WETLAND PLANTS: BIOLOGY AND ECOLOGY - CHAPTER 2 pdf

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Wetland Plant Communities
I. Wetland Plant Habitats
Wetland plants grow in a variety of climates, from the tropics to polar regions — wherever
the water table is high enough, or the standing water is shallow enough, to support them.
Each species is adapted to a range of water depths and many do not survive outside of that
range for extended periods. For example, Hydrilla verticillata (hydrilla) thrives when fully
submerged; Typha angustifolia (narrow-leaved cattail) can grow in water over 1 m in depth,
but its leaves are emergent; and others, like Larix laricina, the tamarack tree of northern
peatlands, are fully emergent and normally do not grow where water covers the soil sur-
face. All rooted wetland plants are adapted to at least periodically saturated substrates
where soil oxygen levels are low to non-existent.
The terms for different types of wetlands help to pinpoint the differences between wet-
land communities and can be defined, at least in part, by the type of vegetation that grows
there. For example, swamp denotes a wet area where trees or shrubs dominate the canopy,
such as a cypress swamp, while a marsh is dominated by herbaceous species, such as a cat-
tail marsh. Names given to some wetland types denote either the source or the chemistry
of the water, such as riparian wetland, or salt marsh.
Wetlands are recognized as vital ecosystems that support a wide array of unique plants
especially adapted to wet conditions. Wetland plants, in turn, support high densities of
fish, invertebrates, amphibians, reptiles, mammals, and birds. Wetland conditions such as
shallow water, high plant productivity, and anaerobic substrates provide a suitable envi-
ronment for important physical, biological, and chemical processes. Because of these
processes, wetlands play a vital role in global nutrient and element cycles. Wetlands also
provide key hydrologic benefits: flood attenuation, shoreline stabilization, erosion control,
groundwater recharge and discharge, and water purification (Mitsch and Gosselink 2000).
In addition, they provide economic benefits by supporting fisheries, agriculture, timber,
recreation, tourism, transport, water supply, and energy resources such as peat (T.J. Davis
1993).
II. Wetland Definitions and Functions
The term wetland envelops a wide variety of habitats, from mangroves along tropical

shorelines to peatlands that lie just south of the Arctic. The following definitions help iden-
tify commonalties among these vastly different ecosystems.
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A. Ecological Definition
The determining factor in the wetland environment is water. To a great extent, hydrology
determines soil chemistry, topography, and vegetation. All wetlands have water inputs
that exceed losses, at least seasonally. It is difficult to say exactly how much water an area
must have at any given time in order to be a wetland. Indeed, Cowardin and others (1979)
state that a “single, correct, indisputable, and ecologically sound” definition of wetlands
does not exist, mostly because the line between wet and dry environments is not easily
drawn. Moisture levels vary along a continuum that shifts in time and space. Wetlands
may have standing water throughout the year, or only during a portion of the year. Those
influenced by tide may have water at each high tide, or only at each spring tide.
In all wetlands, the substrate is saturated enough of the time that plants not adapted to
saturated conditions cannot survive. Saturated conditions lead to low oxygen (hypoxia) or
a lack of oxygen (anaerobiosis or anoxia) in the soil pore spaces. Scarcity of oxygen brings
about reducing conditions, in which reduced forms of elements (e.g., nitrogen, manganese,
iron, sulfur, and carbon) are present (Gambrell and Patrick 1978). Such substrates are
termed hydric soils. Wetland plants have adaptations to waterlogging and hydric soils that
allow them to persist. Wetlands, then, are ecosystems in which there is sufficient water to
sustain both hydric soils and the plants that are adapted to them.
B. Legal Definitions
Legal or formal definitions of wetlands have been adopted in a number of countries. In the
U.S., a legal definition of wetlands is needed because wetlands are protected areas, regu-
lated by government agencies. Wetland definitions help classify areas so that the appro-
priate protections or uses can be determined. Many nations have wetland definitions, and
each country’s definition tends to focus on the characteristics of that country’s wetlands
(Scott and Jones 1995). The international definition adopted by the Ramsar Convention of
1971 (Matthews 1993) is often the basis for the definition used by individual countries.

