the differences in tree communities found between different
tidal várzea are attributed to slight differences in elevation
(Cattanio et al., 2002), which influences flooding.
Climate change impacts
Tidal freshwater forested wetlands are generally adapted
to tolerate short-term increases in salinity that arise from
storm surges and/or decreased river flow during droughts,
but their proximity to the coast also makes them prone to
the chronic salinization driven by sea-level rise and land
subsidence (Conner et al., 2007). The conversion of habitat from tidal freshwater to oligohaline manifests at
sustained average annual porewater salinity around 2 g/l
as trees die off and tidal swamps convert to brackish
marshes or open water (Hackney et al., 2007). Tidal freshwater forested wetlands would be expected to respond to
rising sea levels and the expanded “reach” of tidal influence by migrating upstream (Krauss et al., 2009). However, the extent of their migration is limited in many
places by the presence of levees built for flood control,
human development, and/or agricultural production
(Doyle et al., 2010). By one modeling account, the extent
of tidal freshwater forested wetlands in the southeastern
United States is expected to decrease by 24–34 % by
2100 (Craft et al., 2009). It is likely that this habitat type
is undergoing similar pressures globally with coastal
development and sea-level rise.
Tidal saltwater forested wetlands (mangroves)
Mangroves are found in the intertidal zone (at the edge of
continental land masses or islands) of low energy coasts.
The hydrologic environment that mangroves occupy is similar to that of salt marshes, and the two habitat types are
dynamic with regard to their shared boundaries. However,
mangroves are more restricted globally due to their limited
cold tolerance. Mangrove forests are most pervasive in
tropical climates where they can readily outcompete salt
marshes (Saintilan et al., 2009), but they are found in subtropical and warm temperate climates as well. In total, there
are approximately 73 species and/or hybrids of mangroves
(Duke et al., 1998) found in 123 countries, occupying
roughly 13.7–15.2 M ha of intertidal, estuarine habitat
worldwide (Spalding et al., 2010).
Mangroves occupy a wide variety of soil types, ranging
from coralline soils that can be very alkaline (pH 8.5) and
nutrient deficient to highly organic soils that can be very
acidic (pH 5.8) and nutrient rich (Alongi, 2009). However,
many mangrove forests are located on soils that are nutrient
poor (Reef et al., 2010). Species distributions are often
related to nutrient availability, and trade-offs exist between
having morphological and physiological adaptations to tolerate low-nutrient environments and the ability to
outcompete for dominance in high-nutrient environments
(Krauss et al., 2008). Adaptations best suited for low nutrient conditions include the following: thick, carbon-rich
evergreen leaves, efficient nutrient resorption prior to leaf
fall, high biomass allocation to roots relative to shoots,
and recolonization of previously used root channels (Reef
et al., 2010); several Rhizophora species are well adapted
for low-nutrient conditions. Mangroves that do well in
high-nutrient conditions (e.g., Avicennia spp.) exhibit rapid
growth, increased leaf area relative to stems and roots, thinner leaves with lower tannin concentrations, and greater
photosynthesis and growth relative to the amount of water
used (i.e., water use efficiency) (Krauss et al., 2008).
Hydrology and community composition
Mangrove forests have a remarkable zonation pattern relative to distance from open ocean water, with each zone
often dominated by a single tree species and frequently
sharp (but dynamic) boundaries between zones (Smith,
1992). This zonation pattern correlates with the frequency
and duration of tidal immersion, which directly affects the
degree of waterlogging (i.e., soil saturation), availability
of nutrients, and salinity of the floodwater and soil
porewater (Ball, 1988).
When a wetland is flooded, the oxygen available in the
soil and water column is quickly depleted, resulting in
oxygen deficiencies. Roots require oxygen for respiration
using normal aerobic metabolic pathways, and many
mangroves have structural adaptations to cope with
oxygen deficiencies. Shallow root systems, extensive
aerenchyma, and lenticels all increase the amount of
oxygen available to roots (Ball, 1988).
Oxygen availability also affects soil nutrient availability. The availability of nitrogen and phosphorous, the
two essential soil nutrients most widely linked to rates
of plant growth, can change coincident with the amount
of time the soil is flooded (see Mitsch and Gosselink,
2000). For instance, nitrogen that is bioavailable (able
to be taken up by the roots and used for growth; i.e.,
nitrate or NO 3
À ) reduces to biologically unavailable
forms such as nitrite (NO 2
À ), nitrous oxide (N 2 O),
dinitrogen (N 2 ), and/or ammonium (NH 4
+ ) with
prolonged flooding. Phosphorus can become less bioavailable under anaerobic conditions by precipitating out with
ferric iron, calcium, and aluminum, or binding onto clay
particles, organic peat, and ferric and aluminum hydroxides
and oxides. Because oxygen availability in the soil is
dependent upon flooding frequency and duration, the
amount of bioavailable nitrogen and phosphorous varies
along the tidal immersion gradient. In general, mangrove
growth is considered to be nitrogen limited in the anaerobic
sediments at positions closest to the open ocean, while the
availability of phosphorus limits mangrove growth
in more oxidized soils at positions more landward
(Boto and Wellington, 1983).
As facultative halophytes, mangroves are generally
found in saline environments, but species exhibit a wide
variety of growth responses to salinity. Optimal salinities
for growth range from 2 to 26 g/l (5–75 % seawater), but
most can also grow in freshwater (Krauss and Ball,
2013). All mangroves accumulate ions for osmoregulation
but differ in the extent to which ions can be accumulated
336
FORESTED WETLAND HABITAT
tidal várzea are attributed to slight differences in elevation
(Cattanio et al., 2002), which influences flooding.
