Tidal freshwater forested wetlands
These wetlands occur within floodplains of coastal rivers
at the upper boundary of tidal influence, often just
upstream of tidal marshes. Larger areal distributions of
tidal freshwater forested wetlands are generally found in
areas that have large tidal ranges coupled with high river
discharge (Conner et al., 2007). However, they are not
restricted to these conditions and can occur in smaller
watersheds. Conditions are favorable for this habitat type
worldwide, but most published accounts describe conditions in the southeastern United States. Studies outside
the United States have been limited to Central America
and the Amazonian coast in South America, with limited
descriptions (see Verhoeven et al., 2001; Struyf et al.,
2009) of willow (Salix spp.)-dominated tidal freshwater
forested wetlands in Europe.
A frequently described characteristic of these wetlands
includes the prominent development of microtopographic
patterning under purely freshwater conditions. This patterning is often called hummock and hollow topography
(Rheinhardt and Hershner, 1992) and is common in these
wetlands when occurring in non-stressed states (Conner
et al., 2007). Hollows are low-lying, flat areas that are
mostly bare mud or contain herbaceous vegetation similar
to tidal freshwater marshes. Hummocks are raised
microsites that average 15–20 cm high and are roughly
1–10 m
2 in size. Hollows are flooded during most flood
tides and remain saturated within 20 cm of the surface
nearly 100 % of the time. In contrast, hummocks
typically flood less frequently, to lesser depths, and
remain saturated for shorter times, which increases
oxygen penetration and affects nutrient availability
(Courtwright and Findlay, 2011). As a result, hummocks
often contain a greater diversity of tree and shrub species,
especially in remote backswamp areas (Duberstein and
Conner, 2009).
Hydrology and community composition
Rivers flowing toward the ocean are impeded during
flooding tides, first resulting in flow reversals within the
channel and then rising water levels, often resulting in
overbank flooding onto the floodplain. The frequency,
depth, and duration of flooding are determined primarily
by lunar- and wind-driven tides (Conner et al., 2007),
and the salinity of the floodwater ranges from full-strength
seawater (35 g/l) to completely fresh (<0.5 g/l) depending
upon the relative contributions of seawater versus fresh
river water over multiple tidal cycles. However, floodwater over the soil surface in tidal freshwater forested wetlands typically has low salinity, thus keeping the soil
porewater fresh or nearly so. Storm surges can bring
pulses of saline water into tidal freshwater forested wetlands, but high salinity floodwater usually leaves the
floodplain relatively quickly, allowing for freshwater
flushing. Tidal freshwater forested wetlands have, by
definition, average annual soil porewater salinities
<0.5 g/l; however, sometimes, salinity pulses, e.g., from
storm surge or drought, are incorporated into the
porewater, and when that happens, habitat change to
oligohaline (low salinity, 0.5–5.0 g/l) marsh can occur
(Brinson et al., 1985).
Tidal freshwater forested wetlands within the southeastern United States vary more in the relative dominance
and density of tree species, rather than presence or
absence, with some exceptions. The most common canopy trees include swamp tupelo (Nyssa biflora), water
tupelo (Nyssa aquatica), baldcypress (Taxodium
distichum), pumpkin ash (Fraxinus profunda), Carolina
ash (Fraxinus caroliniana), green ash (Fraxinus
pennsylvanica), and red maple (Acer rubrum). Shrub
species tend to vary more between river systems, but
the most ubiquitous are hazel alder (Alnus serrulata)
and wax myrtle (Morella cerifera). Atlantic white cedar
(Chamaecyparis thyoides) is rare in tidal freshwater areas
but can be found in isolated stands in the southeastern
United States. It occurs in fairly monotypic stands along
the coast of North Carolina or as part of a diverse mix of
hardwood species restricted to parts of Mississippi near
the Alabama border (Conner et al., 2007). Swamps in
Louisiana contain primarily baldcypress and water
tupelo, but in other parts of the southeastern United
States, the diversity of trees and specific assemblages follow a salinity and flood frequency gradient (Conner et al.,
2007) with the most frequently flooded and most saline
stands consisting primarily of baldcypress in the canopy
with wax myrtle in the understory (Krauss et al., 2009).
Central American and Amazonian tidal freshwater forested wetlands are often managed for agriculture (e.g.,
cacao, assai), though some unmanaged landscapes still
exist. There are two general types of unmanaged tidal
freshwater forested wetlands in Central and South America: palm swamps and hardwood swamps. Tidal palm
swamps generally occur as low-diversity patches within
hardwood swamps, with virtual monocultures of the dominant tree or palm species (Prance, 1979). Hardwood
swamps in Central America also have very low diversity
(Ellison, 2004). Amazonian tidal várzea are dominated
by relatively few species as well but have higher total
diversity than any other tidal freshwater forested wetland
reported thus far in the scientific literature when left
unmanaged (see Almeida et al., 2004). Tidal palm swamps
in Honduras are dominated by the spiny palm (Bactris
minor), whereas yolillo palm (Raphia taedigera) dominates
in Costa Rica and Amazonia. Amazon palm swamps can
also be dominated by muriti (Mauritia flexuosa), assai
(Euterpe oleracea), or troolie (Manicaria saccifera). Hardwood tidal swamps in Honduras are dominated by
dragonsblood tree (Pterocarpus officinalis) with coin vine
(Dalbergia ecastophyllum, a shrub) common in the understory. Tree species in tidal várzea vary widely between
stands and subregions, but perhaps the most common
include baboonwood (Virola surinamensis), tornillo
(Cedrelinga catenaeformis), silk cotton tree (Ceiba
pentandra), and pracuiba (Mora paraensis) (Prance, 1979).
