coasts of the Atlantic. Movements of the Earth’s plates
were responsible for the separation of what was once
a continuous global distribution; as the various plates
separated, some species invariably became extinct, while
others diversified regionally. Today, there is a clear separation between the Indo-West Pacific (IWP) and the
Atlantic-Caribbean-East Pacific (ACEP) biogeographic
regions, as cold waters prevent contact and dispersal
between the southern Indian and Atlantic Oceans. Thus,
there are six subregions (Western Americas, Eastern
Americas, Western Africa, Eastern Africa, Indo-Malesia,
Australasia) that fall into the distinct IWP and ACEP
regions (Figure 1). Constraints on their global distribution
include temperature, rainfall, and human impacts (Duke
et al., 1998). Although quite variable geographically, both
the number of mangrove species and their total area
decline with increasing latitude or decreasing rainfall or
both. A poleward expansion of mangroves on at least five
continents in relation to the poleward extension of temperature thresholds has occurred concurrent with sea-level
rise (Saintilan et al., 2014).
Despite their importance along tropical and subtropical
coasts, mangroves are disappearing at an alarming rate due
to clearing for coastal development, for aquaculture, and
for timber and fuel production (Daru et al., 2013; Polidoro
et al., 2010). A global loss rate of 1–2 % has been cited
(Spalding et al., 2010), but some areas experience little
loss, while others are losing a greater percentage of total
area. Approximately 15 % of the world’s mangrove species are at a high threat of extinction (Polidoro et al.,
2010), especially along the Atlantic and Pacific coasts of
Central America, where up to 40 % of mangrove species
present are currently at risk. The most landward forests
are most often the most threatened with clearing.
Adaptations
Mangroves have many morphological, reproductive, and
physiological attributes that make them well adapted to
salty soils. These specializations include aerial roots,
viviparous embryos, and tidal dispersal of propagules.
Mangroves exhibit a number of different strategies to deal
with salt, such as salt avoidance and regulation coupled
with mechanisms of tolerance; resistance strategies
include exclusion, extrusion, storage, succulence, compartmentalization, and osmoregulation (Popp, 1995). Salt
tolerance varies greatly among species and there are wide
species-specific variations in growth responses. Some
species such as Avicennia marina do not grow in freshwater and may be obligate halophytes, whereas other species
grow well in freshwater and do not have an obligatory
need for more than trace amounts of salt. The presence
of salt can constrain water relations, as a positive water
balance and photosynthesis can only be maintained if the
potentials in the plant are lower than in the soil; in highsalinity soils, maintaining water balance presents
a problem of trying to take up essential inorganic ions to
maintain osmotic balance while trying to avoid adverse
effects of high ionic levels in the cytoplasm (Lovelock
and Ball, 2002).
Because the metabolic cost of maintaining water balance is high, mangroves display a number of features to
minimize water loss, including low transpiration rates
and sclerophylly, expressed as mangrove leaves being
thick-walled, usually with a multilayered epidermis covered by a thick, waxy, lamellar cuticle that helps to minimize evaporation. On the lower leaf surface, there is
usually a dense field of hairs (e.g., Avicennia, Pemphis)
or scales (e.g., Camptostemon) that cover salt glands and
stomata to reduce water loss from these openings. Sunken
stomata, waxy coatings, a thick cuticle, and widespread,
cutinized, and sclerenchymatous cells are xenic characteristics for dealing with a physiologically dry environment
(Saenger, 2002). Low transpiration rates are imposed by
high salt concentrations. When salinity is lower due to
high rainfall in the wet season, transpiration rates and
stomatal conductance can be high. However, mangroves
overall follow a very conservative water-use strategy.
Conserving water reflects a trade-off between the need
for the stomata to open to maintain intercellular CO 2 concentrations and the simultaneous loss of water vapor; thus,
carbon gain is balanced by some water loss. Low stomatal
conductance limits such water loss but also restricts the
uptake of CO 2 . This dilemma results in low intercellular
CO 2 concentrations, low assimilation rates, and high
water-use efficiencies – the ratio of carbon assimilated to
water used (Saenger, 2002). High water-use efficiency is
achieved by adaptive traits such as specialized leaf and
stomatal anatomy, high levels of photooxidative protection, hydraulic architecture (small vessels and dense
wood), and greater carbon investment in roots than aboveground tree parts (Feller et al., 2010). Physiological stress
is minimized and water-use efficiency is maximized by the
ability of many species to adjust the angle of their leaves to
avoid high temperatures and maximize heat loss; this
adaptation also has a metabolic cost as maintaining
a favorable leaf angle comes at the expense of light
harvesting and assimilative capacity. Various leaf sizes
have also evolved to help achieve a balance between maximizing carbon uptake, minimizing leaf temperatures, and
minimizing water expenditure.