1. United States Army Corps of Engineers’ Definition
In the U.S., wetlands are legally defined by government agencies actively involved in wet-
land identification, protection, and the issuance of permits to people who seek to alter wet-
lands. The U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency
define wetlands as:
those areas that are inundated or saturated by surface or ground water at a frequency
and duration sufficient to support, and that under normal circumstances do support, a
prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands gen-
erally include swamps, marshes, bogs, and similar areas (Federal Interagency Committee for
Wetland Delineation 1989).
This definition is used for the delineation of wetlands throughout the U.S. Disputes con-
cerning wetland boundaries often arise because wetlands do not have distinct edges.
Three components of the wetland ecosystem are taken into consideration by the U.S.
definition: hydrology, soil, and vegetation (see Chapter 10, Wetland Plants as Biological
Indicators). Specific indicators of all three must be present during some part of the grow-
ing season for an area to be a wetland, unless the site has been significantly altered.
Indicators of wetland hydrology include the presence of standing or flowing water or
tides, but water may also be below the soil surface in a wetland. Secondary indicators of
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water level may also be used to establish wetland hydrology, such as water marks, drift
lines, debris lodged in trees or elsewhere, and layers of sediment that form a crust on the
soil surface. Hydric soils develop under low oxygen conditions that bring about diagnos-
tic soil colors, textures, or odors. Other soil indicators include partially decomposed plant
material in the soil profile (as in peatlands) or decomposing plant litter at the surface of the
soil profile. The dominance of wetland vegetation (and the absence or rarity of upland veg-
etation) indicates a wetland.
2. U.S. Fish and Wildlife Classification of Wetlands
For the purpose of wetland and deepwater habitat classification, the U.S. Fish and Wildlife
Service (Cowardin et al. 1979) defined wetlands as:

… lands transitional between terrestrial and aquatic systems where the water table is
usually at or near the surface or the land is covered by shallow water. For purposes of this
classification, wetlands must have one or more of the following three attributes: (1) at least
periodically, the land supports predominantly hydrophytes; (2) the substrate is predomi-
nantly undrained hydric soil; and (3) the substrate is nonsoil and is saturated with water or
covered by shallow water at some time during the growing season of each year.
This definition is the basis for a detailed classification of wetlands (the Cowardin system,
1979) that was a first step in compiling an inventory of all U.S. wetlands (the National
Wetlands Inventory).
3. International Definition
In 1971, an international convention on wetlands was held in Ramsar, Iran by the
International Union for the Conservation of Nature and Natural Resources (IUCN). An
international treaty on wetlands, the Convention on Wetlands of International Importance
Especially as Waterfowl Habitat, also known as the Ramsar Convention, was signed there.
It “provided the framework for international cooperation for the conservation and wise
use of wetlands and their resources” (Matthews 1993). Under the Ramsar Convention wet-
lands are defined as:
… areas of marsh, fen, peatland or water, whether natural or artificial, permanent or tem-
porary, with water that is static or flowing, fresh, brackish or salt, including areas of marine
water the depth of which at low tide does not exceed six meters.
In addition, wetlands “may incorporate riparian and coastal zones adjacent to the wet-
lands, and islands or bodies of marine water deeper than six meters at low tide lying
within the wetlands.”
The Ramsar Convention definition of wetlands is broader than the U.S. Army Corps of
Engineers’ definition as it includes coral reefs and other deeper water habitats. The inclu-
sion of more habitat types in the definition allows the convention to protect a greater area.
All signatory nations agree to designate at least one site for inclusion on the Ramsar List.
Inclusion confers international recognition on a site and obliges the government to main-
tain and protect the wetland. As of February 2000, there were 118 contracting parties with
1,016 sites on the Ramsar List for a total area of over 72.8 million ha (Ramsar Convention

