continual push and pull
of the tides, wave action, coastal winds and tropical
storms. It makes evolutionary sense for mangroves to
invest in a large belowground pool of carbon biomass
as an effective counterbalance to litter and carbon dis-
solved in interstitial water that is
lost via the tides. Whereas tropical
humid forests recycle nutrients by
rapid soil decomposition of litter in
a relatively thin humus layer, man-
groves reclaim elements by way of
very tight cycling between roots and microbes several
meters deep into the soil, possibly to curtail losses and
to minimize energetic costs.
Mechanisms facilitating sediment accumulation
Lying at the interface between land and sea, it is hardly
surprising that mangroves accumulate sediment and
associated particulate elements, such as inorganic and
organic carbon. What is surprising is that their presence
actively facilitates the accumulation of materials [26].
Carbon is accumulated in mangroves by direct inputs
of mangrove carbon to the soil pool and by increasing
rates of mass sediment accumulation. Carbon produced
by mangroves does have other flow pathways, such as
consumption by living organisms, especially microbes.
Carbon consumed is remineralized and either emitted
back to the atmosphere as CO
2
or exported by dissolved
inorganic carbon. Dissolved and particulate organic
carbon is also exported by tides where it can be either
deposited or eaten or mineralized offshore.
The amount of carbon stored in mangrove soils varies
widely, from <0.1% by soil dry weight to >40% with a
grand median of 2.2% [8]. A highly variable proportion
of this carbon is mangrove-derived as organic matter is
brought in by the tides from adjacent seagrass mead-
ows, coral reefs, macroalgae, rivers and from land-based
sources, and other marine environments [8]. The frac-
tion of mangrove-derived carbon in forest soils depends
on a number of factors, including location of the forest
Key term
Flocculation: Physical, chemical and
microbial processes by which particles
are cemented together; the term ‘floc’
refers to the cemented tuft-like mass.
Table1. Whole-ecosystem inventories of above- and below-ground carbon biomass and soil carbon for natural and replanted
mangrove forests.
Location Dominant species Age
(years)
Total
(tCha
-1
)
AGB
(tCha
-1
)
BGB and
soil (tCha
-1
)
Roots/AGB
(tCha
-1
)
Roots
(tCha
-1
)
Soil
(tCha
-1
)
Soil depth
(cm)
Peninsular Malaysia Rhizophora apiculata 80 2205 312 1893 NA NA NA 3800
R. apiculata 18 1117 193 924 NA NA NA 4000
R. apiculata 5 479 87 392 NA NA NA 2800
Southern Vietnam R. apiculata 6 1179 54 1125 NA NA NA 3400
R. apiculata 20 979 72 907 NA NA NA 2750
R. apiculata 35 1904 153 1752 NA NA NA 3600
Southern
China
Kandelia candel NA 619 64 555 2.0 130 425 1850
K. candel NA 391 43 348 2.2 94 254 1900
K. candel NA 332 7 325 1.1 8 317 1175
Indonesia Avicennia marina NA 437 24 413 NA NA NA 80
Rhizophora stylosa NA 703 19 684 NA NA NA 62
Sonneratia caseolaris NA 654 28 626 NA NA NA 1450
Southern Thailand R. apiculata 25 808 138 670 1.0 142 528 1900
R. apiculata 5 579 20 559 2.9 57 502 800
Ceriops decandra 3 600 29 571 4.4 127 444 1000
Western Australia R. stylosa NA 863 115 621 1.1 127 621 1500
A. marina NA 662 55 515 1.7 92 515 775
Queensland, Australia R. stylosa NA 2139 297 1842 1.1 312 1530 3500
AGB: Aboveground biomass; BGB: Belowground biomass; NA: Not available.
Data from [48,50–54,101].

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in relation to the open coast, dis-
tance to adjacent aquatic habitats,
tidal amplitude, forest position in
the tidal seascape and productivity
of primary producers [27].