Morphological and physiological adaptations to maximize root aeration are a key feature of mangroves to deal
with the problem of the lack of oxygen and the presence
of potentially toxic metabolites in waterlogged saline
soils. Morphological adaptations include relatively high
root/shoot ratios as well as a range of aboveground root
systems, such as pneumatophores (e.g., Avicennia,
Sonneratia) that break the soil surface from the cable
roots; stilt roots (e.g., Rhizophora) that branch off from
the lower trunk and descent into the substrate; knee roots
(e.g., Bruguiera) that break the surface but curve back
down into the soil; buttress roots (Xylocarpus, Heritiera)
that also branch off from the trunk but do so as flattened,
triangular structures; and aerial roots that originate from
the trunk or lower branches but usually do not reach the
MANGROVES
395
were responsible for the separation of what was once
a continuous global distribution; as the various plates
separated, some species invariably became extinct, while
others diversified regionally. Today, there is a clear separation between the Indo-West Pacific (IWP) and the
Atlantic-Caribbean-East Pacific (ACEP) biogeographic
regions, as cold waters prevent contact and dispersal
between the southern Indian and Atlantic Oceans. Thus,
there are six subregions (Western Americas, Eastern
Americas, Western Africa, Eastern Africa, Indo-Malesia,
Australasia) that fall into the distinct IWP and ACEP
regions (Figure 1). Constraints on their global distribution
include temperature, rainfall, and human impacts (Duke
et al., 1998). Although quite variable geographically, both
the number of mangrove species and their total area
decline with increasing latitude or decreasing rainfall or
both. A poleward expansion of mangroves on at least five
continents in relation to the poleward extension of temperature thresholds has occurred concurrent with sea-level
rise (Saintilan et al., 2014).
Despite their importance along tropical and subtropical
coasts, mangroves are disappearing at an alarming rate due
to clearing for coastal development, for aquaculture, and
for timber and fuel production (Daru et al., 2013; Polidoro
et al., 2010). A global loss rate of 1–2 % has been cited
(Spalding et al., 2010), but some areas experience little
loss, while others are losing a greater percentage of total
area. Approximately 15 % of the world’s mangrove species are at a high threat of extinction (Polidoro et al.,
2010), especially along the Atlantic and Pacific coasts of
Central America, where up to 40 % of mangrove species
present are currently at risk. The most landward forests
are most often the most threatened with clearing.
Adaptations
Mangroves have many morphological, reproductive, and
physiological attributes that make them well adapted to
salty soils. These specializations include aerial roots,
viviparous embryos, and tidal dispersal of propagules.
Mangroves exhibit a number of different strategies to deal
with salt, such as salt avoidance and regulation coupled
with mechanisms of tolerance; resistance strategies
include exclusion, extrusion, storage, succulence, compartmentalization, and osmoregulation (Popp, 1995). Salt
tolerance varies greatly among species and there are wide
species-specific variations in growth responses. Some
species such as Avicennia marina do not grow in freshwater and may be obligate halophytes, whereas other species
grow well in freshwater and do not have an obligatory
need for more than trace amounts of salt. The presence
of salt can constrain water relations, as a positive water
balance and photosynthesis can only be maintained if the
potentials in the plant are lower than in the soil; in highsalinity soils, maintaining water balance presents
a problem of trying to take up essential inorganic ions to
maintain osmotic balance while trying to avoid adverse
effects of high ionic levels in the cytoplasm (Lovelock
and Ball, 2002).
Because the metabolic cost of maintaining water balance is high, mangroves display a number of features to
minimize water loss, including low transpiration rates
and sclerophylly, expressed as mangrove leaves being
thick-walled, usually with a multilayered epidermis covered by a thick, waxy, lamellar cuticle that helps to minimize evaporation. On the lower leaf surface, there is
usually a dense field of hairs (e.g., Avicennia, Pemphis)
or scales (e.g., Camptostemon) that cover salt glands and
stomata to reduce water loss from these openings. Sunken
stomata, waxy coatings, a thick cuticle, and widespread,
cutinized, and sclerenchymatous cells are xenic characteristics for dealing with a physiologically dry environment
(Saenger, 2002). Low transpiration rates are imposed by
high salt concentrations. When salinity is lower due to
high rainfall in the wet season, transpiration rates and
stomatal conductance can be high. However, mangroves
overall follow a very conservative water-use strategy.
Conserving water reflects a trade-off between the need
for the stomata to open to maintain intercellular CO 2 concentrations and the simultaneous loss of water vapor; thus,
carbon gain is balanced by some water loss. Low stomatal
conductance limits such water loss but also restricts the
uptake of CO 2 . This dilemma results in low intercellular
CO 2 concentrations, low assimilation rates, and high
water-use efficiencies – the ratio of carbon assimilated to
water used (Saenger, 2002). High water-use efficiency is
achieved by adaptive traits such as specialized leaf and
stomatal anatomy, high levels of photooxidative protection, hydraulic architecture (small vessels and dense
wood), and greater carbon investment in roots than aboveground tree parts (Feller et al., 2010). Physiological stress
is minimized and water-use efficiency is maximized by the
ability of many species to adjust the angle of their leaves to
avoid high temperatures and maximize heat loss; this
adaptation also has a metabolic cost as maintaining
a favorable leaf angle comes at the expense of light
harvesting and assimilative capacity. Various leaf sizes
have also evolved to help achieve a balance between maximizing carbon uptake, minimizing leaf temperatures, and
minimizing water expenditure.
Morphological and physiological adaptations to maximize root aeration are a key feature of mangroves to deal
with the problem of the lack of oxygen and the presence
of potentially toxic metabolites in waterlogged saline
soils. Morphological adaptations include relatively high
root/shoot ratios as well as a range of aboveground root
systems, such as pneumatophores (e.g., Avicennia,
Sonneratia) that break the soil surface from the cable
roots; stilt roots (e.g., Rhizophora) that branch off from
the lower trunk and descent into the substrate; knee roots
(e.g., Bruguiera) that break the surface but curve back
down into the soil; buttress roots (Xylocarpus, Heritiera)
that also branch off from the trunk but do so as flattened,
triangular structures; and aerial roots that originate from
the trunk or lower branches but usually do not reach the
MANGROVES
395