Much like tidal freshwater swamps in the United States,
FORESTED WETLAND HABITAT
335
These wetlands occur within floodplains of coastal rivers
at the upper boundary of tidal influence, often just
upstream of tidal marshes. Larger areal distributions of
tidal freshwater forested wetlands are generally found in
areas that have large tidal ranges coupled with high river
discharge (Conner et al., 2007). However, they are not
restricted to these conditions and can occur in smaller
watersheds. Conditions are favorable for this habitat type
worldwide, but most published accounts describe conditions in the southeastern United States. Studies outside
the United States have been limited to Central America
and the Amazonian coast in South America, with limited
descriptions (see Verhoeven et al., 2001; Struyf et al.,
2009) of willow (Salix spp.)-dominated tidal freshwater
forested wetlands in Europe.
A frequently described characteristic of these wetlands
includes the prominent development of microtopographic
patterning under purely freshwater conditions. This patterning is often called hummock and hollow topography
(Rheinhardt and Hershner, 1992) and is common in these
wetlands when occurring in non-stressed states (Conner
et al., 2007). Hollows are low-lying, flat areas that are
mostly bare mud or contain herbaceous vegetation similar
to tidal freshwater marshes. Hummocks are raised
microsites that average 15–20 cm high and are roughly
1–10 m
2 in size. Hollows are flooded during most flood
tides and remain saturated within 20 cm of the surface
nearly 100 % of the time. In contrast, hummocks
typically flood less frequently, to lesser depths, and
remain saturated for shorter times, which increases
oxygen penetration and affects nutrient availability
(Courtwright and Findlay, 2011). As a result, hummocks
often contain a greater diversity of tree and shrub species,
especially in remote backswamp areas (Duberstein and
Conner, 2009).
Hydrology and community composition
Rivers flowing toward the ocean are impeded during
flooding tides, first resulting in flow reversals within the
channel and then rising water levels, often resulting in
overbank flooding onto the floodplain. The frequency,
depth, and duration of flooding are determined primarily
by lunar- and wind-driven tides (Conner et al., 2007),
and the salinity of the floodwater ranges from full-strength
seawater (35 g/l) to completely fresh (<0.5 g/l) depending
upon the relative contributions of seawater versus fresh
river water over multiple tidal cycles. However, floodwater over the soil surface in tidal freshwater forested wetlands typically has low salinity, thus keeping the soil
porewater fresh or nearly so. Storm surges can bring
pulses of saline water into tidal freshwater forested wetlands, but high salinity floodwater usually leaves the
floodplain relatively quickly, allowing for freshwater
flushing. Tidal freshwater forested wetlands have, by
definition, average annual soil porewater salinities
<0.5 g/l; however, sometimes, salinity pulses, e.g., from
storm surge or drought, are incorporated into the
porewater, and when that happens, habitat change to
oligohaline (low salinity, 0.5–5.0 g/l) marsh can occur
(Brinson et al., 1985).
Tidal freshwater forested wetlands within the southeastern United States vary more in the relative dominance
and density of tree species, rather than presence or
absence, with some exceptions. The most common canopy trees include swamp tupelo (Nyssa biflora), water
tupelo (Nyssa aquatica), baldcypress (Taxodium
distichum), pumpkin ash (Fraxinus profunda), Carolina
ash (Fraxinus caroliniana), green ash (Fraxinus
pennsylvanica), and red maple (Acer rubrum). Shrub
species tend to vary more between river systems, but
the most ubiquitous are hazel alder (Alnus serrulata)
and wax myrtle (Morella cerifera). Atlantic white cedar
(Chamaecyparis thyoides) is rare in tidal freshwater areas
but can be found in isolated stands in the southeastern
United States. It occurs in fairly monotypic stands along
the coast of North Carolina or as part of a diverse mix of
hardwood species restricted to parts of Mississippi near
the Alabama border (Conner et al., 2007). Swamps in
Louisiana contain primarily baldcypress and water
tupelo, but in other parts of the southeastern United
States, the diversity of trees and specific assemblages follow a salinity and flood frequency gradient (Conner et al.,
2007) with the most frequently flooded and most saline
stands consisting primarily of baldcypress in the canopy
with wax myrtle in the understory (Krauss et al., 2009).
Central American and Amazonian tidal freshwater forested wetlands are often managed for agriculture (e.g.,
cacao, assai), though some unmanaged landscapes still
exist. There are two general types of unmanaged tidal
freshwater forested wetlands in Central and South America: palm swamps and hardwood swamps. Tidal palm
swamps generally occur as low-diversity patches within
hardwood swamps, with virtual monocultures of the dominant tree or palm species (Prance, 1979). Hardwood
swamps in Central America also have very low diversity
(Ellison, 2004). Amazonian tidal várzea are dominated
by relatively few species as well but have higher total
diversity than any other tidal freshwater forested wetland
reported thus far in the scientific literature when left
unmanaged (see Almeida et al., 2004). Tidal palm swamps
in Honduras are dominated by the spiny palm (Bactris
minor), whereas yolillo palm (Raphia taedigera) dominates
in Costa Rica and Amazonia. Amazon palm swamps can
also be dominated by muriti (Mauritia flexuosa), assai
(Euterpe oleracea), or troolie (Manicaria saccifera). Hardwood tidal swamps in Honduras are dominated by
dragonsblood tree (Pterocarpus officinalis) with coin vine
(Dalbergia ecastophyllum, a shrub) common in the understory. Tree species in tidal várzea vary widely between
stands and subregions, but perhaps the most common
include baboonwood (Virola surinamensis), tornillo
(Cedrelinga catenaeformis), silk cotton tree (Ceiba
pentandra), and pracuiba (Mora paraensis) (Prance, 1979).
Much like tidal freshwater swamps in the United States,
FORESTED WETLAND HABITAT
335