Unconsolidated sediments accu-
mulate in relation to the movement
of the turbidity maximum zone,
where incoming bottom flow meets
outward river flow. Tidal mixing
and pumping within the moving
zone facilitate particle occulation
and settlement. Flocculation of
particles begins at salinities <1, and
small flocs and free particles move
downstream where they aggregate
with local particles [28]. As flocs get
larger, they move toward the river
bed where they are entrained back
upstream by baroclinic circulation
and even further upstream at flood
tide due to tidal pumping [28]. As these flocs move into
the forest on flood tides, turbulence generated by flow
around the trees helps to maintain flocs in suspension
[2]. Settling occurs quickly, facilitated by the sticking of
microbial mucus in the soil surface and by pelletization
by invertebrate excreta. Large quantities of nonfloccu-
lated particles are re-exported from the forest on ebb
tide, but most stick to mucus at the water surface.
Mangroves thus actively capture silt, clay and organic
particles, and are not just passive importers of fine par-
ticles [2,28]; mangrove vegetation has a profound impact
on sedimentation. Large trees with complex root sys-
tems, such as Rhizophora species, facilitate the deposi-
tion of particles to a much larger extent than trees that
are smaller and of much simpler architecture, such as
Ceriops species. Until slack water, turbulent wakes cre-
ated by tree trunks, prop roots and pneumatophores
maintain particles in suspension, but most flocs settle
within 30 min just before slack high tide [28]. Despite the
pull of the ebb tide, most flocs are retained within the
forest as water motion and turbulence necessary for their
resuspension is inhibited by the high vegetation density.
Rates of soil accretion & carbon sequestration
Mangroves accumulate carbon in tree biomass, but
much of this carbon is eventually lost in the short- and
medium-term by way of clear-cutting and human use,
decomposition and export to adjacent ecosystems. Over
the long term, carbon is stored primarily belowground as
soil carbon and, eventually, under the right conditions,
as peat. There are a number of methods to measure
soil accretion [29], but some are either highly inaccurate
(a mass balance approach where carbon inputs minus
carbon losses equal carbon either buried or unaccounted
for) or reflect mostly modern rates of accumulation
(measurement of short-term sediment accumulation
using sediment traps or changes in depth of the soil
profile). Analysis of radioactive elements produced by
fallout (excess
210
Pb and
137
Cs) from atomic bomb test-
ing in the atmosphere coupled with estimates of soil
carbon concentrations provide longer term estimates
of accumulation and a chronology of sedimentation of
up to a century. Such methods also have their pitfalls,
including reliance on expensive analytical equipment,
difficulty in interpreting radiotracer profiles in biotur-
bated and disturbed soils and in soils where there are
vertical changes in grain size, and problems with error
induced by compaction of sediments as a result of the
coring process [29].
The rate of soil accretion in mangrove forests averages
5 mm year
-1
, with 94 measurements out of a total of
139 ranging from 0.1 to 10.0 mm year
-1
(Figure2). The
median value is 2.7 mm year
-1
with a few measurements
showing net erosion (minimum value = -11.0 mm year
-1
)
or massive accretion (46.3 mm year
-1
) in highly-impacted
estuaries, such as those in southern China [30].
Frequency of tidal inundation is the primary factor
controlling the rate of accretion [31–33] . Less frequent
inundation by tides means less input of sediment par-
ticles; forests located in the high intertidal area experi-
ence less soil accretion than forests closer to mean sea
level, such as fringing stands at the sea–forest interface.
In fact, mangrove carbon often accumulates on adjacent
margins and intertidal mudflats [34]. Often overlooked,
because empirical data are rare, are contributions to ver-
tical accretion from the growth of belowground roots
Boreal Temperate Tropical Mangroves
0
200
400
600
800
1000
1200
Ecosystem carbon storage (tonnes ha
-1
)
Aboveground carbon storage
Belowground carbon storage
Figure1. Dierences in whole-ecosystem carbon stocks among boreal, temperate and
tropical terrestrial forests, and subtropical and tropical mangrove forests.
Reprinted with permission from Macmillan Publishers Ltd: Nature Geoscience
[21] © (2011).
Mangrove data taken from supplementary data
[102] in[21], and [48,50–54].