Bureau 2000).
The Ramsar Convention emphasizes the “wise use” and “sustainable development”
of wetlands rather than conservation. They define wise use as the “sustainable utilization
[of wetlands] for the benefit of mankind in a way compatible with the maintenance of the
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natural properties of the ecosystem.” Sustainable utilization of a wetland is defined as
“human use of a wetland so that it may yield the greatest continuous benefit to present
generations while maintaining its potential to meet the needs and aspirations of future
generations” (T.J. Davis 1993). In order to use a wetland wisely, a thorough understanding
of its functions within the landscape is essential.
C. Functions of Wetlands
Whether wetlands are bordered by upland forest, desert, tundra, agricultural land, urban
areas, or ocean, they often perform similar roles, or functions, within the broader land-
scape. All wetland functions are related to the presence, quantity, quality, and movement
of water in wetlands (Carter et al. 1979). Functions are linked to the self-maintenance of the
wetland and its relationship to its surroundings (Mitsch and Gosselink 2000). The func-
tions of wetlands can be categorized into three main categories: hydrology, biogeochem-
istry, and habitat (Walbridge 1993). Wetland functions do not necessarily affect humans
directly. Another term, values, refers to the benefits society derives from wetlands. Wetland
values are closely tied to functions (Table 2.1).
1. Hydrology
Hydrologic functions of wetlands include the recharge and discharge of ground water
supplies, floodwater conveyance and storage, and shoreline and erosion protection.
TABLE 2.1
Functions and Values Commonly Attributed to Wetlands
Function Societal Value
Hydrology Flood mitigation
Groundwater recharge
Shoreline protection

Biogeochemistry
Sediment deposition Improved water quality
Phosphorus sorption
Nitrification
Denitrification
Sulfate reduction
Nutrient uptake
Sorption of metals
Carbon storage Global climate mitigation
Methane production
Plant and animal habitat Timber production
Agricultural crops (rice, cranberries, etc.)
Animal pelts (furs and skins)
Commercial fish/shellfish production
Recreational hunting and fishing
Adapted from Walbridge 1993.
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a. Groundwater Supply
Groundwater may move into a wetland via springs or seeps (groundwater discharge) and
water from the wetland may seep into the groundwater (groundwater recharge).
Groundwater can be recharged from depressional wetlands if the water level in the wet-
land is above the water table of the surrounding soil. Recharge is important for replenish-
ing aquifers for water supply. At some sites, both recharge and discharge occur. For exam-
ple, in Florida cypress ponds, the water level is continuous with the water table of the
surrounding landscape. When the water table rises due to rainfall, groundwater moves
into the cypress pond. In dry periods, the water movement is reversed, and the aquifer is
recharged (Ewel 1990a).
b. Flood Control
Wetlands can temporarily store excess water and release it slowly over time, thus buffer-

ing the impact of floods. Intact and undeveloped riparian wetlands can prevent damaging
floods along rivers (Sather and Smith 1984). Depressional wetlands such as cypress ponds
or prairie potholes have the capacity to receive and store at least twice as much water as a
site filled with soil (Ewel 1990a). Some wetlands are not able to store excess water. If wet-
lands are impounded in order to store more floodwater than they normally would, signif-
icant changes in the plant community can result (Thibodeau and Nickerson 1985).
c. Erosion and Shoreline Damage Reduction
Wetlands along rivers, lakes, and seafronts can protect the shoreline by absorbing the
energy of waves and currents. Wetlands along shorelines are dynamic systems, generally
reaching equilibrium between accretion and erosion of substrate. Structures used for
shoreline protection, such as bulkheads or jetties, can destroy the shoreline habitat by
interrupting this equilibrium. These structures can also channel sediment into navigable
waterways, where the cost of dredging is added to the cost of shoreline protection
(Adamus and Stockwell 1983). Mangroves along tropical shorelines provide a good exam-
ple of the erosion protection that wetlands can provide. Their extensive roots help stabilize
sediments and prevent wave damage to inland areas (Odum and McIvor 1990). In China,
wetlands have been created for shoreline reclamation and stabilization using vast
plantings of Spartina alterniflora (cordgrass [Chung 1993]).
2. Biogeochemistry
A number of important biogeochemical processes are favored in wetlands due to shallow
water (which maximizes the sediment-to-water interface), high primary productivity, the
presence of both aerobic and anaerobic sediments, and the accumulation of litter (Mitsch
and Gosselink 2000). These conditions often lead to a natural cleansing of the water that
flows into wetlands. Incoming suspended solids settle from the water column due to the
reduced water velocity found in wetlands (Johnston et al. 1984; Fennessy et al. 1994b).
Materials associated with solids, such as phosphorus, are also removed from the water col-
umn in wetlands (Johnston 1991; Mitsch et al. 1995). Nitrogen is transformed through
microbial processes (e.g., nitrification followed by denitrification; Faulkner and
Richardson 1989) which require the presence of both aerobic and anaerobic substrates.
Plant uptake and plant tissue accumulation can also remove nitrogen and phosphorus

