aboveground net primary production (AGNPP) averages
11 t DW ha
À1 year
À1 (Alongi, 2009). This AGNPP rate
compares favorably with the mean AGNPP rate for tropical terrestrial forests of 12 t DW ha
À1 year
À1
. Production
rates of both mangroves and other tropical forests overlap
highlighting the fact that similar ecological and physiological factors limit production of all trees. Some forests of
both habitats in the dry tropics are not very productive
compared with the most luxuriant forests in wet tropical
regions. Mangroves, like other forests, vary in size and
age and in the balance between production and respiration.
Measurements of belowground primary production are
lacking for all tropical forests, as are measurements of
dark leaf respiration and respiration of roots and woody
parts, so true estimates of total forest net primary production are sparse. Despite these constraints, mangrove
AGNPP declines with increasing distance from the equator, mirroring the decline in mangrove biomass (Alongi,
2009).
The light response curves of mangrove leaves are similar to other tropical plants in which saturation is reached
at 300–400 mmol photons m
À2 s
À1 after a steep linear
increase to this threshold. Maximum CO 2 assimilation
rates can often exceed 25 mmol CO 2 m
À2 s
À1 although
most rates lie between 5 and 20 mmol CO 2 m
À2 s
À1 as saturation is reached at comparatively low light conditions
due to low stomatal conductance and intercellular CO 2
concentrations. Rates of leaf photosynthesis decline with
increasing salinity and increasing vapor pressure deficit.
Despite these limitations, the median rate of mangrove leaf
photosynthesis (12 mmol CO 2 m
À2 s
À1
) is equal to the
median rate (11 mmol CO 2 m
À2 s
À1
) for shade-intolerant
terrestrial trees.
The rate of dark leaf respiration in mangroves ranges
from 0.2 to 1.4 mmol CO 2 m
À2 s
À1 with photosynthesis to
respiration (P/R) ratios ranging from 2.1 to 11.2, which is
at the upper end of the range of values for tropical terrestrial
species. A few root respiration measurements have
been made, mainly on Avicennia marina pneumatophores, with highly variable rates among all species
(Rhizophora mangle, 0.5–6 nmol CO 2 g
À1 root FW s
À1
;
Avicennia marina, 2–3 mmol CO 2 g
À1 root FW h
À1
). These
rates are at the lower end of values for other tropical trees.
The uptake and assimilation of micro- and macronutrients play a key role in determining mangrove growth and
production, as mangroves are often limited by the availability of nitrogen (N), phosphorus (P), and iron (Fe).
These and many other elements are required to synthesize
cells and to manufacture structural and reproductive tissue
(Reef et al., 2010). The critical need for N and P has
been demonstrated for a number of mangroves, with clear
interactive effects among different mangrove species,
nutrients, and environmental factors such as salinity, temperature, soil type, and frequency of tidal inundation
(Feller et al., 2010).
The ordinarily high photosynthetic rates for mangroves
drive a high requirement for nutrients; nutrient-use
efficiencies and rates of nutrient resorption by leaves are
correspondingly high. Differences among species in
nutrient-use and resorption rates can be explained by differences in the way species allocate nutrients as well as
species differences in leaf life spans and how energy and
nutrients are vested in chemical defenses. With such
a high requirement for nutrients, mangroves have evolved
a number of conservation mechanisms (in addition to high
resorption efficiencies), including large reservoirs of dead
roots belowground, tidal export of predominantly refractory matter, and highly efficient nutrient cycles (Reef
et al., 2010).
Phytoplankton and algae living on the forest floor and
as epiphytes on aboveground tree parts are additional
sources of fixed carbon in mangroves. Algal production
in mangrove waters and under the canopy is light limited,
and while algal productivity can be high, it is usually
dwarfed by tree production (Alongi, 2009).
Nutrient cycling and sources for secondary
consumers
The cycling of essential elements such as N in mangrove
ecosystems is highly complex and internally regulated
by the trees and their interrelationships with soil, water,
and microbiota – most of whom are responsible for the
bulk of nutrient transformations and recycling processes.
Concentrations of dissolved and particulate N and P are
low in tropical waters and mangrove soils, but cycle
quickly, to the extent that residence times for many nutrient pools are on the order of minutes to hours. Nutrients
are transformed, taken up, and assimilated by the trees
and other biota both rapidly and efficiently, despite the fact
that acquiring nutrients is not a straightforward process. In
mangrove soils, for instance, Fe oxyhydroxides and metal
sulfide complexes readily bind to organic nutrients,
thereby limiting the amount available to the plant.
Geochemical redox reactions in soils and sediments are
complex and involve a wide array of elements and compounds restricting the uptake of dissolved organic and
inorganic nutrients.
Conserving nutrients is advantageous, so large belowground reservoirs of dead roots and maximizing nutrient
storage in the youngest tree parts are efficient retention
mechanisms. Another effective conservation strategy is
to increase the efficiency of various metabolic processes
and the utilization of nutrient pools. Mangroves invest
a large proportion of root metabolism in the uptake and
assimilation of soil ammonium, which translates into
a comparatively low-energy investment compared with
using nitrate or possibly DON.