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Review Alongi
and surface growth of microbial mats and turf algae
and accumulation of litter. In some cases, such as in the
Caribbean, contributions from these biological sources
can be greater than accretion of mineral particles [33].
On other islands such those in the Federates States of
Micronesia, natural subsidence plays a key role in overall
rates of net elevation [35], although not the actual rates
of soil accretion; nevertheless, such changes are impor-
tant in determining the susceptibility of mangroves
to changes in sea level [35]. Rates of soil accretion can
vary over long timescales. In lagoon mangroves on the
Yucatan Peninsula in Mexico, for instance, natural vari-
ations in accumulation rates and sources of soil carbon
were detected over the past 160 years [36]. These changes
corresponded to fluctuations in climatic variability in
the region.
Mangrove sedimentation in rela-
tion to sea level rise was assessed
by Alongi, who found that most
mangrove forests were currently
keeping pace with local rises in sea
level [37]. However, there are a num-
ber of regions where sedimentation
rates are lower than the rates of
regional relative sea level rise, such
as on some Pacific Islands [36] and
at a number of mangrove stands
in the Caribbean [33,38], although
accretion rates at a number of these
endangered forests is higher than
the eustatic sea level rise.
Available data on burial rates of
carbon in mangrove ecosystems
were first compiled by Twilley et al.
[39], later updated by Jennerjahn
and Ittekot [40] and Duarte et al.
[9], based on data in Chmura et al.
[41]. Despite the different databases
and methods used, all derived a
similar estimate of a global car-
bon burial rate of approximately
23 TgC year
-1
, which is equiva-
lent to a rate of 167 gC m
-2
year
-1
assuming a total mangrove area of
137,760 km
2
[5]. Bouillon et al. [26,27]
and Alongi [1] derived carbon
burial rates of 18.4 TgC year
-1
(= 134 g C m
-2
year
-1
) and 29 TgC year
-1
(= 211 g C m
-2
year
-1
), respectively.
Adding more recent data derived
from radiochemical methods, we
can revise the mean global burial
rate for soil carbon to 24 TgC year
-1
,
equivalent to 174 g C m
-2
year
-1
with
values ranging from 10 to 920 g C m
-2
year
-1
; the median
burial rate was 16 TgC year
-1
(= 115 g C m
-2
year
-1
).
Like the sediment accretion data, the standard devia-
tion exceeds the mean reflecting the high level of
variability (and uncertainty) in carbon burial rates
among forests worldwide. Nevertheless, most individ-
ual estimates (47 of a total of 66 measurements) are
<200 g C m
-2
year
-1
, with a minority of forests accumu-
lating soil carbon faster (Figure3), mostly in catchments
heavily impacted by human activities, such as those in
southern China [30] and in southeast Asia [42].
Signicance of mangroves to terrestrial &
marine carbon sequestration
How do these new estimates of carbon sequestration
compare with other forested and coastal ecosystems?
Globally, are mangroves a significant sink for carbon?
Does their loss represent a significant return of CO
2
to the atmosphere?
The data presented here confirm the notion that
mangroves are among the most carbon-rich eco systems
in the tropics. But at a global level, mangroves occupy
only approximately 137, 760 km
2
, and a simple scal-
ing up of the mean carbon burial rate equates to a
global carbon sequestration rate of 13.53 Gt year
-1
. The
same exercise for boreal, temperate and tropical terres-
trial forests extrapolates to global sequestration rates
of 451.1, 327.6, and 422.4 Gt year
-1
, respectively [43].
Sediment accretion rates (mm year
-1
)
Number of measurements
-20 -10 0 10 20 30 40 50
0
20
40
60
80
100
Mean = 4.996 mm year
-1
SD of mean = 7.402 mm year
-1
Confidence interval of mean = 1.241 mm year
-1
Median = 2.7 mm year
-1
Minimum = -11.0 mm year
-1
Maximum = 46.3 mm year
-1
Figure2. Sediment accretion rates measured in various mangrove forests
worldwide(n=139).