from the water; however, this process can be reversed when plants die back after the grow-
ing season (Howarth and Fisher 1976; Richardson 1985; Peverly 1985). Wetlands also play
a role in the global cycling of sulfur and carbon as their anaerobic forms are produced
under wetland conditions (see Chapter 3, Section III.A.1, Reduced Forms of Elements).
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The capacity of wetlands to purify water is one of the most important societal values
wetlands provide. Water quality improvements within wetlands are well documented
(Engler and Patrick 1974; Odum et al. 1977; Mitsch et al. 1979; Dierberg and Brezonik 1983;
Nichols 1983; Kadlec 1987; Knight et al. 1987; Brodrick et al. 1988; Mitsch 1992; Mitsch et
al. 1995; Cronk 1996; Fennessy and Cronk 1997) and both natural and constructed wet-
lands are used worldwide to treat wastewater from industrial, agricultural, and domestic
sources (Kadlec and Knight 1996; see Chapter 9, Section II, Treatment Wetlands).
3. Habitat
a. Wildlife and Fish Habitat
Because many wetlands are highly productive ecosystems, they support a large number of
fish and wildlife species. Some animals, such as many fish, reptiles, and amphibians,
depend exclusively on wetland habitats. Others utilize wetlands for only short periods of
their life cycles (breeding, resting grounds) and some use wetlands as a source of food and
water. Wetlands provide a habitat for many endangered and threatened animal species
such as whooping cranes (Grus americana; U.S. Fish and Wildlife Service 1980), wood
storks (Mycteria americana), crocodiles (Crocodylus acutus), snail kites (Rostrhamus socia-
bilis; U.S. National Park Service 1997), and Florida panthers (Puma concolor coryi; Maehr
1997). Hunters use wetland areas extensively for both waterfowl and deer, and their activ-
ities provide an economic value to the wildlife function of wetlands. Many animals such
as muskrats, beavers, mink, and alligator are harvested for the fur and leather industries,
worth millions of dollars annually. Both commercial and sports fisheries depend on the
fish and shellfish of wetlands.
b. Plant Habitat
Wetland plant communities are among the most highly productive ecosystems in the