Climate change impacts
Tidal freshwater forested wetlands are generally adapted
to tolerate short-term increases in salinity that arise from
storm surges and/or decreased river flow during droughts,
but their proximity to the coast also makes them prone to
the chronic salinization driven by sea-level rise and land
subsidence (Conner et al., 2007). The conversion of habitat from tidal freshwater to oligohaline manifests at
sustained average annual porewater salinity around 2 g/l
as trees die off and tidal swamps convert to brackish
marshes or open water (Hackney et al., 2007). Tidal freshwater forested wetlands would be expected to respond to
rising sea levels and the expanded “reach” of tidal influence by migrating upstream (Krauss et al., 2009). However, the extent of their migration is limited in many
places by the presence of levees built for flood control,
human development, and/or agricultural production
(Doyle et al., 2010). By one modeling account, the extent
of tidal freshwater forested wetlands in the southeastern
United States is expected to decrease by 24–34 % by
2100 (Craft et al., 2009). It is likely that this habitat type
is undergoing similar pressures globally with coastal
development and sea-level rise.
Tidal saltwater forested wetlands (mangroves)
Mangroves are found in the intertidal zone (at the edge of
continental land masses or islands) of low energy coasts.
The hydrologic environment that mangroves occupy is similar to that of salt marshes, and the two habitat types are
dynamic with regard to their shared boundaries. However,
mangroves are more restricted globally due to their limited
cold tolerance. Mangrove forests are most pervasive in
tropical climates where they can readily outcompete salt
marshes (Saintilan et al., 2009), but they are found in subtropical and warm temperate climates as well. In total, there
are approximately 73 species and/or hybrids of mangroves
(Duke et al., 1998) found in 123 countries, occupying
roughly 13.7–15.2 M ha of intertidal, estuarine habitat
worldwide (Spalding et al., 2010).
Mangroves occupy a wide variety of soil types, ranging
from coralline soils that can be very alkaline (pH 8.5) and
nutrient deficient to highly organic soils that can be very
acidic (pH 5.8) and nutrient rich (Alongi, 2009). However,
many mangrove forests are located on soils that are nutrient
poor (Reef et al., 2010). Species distributions are often
related to nutrient availability, and trade-offs exist between
having morphological and physiological adaptations to tolerate low-nutrient environments and the ability to
outcompete for dominance in high-nutrient environments
(Krauss et al., 2008). Adaptations best suited for low nutrient conditions include the following: thick, carbon-rich
evergreen leaves, efficient nutrient resorption prior to leaf
fall, high biomass allocation to roots relative to shoots,
and recolonization of previously used root channels (Reef
et al., 2010); several Rhizophora species are well adapted
for low-nutrient conditions. Mangroves that do well in
high-nutrient conditions (e.g., Avicennia spp.) exhibit rapid
growth, increased leaf area relative to stems and roots, thinner leaves with lower tannin concentrations, and greater
photosynthesis and growth relative to the amount of water
used (i.e., water use efficiency) (Krauss et al., 2008).
Hydrology and community composition
Mangrove forests have a remarkable zonation pattern relative to distance from open ocean water, with each zone
often dominated by a single tree species and frequently
sharp (but dynamic) boundaries between zones (Smith,
1992). This zonation pattern correlates with the frequency
and duration of tidal immersion, which directly affects the
degree of waterlogging (i.e., soil saturation), availability
of nutrients, and salinity of the floodwater and soil
porewater (Ball, 1988).
When a wetland is flooded, the oxygen available in the
soil and water column is quickly depleted, resulting in
oxygen deficiencies. Roots require oxygen for respiration
using normal aerobic metabolic pathways, and many
mangroves have structural adaptations to cope with
oxygen deficiencies. Shallow root systems, extensive
aerenchyma, and lenticels all increase the amount of
oxygen available to roots (Ball, 1988).
Oxygen availability also affects soil nutrient availability. The availability of nitrogen and phosphorous, the
two essential soil nutrients most widely linked to rates
of plant growth, can change coincident with the amount
of time the soil is flooded (see Mitsch and Gosselink,
2000). For instance, nitrogen that is bioavailable (able
to be taken up by the roots and used for growth; i.e.,
nitrate or NO 3
À ) reduces to biologically unavailable
forms such as nitrite (NO 2
À ), nitrous oxide (N 2 O),
dinitrogen (N 2 ), and/or ammonium (NH 4
+ ) with
prolonged flooding. Phosphorus can become less bioavailable under anaerobic conditions by precipitating out with
ferric iron, calcium, and aluminum, or binding onto clay
particles, organic peat, and ferric and aluminum hydroxides
and oxides. Because oxygen availability in the soil is
dependent upon flooding frequency and duration, the
amount of bioavailable nitrogen and phosphorous varies
along the tidal immersion gradient. In general, mangrove
growth is considered to be nitrogen limited in the anaerobic
sediments at positions closest to the open ocean, while the
availability of phosphorus limits mangrove growth
in more oxidized soils at positions more landward
(Boto and Wellington, 1983).
As facultative halophytes, mangroves are generally
found in saline environments, but species exhibit a wide
variety of growth responses to salinity. Optimal salinities
for growth range from 2 to 26 g/l (5–75 % seawater), but
most can also grow in freshwater (Krauss and Ball,
2013). All mangroves accumulate ions for osmoregulation
but differ in the extent to which ions can be accumulated
336
FORESTED WETLAND HABITAT