Data from
[1,24,28–31,33,35,38,48,51,55].
Key terms
REDD+: Acronym for Reducing
Emissions from Deforestation and
Forest Degradation. The + refers to the
additional steps of conservation and the
sustainable management of forests and
enhancement of forest carbon stocks.
Blue carbon: Term coined to refer to
steps designed to enhance the
acquisition and storage of carbon in
aquatic ecosystems, especially in coastal
habitats such as seagrass beds and
mangrove forests.

Carbon sequestration in mangrove forests Review
future science group
www.future-science.com
319
Thus, mangroves account for
approximately 3% of carbon seques-
tered by the world’s tropical forests,
although they account for <1% of
total area of tropical forests.
These data do, however, suggest
the potential for significant GHG
emissions if the high per-hectare
carbon stocks of mangroves are
disturbed. Losses of mangroves by
clearing, conversion to industrial
estates/aquaculture and changes in
drainage patterns lead to dramatic
changes in soil chemistry and usu-
ally result in rapid emission rates of
GHGs, especially CO
2
. For exam-
ple, deforesting mangroves that
grow on peat soils results in CO
2
emissions comparable to rates esti-
mated from collapse of terrestrial
peat soils [44 ]. Lovelock et al. mea-
sured CO
2
emissions from cleared
mangrove peat soils in Belize on
the order of 2900 tC km
-2
year
-1
[4 4]; this value compares well
with CO
2
emissions measured
from hurricane-damaged and
aquaculture-impacted mangroves
(1500–1750 tC km
-2
year
-1
), rain-
forests drained for agriculture (3200 tC km
-2
year
-1
)
and thawed Arctic tundra (150–430 tC km
-2
year
-1
).
Donato et al. [21] calculated a plausible range of CO
2
emissions of 112–392 tC released per hectare of man-
grove forest and soils cleared, which gives a global emis-
sions range of 0.02–0.12 PgC year
-1
, assuming current
deforestation rates (1–2% per year) and global area.
This range is equivalent to at least 2–10% of global
deforestation emissions (~1.2 PgC year
-1
[45]) and up to
50% of emissions from the world’s tropical peatlands
(0.24 PgC year
-1
[46]). These values are only indicative,
as large uncertainties remain, including the accuracy
of forest areas, temporal and spatial variations in fluxes
and standing stocks, local and regional differences in
the modes of disturbance, and variations in the depth
to which soil is dredged.
If the contribution of mangroves to global forest car-
bon sequestration is very small, their contribution to
carbon burial in the global coastal ocean is considerably
greater. Compared with other coastal ecosystems, man-
groves contribute an average of 14% to carbon seques-
tration in the world’s oceans, although accounting for
only 0.5% of total coastal ocean area (Table2).
Even considering the large uncertainties in these esti-
mates, the average burial rate of carbon in mangroves is
much greater than that from all other habitats, except
for salt marshes. Therefore, considering the data in
Figure1 and in Table2, mangrove forests have the high-
est area rates of carbon sequestration compared with
any other ecosystem, terrestrial or marine, contributing
disproportionately as a carbon sink.
Future perspective
Mangroves are currently being advanced as an essential
component of climate change strategies such as REDD+
and blue carbon. McLeod et al. [47] and Alongi [48] have
recently identified specific actions and issues that need
to be addressed in blue carbon projects:
 Careful site selection, preferably at the seaward edge,
based on drivers thought to affect carbon sequestra-
tion rates, such as frequency of tidal inundation, pri-
mary productivity and rates of exchange with adjacent
ecosystems, as not all mangroves accumulate carbon;
 Measure and map the spatial and temporal variations
in carbon stocks and burial rates, relating these fac-
tors to environmental and ecological drivers, possibly
determining a set of indicators that can be used to
quickly estimate changes in carbon stocks and
fluxes;
Number of measurements
Annual rates of carbon burial (g m
-2
year
-1
)
0 200 400 600 800 1000
0
5
10
15
20
25
30
Mean = 174.1 mm year
-1
SD of mean = 184.1 mm year
-1
Confidence interval of mean = 45.3 mm year
-1
Median = 114.5 mm year
-1
Minimum = 10.0 mm year
-1
Maximum = 920.0 mm year
-1
Figure3. Annual rates of carbon burial estimated in various mangrove forests
worldwide(n=66).