world (Mitsch and Gosselink 2000). The production of biomass and the export of organic
carbon to downstream areas make wetlands an integral part of a landscape’s food web.
The high usage of wetlands by wildlife attests to wetland plants’ importance and diversity.
Wetland plant products such as timber from bottomland swamps, peat from bogs, and
many plant food products such as Oryza sativa (rice), Trapa bispinosa (water chestnut), and
various species of Vaccinium (blueberries and cranberries) are harvested throughout the
world. In many areas, farm animals graze wetland plants.
Wetland plant habitat is threatened by changes in wetland hydrology, eutrophication,
the invasion of exotic plants, and other human-induced disturbances such as agriculture
and development (Wisheu and Keddy 1994). Although many wetland plants are listed by
the U.S. Fish and Wildlife Service as rare or endangered, wetland management plans rarely
mention the conservation of rare species (Lovett-Doust and Lovett-Doust 1995; see
Chapter 1, Table 1.3).
III. Broad Types of Wetland Plant Communities
One of the challenges wetland ecologists face is classifying wetlands so that plant commu-
nities, soil types, and hydrologic influences can be described, managed, mapped, or quan-
tified. The variety of wetland types is enormous, and all wetland classifications must
impose subjective boundaries on types. The sources and amounts of water vary over a wide
range even within the same type of wetland. In addition, wetlands are found along succes-
sional gradients, further complicating their classification. Nonetheless, classification of
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wetlands is useful in order to describe their characteristics and manage them effectively
(Cowardin et al. 1979).
Several wetland classification schemes have been used, some for specific regions, coun-
tries, or states, and some for certain types of wetlands, such as peatlands (Shaw and
Fredine 1956; Taylor 1959; Bellamy 1968; Stewart and Kantrud 1971; Golet and Larson
1974; Cowardin et al. 1979; Beadle 1981; Zoltai 1983). Internationally, a number of nations
have classified and inventoried their wetlands, including Canada, Greece, Indonesia, and
South Africa. Some of these countries have used the Ramsar definition as a starting point

and adapted it to local conditions. For example, Canada’s classification system has five
wetland classes and 70 wetland forms, half of which are types of northern peatlands.
Indonesia has classified wetlands into six mangrove forest types and eight freshwater
forested wetland types (Scott and Jones 1995).
In the U.S., the first well-known official wetland classification was published by the
U.S. Fish and Wildlife Service in 1956 (Shaw and Fredine 1956). In this publication, known
as Circular 39, wetlands were categorized into four broad types: inland fresh areas, inland
saline areas, coastal fresh areas, and coastal saline areas. Each of these was further divided
for a total of 20 wetland types. This classification scheme was influential in the beginning
of federal wetland protection. Other classifications were statewide and were based on
regional wetland characteristics.
In order to better define and inventory the wetlands of the U.S., the U.S. Fish and
Wildlife Service developed a classification of wetlands and deepwater habitats based on
the geologic and hydrologic origins of wetlands (Cowardin et al. 1979). This classification
is beneficial because it eliminates the reliance on regional terms that may be meaningless
in other parts of the country. In the Cowardin classification scheme, the major systems of
wetland and deepwater habitat types are marine, estuarine, lacustrine, palustrine, and river-
ine. Systems are wetlands that share similar hydrologic, geomorphologic, chemical, or bio-
logical factors. The Cowardin system includes deepwater habitats (e.g., coral reefs), and
those where plants do not grow, such as coastal sand flats or rocky shores.
A more recently developed classification scheme, called the hydrogeomorphic (HGM)
setting of a wetland, is based on three parameters: the wetland’s geomorphic setting
within the landscape (i.e., riverine, depressional, lacustrine fringe), its water source, and
the internal movement of water within the wetland, known as its hydrodynamics. As a
classification system, the HGM approach emphasizes the topographic setting and the
hydrology of the wetland that in turn affect its functions (Brinson 1993a). In this scheme,
the presence of vegetation is seen as a result of the long-term interaction of climate and
landscape position that also control wetland hydrology.
Alternatively, an approach based on the hydrogeologic setting (HGS) refers to the fac-
tors, both regional and local, that drive wetland hydrology and chemistry. It places an