The soil N cycle in mangroves indicates that ammonium production (ammonification) is the dominant
N transformation process, with proportionally little loss
to the atmosphere via anammox (anaerobic ammonium
oxidation) and denitrification (production of N 2 gas). High
rates of ammonification are supported by dissolved
organic nitrogen (DON) released by roots. The uptake of
ammonium is fast enough compared to its production that
400
MANGROVES
11 t DW ha
À1 year
À1 (Alongi, 2009). This AGNPP rate
compares favorably with the mean AGNPP rate for tropical terrestrial forests of 12 t DW ha
À1 year
À1
. Production
rates of both mangroves and other tropical forests overlap
highlighting the fact that similar ecological and physiological factors limit production of all trees. Some forests of
both habitats in the dry tropics are not very productive
compared with the most luxuriant forests in wet tropical
regions. Mangroves, like other forests, vary in size and
age and in the balance between production and respiration.
Measurements of belowground primary production are
lacking for all tropical forests, as are measurements of
dark leaf respiration and respiration of roots and woody
parts, so true estimates of total forest net primary production are sparse. Despite these constraints, mangrove
AGNPP declines with increasing distance from the equator, mirroring the decline in mangrove biomass (Alongi,
2009).
The light response curves of mangrove leaves are similar to other tropical plants in which saturation is reached
at 300–400 mmol photons m
À2 s
À1 after a steep linear
increase to this threshold. Maximum CO 2 assimilation
rates can often exceed 25 mmol CO 2 m
À2 s
À1 although
most rates lie between 5 and 20 mmol CO 2 m
À2 s
À1 as saturation is reached at comparatively low light conditions
due to low stomatal conductance and intercellular CO 2
concentrations. Rates of leaf photosynthesis decline with
increasing salinity and increasing vapor pressure deficit.
Despite these limitations, the median rate of mangrove leaf
photosynthesis (12 mmol CO 2 m
À2 s
À1
) is equal to the
median rate (11 mmol CO 2 m
À2 s
À1
) for shade-intolerant
terrestrial trees.
The rate of dark leaf respiration in mangroves ranges
from 0.2 to 1.4 mmol CO 2 m
À2 s
À1 with photosynthesis to
respiration (P/R) ratios ranging from 2.1 to 11.2, which is
at the upper end of the range of values for tropical terrestrial
species. A few root respiration measurements have
been made, mainly on Avicennia marina pneumatophores, with highly variable rates among all species
(Rhizophora mangle, 0.5–6 nmol CO 2 g
À1 root FW s
À1
;
Avicennia marina, 2–3 mmol CO 2 g
À1 root FW h
À1
). These
rates are at the lower end of values for other tropical trees.
The uptake and assimilation of micro- and macronutrients play a key role in determining mangrove growth and
production, as mangroves are often limited by the availability of nitrogen (N), phosphorus (P), and iron (Fe).
These and many other elements are required to synthesize
cells and to manufacture structural and reproductive tissue
(Reef et al., 2010). The critical need for N and P has
been demonstrated for a number of mangroves, with clear
interactive effects among different mangrove species,
nutrients, and environmental factors such as salinity, temperature, soil type, and frequency of tidal inundation
(Feller et al., 2010).
The ordinarily high photosynthetic rates for mangroves
drive a high requirement for nutrients; nutrient-use
efficiencies and rates of nutrient resorption by leaves are
correspondingly high. Differences among species in
nutrient-use and resorption rates can be explained by differences in the way species allocate nutrients as well as
species differences in leaf life spans and how energy and
nutrients are vested in chemical defenses. With such
a high requirement for nutrients, mangroves have evolved
a number of conservation mechanisms (in addition to high
resorption efficiencies), including large reservoirs of dead
roots belowground, tidal export of predominantly refractory matter, and highly efficient nutrient cycles (Reef
et al., 2010).
Phytoplankton and algae living on the forest floor and
as epiphytes on aboveground tree parts are additional
sources of fixed carbon in mangroves. Algal production
in mangrove waters and under the canopy is light limited,
and while algal productivity can be high, it is usually
dwarfed by tree production (Alongi, 2009).
Nutrient cycling and sources for secondary
consumers
The cycling of essential elements such as N in mangrove
ecosystems is highly complex and internally regulated
by the trees and their interrelationships with soil, water,
and microbiota – most of whom are responsible for the
bulk of nutrient transformations and recycling processes.
Concentrations of dissolved and particulate N and P are
low in tropical waters and mangrove soils, but cycle
quickly, to the extent that residence times for many nutrient pools are on the order of minutes to hours. Nutrients
are transformed, taken up, and assimilated by the trees
and other biota both rapidly and efficiently, despite the fact
that acquiring nutrients is not a straightforward process. In
mangrove soils, for instance, Fe oxyhydroxides and metal
sulfide complexes readily bind to organic nutrients,
thereby limiting the amount available to the plant.
Geochemical redox reactions in soils and sediments are
complex and involve a wide array of elements and compounds restricting the uptake of dissolved organic and
inorganic nutrients.
Conserving nutrients is advantageous, so large belowground reservoirs of dead roots and maximizing nutrient
storage in the youngest tree parts are efficient retention
mechanisms. Another effective conservation strategy is
to increase the efficiency of various metabolic processes
and the utilization of nutrient pools. Mangroves invest
a large proportion of root metabolism in the uptake and
assimilation of soil ammonium, which translates into
a comparatively low-energy investment compared with
using nitrate or possibly DON.
The soil N cycle in mangroves indicates that ammonium production (ammonification) is the dominant
N transformation process, with proportionally little loss
to the atmosphere via anammox (anaerobic ammonium
oxidation) and denitrification (production of N 2 gas). High
rates of ammonification are supported by dissolved
organic nitrogen (DON) released by roots. The uptake of
ammonium is fast enough compared to its production that
400
MANGROVES