Data from
[1,9,15–18,21,25,28,30–32, 34,38 ,41,42,48,51,55–59].

Carbon Management (2012) 3(3)
future science group
320
Review Alongi

Remote sensing and aerial photography may be useful
to facilitate changes in restoration/rehabilitation
strategies, and in identifying changes in land use;
 Standardization of methods used to measure biomass
and soil carbon stocks and rates of carbon burial;
 The execution of any scheme must consider modeled
predictions of future climate changes, such as regional
predicted rises in sea level;
 Planting of mixed species to maximize biodiversity,
food web connectivity and net ecosystem production;
 Priority must be given to REDD+ schemes that give
priority to old-growth forests as mangrove carbon
stocks increase with stand age;
 Studies should be conducted concurrently to assess
the conditions that determine whether or not climate
change impacts such as changes in sea.
Future climate scenarios for the ocean are subject to
large uncertainties, but regional changes in ocean cir-
culation, temperature, salinity and pH patterns, and in
sea level, must be considered as likely to have a strong
impact on the ability of mangroves to sequester carbon
[49]. Large uncertainties exist in our knowledge of carbon
sequestration in mangroves, and such limitations must be
factored into the blueprints of any payment for ecosystem
services, blue carbon or REDD+ schemes. Only then
will management of mangrove ecosystems be sustainable.
Financial & competing interests disclosure
The author has no relevant affiliations or financial involvement with
any organization or entity with a financial interest in or financial
conflict with the subject matter or materials discussed in the manu-
script. This includes employment, consultancies, honoraria, stock
ownership or options, expert t estimony, grants or patents received or
pending, or royalties. No writing assistance was utilized in the
production of this manuscript.
Executive summary
Carbon production
 Mangrove net primary production averages 11.1t dry weightha
-1
year
-1
, roughly equivalent to tropical terrestrial forests.
 Mangroves may constitute a carbon sink for up to a century.
Carbon allocation & storage
 Belowground biomass is equivalent to aboveground biomass in mangroves.
 Most carbon in mangroves is stored as large pools of soil carbon and belowground roots.
 Storage of carbon in mangroves averages 937tCha
-1
.
Mechanisms facilitating sediment accretion
 Mangroves actively facilitate accumulation of carbon and other elements associated to ne particles.
Rates of soil accretion & carbon sequestration
 Rates of soil accretion in mangroves average 5mmyear
-1
.
 Frequency if tidal inundation is the main factor controlling accretion.
 Global carbon burial rates for mangroves approximate 24TgCyear
-1
.
Signicance of mangroves to terrestrial & marine carbon sequestration
 Mangroves account for 3% of carbon sequestered by the world’s tropical forests, but 14% of carbon sequestered in the world’s ocean.
 If disturbed, mangroves may emit 0.02–0.12 PgCyear
-1
, equal to 2–10% of global deforestation emissions.
Future perspective
 Mangroves are prime candidates for REDD+ and blue carbon projects, but a number of issues and specic actions must be carefully
addressed prior to commencement of such projects.
Table2. Global contribution of mangroves and other coastal habitats to carbon sequestration in the global
coastal ocean.
Habitat Area (10
12
m
2
) Sequestration rate
(gCm
-2
year
-1
)
Global carbon sequestration
(Tgyear
-1
)
Mangroves 0.14 (0.5%) 174 24 (14%)
Salt marshes 0.22 (0.8%) 150 33 (20%)
Seagrasses 0.3 (1.1%) 54 16 (10%)
Estuaries 1.1 (4.0%) 45 50 (30%)
Shelves 26 (93.6%) 17 44 (26%)
†
Total 167
†
Assumes that depositional areas cover 10% of total shelf area [9].
Data from [41,60–62].

Carbon sequestration in mangrove forests Review
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321
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