emphasis on the surface and subsurface features of the landscape that cause water flow
into wetlands, thus determining the quantity and quality of water that a wetland receives
(Bedford 1999). Winter (1992) defined the HGS in terms of surface relief and slope, soil
thickness and permeability, and the stratigraphy, composition, and hydraulic conductivity
of the underlying geologic materials. He used these parameters to classify sites into one of
24 “type settings” based on unique combinations of physiography and climate. This
framework has a landscape basis and has been proposed for use in classifying wetlands for
research into their diversity and ecological functions.
For the purposes of this book, we describe broad types of systems where wetland
plants grow. We have categorized wetlands into three major wetland plant communities:
(1) marshes, where herbaceous species dominate; (2) forested wetlands, where trees or
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shrubs dominate; and (3) peatlands, where the decomposition of plant matter is slow
enough to allow peat to accumulate. Within these three categories, we further divide our
description of plant communities based on hydrology, salinity, and pH.
A. Marshes
Marshes are dominated by herbaceous species which can include emergent, floating-
leaved, floating, and submerged species. The term marsh covers a broad range of habitat
types, and marshes can be found around the world in both inland and coastal areas.
Further classification is based on hydrology and specific herbaceous type. Many names for
marshes exist due to the numerous possible local plant associations in marshes. For exam-
ple, in the state of Florida, a marsh can be classified as a water lily marsh, a cattail marsh,
a flag marsh, or a sawgrass marsh (after the dominant plant), or a submersed marsh or wet
prairie (after the community type; Kushlan 1990). Coastal marshes and inland marshes are
discussed in more detail below.
1. Coastal Marshes
a. Salt Marshes
Salt marshes occur in coastal areas and are usually protected from direct wave action by
barrier islands, or because they are located within bays or estuaries, or along tidal rivers

(Figure 2.1). However, some are in direct contact with ocean waves on low-energy coast-
lines such as the Gulf of Mexico coast in west Florida and parts of Louisiana, the north
Norfolk coast of Britain, and the coast of the Netherlands (Pomeroy and Wiegert 1981).
Most salt marshes are found north and south of the tropics. In the tropics, mangroves are
able to outcompete marsh plants (Kangas and Lugo 1990), although salt marshes do per-
sist inland from mangroves in tropical (northern) Australia (Finlayson and Von Oertzen
1993) and alongside mangroves in some coastal areas of Mexico (Olmsted 1993). Salt
marshes occur as far north as the subarctic and are particularly extensive around the
Hudson and James Bays of Canada (approximately 300,000 km
2
; Glooschenko et al. 1993).
FIGURE 2.1
Salt marsh in Cape Cod, Massachusetts with Spartina patens (salt marsh hay)
in the foreground and S. alterniflora (cordgrass) near the tidal creek. (Photo by
H. Crowell.)
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The plant communities of salt marshes are subjected to daily and seasonal water level
fluctuations due to tides, and to variations in freshwater inputs from overland runoff. In
addition, plants are adapted to low soil oxygen levels that can lead to high levels of sulfide
(Valiela and Teal 1974). Some salt marsh plants are able to withstand salt concentrations in
the soil pore water that are sometimes higher than that of seawater (i.e., 35 ppt) due to the
deposition of salt and evaporation (Wijte and Gallagher 1996a).
In North America, some of the major remaining areas of salt marshes are on the Atlantic
coast and along the Gulf of Mexico. Along the northern Atlantic shore, the coasts of Labrador,
Newfoundland, and Nova Scotia harbor salt marshes in river deltas and where the wave
energy is low (0 to 2 m amplitude; Roberts and Robertson 1986). South of this region, salt
marshes have been divided into three major types (Chapman 1974; Mitsch et al. 1994):
1. The Bay of Fundy marshes in Canada: These marshes are influenced hydrologi-
cally by rivers and a high tidal range (up to 11 m; Gordon and Cranford 1994)

that erodes the surrounding rocks. The substrate is predominantly red silt.
2. New England marshes (from Maine to New Jersey): These marshes were formed
on marine sediments and marsh peat without as much upland erosion as in the
Bay of Fundy marshes.
3. Coastal Plain marshes: These marshes extend from New Jersey south along the
Atlantic and along the Gulf of Mexico coast to Texas. The tidal range is smaller
and the inflow of silt from the coastal plain is high. Included among these are the
Mississippi River delta wetlands, which are the largest salt marshes in the U.S.
All three of these salt marsh types are dominated by Spartina alterniflora (Figure 2.2).
S. alterniflora is a perennial grass that usually occurs along the seaward edge of salt
marshes (Metcalfe et al. 1986) and can grow in water salinities as high as 60 ppt (Wijte and
FIGURE 2.2
Spartina alterniflora (cordgrass), the dominant plant of
many U.S. east coast and Gulf of Mexico salt marshes.
(Photo by H. Crowell.)
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Gallagher 1996a). Two forms of S. alterniflora often coexist within the same marsh: the tall
and short forms. The tall form (1 to 3 m) grows along the banks of tidal creeks, in the low-
est part of the marsh, closest to the sea. The short form (10 to 80 cm) grows inland from
there (Valiela et al. 1978; Anderson and Treshow 1980; Niering and Warren 1980). More
stressful conditions in the inland area of the low marsh, such as nitrogen limitation (Valiela
and Teal 1974; Gallagher 1975), high salinity (Anderson and Treshow 1980), and low soil
oxygen levels (Howes et al. 1981), may cause the height difference (see Chapter 4, Case
Study 4.A, Factors Controlling the Growth Form of Spartina alterniflora).
Salt marshes provide a striking example of plant species zonation in response to envi-
ronmental variation, with different species occurring at different marsh elevations. Each
species’ habitat can be explained by its tolerance to salinity levels, tidal regime, soil oxy-
gen availability, sulfur levels, or other factors (Partridge and Wilson 1987). In many east-
ern U.S. and Gulf coast salt marshes, a zone of Spartina patens (salt marsh hay) is located

inland from the zone of both forms of S. alterniflora (Bertness and Ellison 1987; Gordon and
Cranford 1994). S. patens may dominate in the better drained and less saline areas of salt
marshes because it outcompetes S. alterniflora in those sites (Bertness and Ellison 1987;
Bertness 1991a, b). Although east coast salt marshes of the U.S. appear to be monospecific
within each of these zones, other salt marsh species are present in smaller numbers, such
as Juncus gerardii (rush), Distichlis spicata (spike grass), and Salicornia europaea (glasswort;
Bertness and Ellison 1987).
On the Pacific coast of the U.S. and Canada, salt marshes are less extensive than in the
east, mostly because the geophysical conditions are not suitable for salt marsh formation.
Crustal rise has resulted in shoreline emergence and a coastline with cliffs and few wide
flat river deltas and estuaries. The majority of Pacific coast salt marshes that did exist have
been filled for development (over 90% in some areas; Dahl and Johnson 1991; Chambers et
al. 1994). Salt marshes still exist in estuaries or protected bays like Tijuana Estuary near San
Diego (Zedler 1977), in northern San Francisco Bay (Mahall and Park 1976), Tomales Bay
north of San Francisco (Chambers et al. 1994), Nehalem Bay in northern Oregon (Eilers
1979), Puget Sound in Washington (Burg et al. 1980), at the head of fjords and on the Queen
Charlotte Islands in British Columbia (Glooschenko et al. 1993), and in Cook Inlet near
Anchorage, Alaska (Vince and Snow 1984).
The plant communities of western salt marshes tend to be more diverse than Atlantic
coast and Gulf of Mexico marshes. Like Atlantic salt marshes, many west coast salt
marshes are dominated by grasses. For example, Spartina foliosa dominates some southern
California marshes (Zedler 1977) as well as marshes near San Francisco (Mahall and Park
1976). Other northern California marshes are dominated by Distichlis spicata (Chambers et
al. 1994), while Salicornia virginica (glasswort) is a dominant species in marshes of both
northern and southern California (Callaway et al. 1990; Zedler 1993; Chambers et al. 1994).
In Oregon, Washington, and British Columbia, the sedge, Carex lyngbyei, dominates salt
marshes (Eilers 1979; Burg et al. 1980; Glooschenko et al. 1993). Alaskan salt marshes are
dominated by the grass, Puccinellia phryganodes, and by various species of Carex (Jefferies
1977; Vince and Snow 1984). Diversity tends to be highest in better drained and less saline
locations (MacDonald and Barbour 1974; Vince and Snow 1984; Chambers et al. 1994).

In western and northern Europe, salt marshes are found along the Atlantic coasts of
Spain, Portugal, France, and Ireland, and along the North Sea and the Baltic Sea. In south-
ern Europe, salt marshes are located within the watershed of the Mediterranean Sea and
in the Rhone River delta (the Camargue; Chapman 1974). Mediterranean salt marshes also
fringe northern Africa along the Tunisian, Moroccan, and Algerian coasts (Britton and
Crivelli 1993). The seaward portions of European salt marshes are often tidal mudflats,
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with sparse vegetation. The equivalent area in eastern U.S. salt marshes is heavily vege-
tated and dominated by Spartina alterniflora. The difference is due to higher tidal fluctua-
tions in many European salt marshes (up to 15 m). While eastern North American salt
marshes are flooded twice daily, many European marshes are only partially flooded, with
their highest areas flooded only during spring tides. The lowest areas of the marsh tend to
be dominated by Spartina maritima in Portugal, Salicornia europaea and Spartina anglica in
France and the United Kingdom, and Salicornia dolichostachya in the Netherlands
(Lefeuvre and Dame 1994).
b. Tidal Freshwater Marshes
Tidal freshwater wetlands are influenced by the daily flux of tides, yet they have a salinity
of less than 0.5 ppt. They are usually located in upstream reaches of rivers that drain into
estuaries or oceans. Their position within the landscape places them at the interface
between the upstream sources of fresh water and the downstream sources of tides. They
occur worldwide, wherever these conditions are met. Tidal freshwater wetlands cover an
estimated 632,000 ha in the U.S., with the majority along the Gulf of Mexico (468,000 ha),
primarily in Louisiana (Mitsch and Gosselink 2000). Along the Atlantic coast, there are
about 164,000 ha with over half (89,000 ha) in New Jersey, and most of the remaining in the
Chesapeake Bay watershed (Odum et al. 1984).
The organisms that inhabit tidal freshwater wetlands originate in upstream freshwater
or in downstream brackish areas. Because of the heterogeneity of habitat conditions, tidal
freshwater wetlands harbor diverse communities of plants and animals. Since salinity and
sulfur stresses are not as profound, macrophyte diversity is higher in tidal freshwater sys-

tems than in salt marshes. Tidal freshwater communities tend to have several plant forms
such as shrubs, floating plants, grasses, and forbs, rather than the monotypic stands of
grasses typical of salt marshes (Whigham et al. 1978; Simpson et al. 1983a; Odum et al.
1984). Many of the plants of tidal freshwater marshes are also found in inland marshes.
Tidal freshwater marshes show distinct vegetation patterns according to moisture lev-
els (Odum et al. 1984; Leck and Simpson 1994). For example, along the Delaware River in
New Jersey, Acnida cannabina (salt marsh water hemp) and Ambrosia trifida (great rag-
weed) grow along banks and levees. Polygonum punctatum (water smartweed) and Bidens
laevis (larger bur marigold) are common along stream channels. B. laevis also grows on the
high marsh with Impatiens capensis (spotted touch-me-not), Peltandra virginica (arrow
arum), Phalaris arundinacea (reed canary grass), Sium suave (water parsnip), and the para-
sitic vine, Cuscuta gronovii (common dodder). Nuphar advena (spatterdock; Figure 2.3) and
Acorus calamus (sweetflag) grow in the tidal channel and adjacent banks. Pilea pumila
(clearweed) grows in elevated sites, Sagittaria latifolia (arrowhead; Figure 2.4) is scattered
in all areas except the stream channel, and the vine, Polygonum arifolium (halberd-leaved
tearthumb), occurs along the entire moisture gradient (Leck and Simpson 1994). In other
Chesapeake Bay area tidal freshwater marshes, tall emergents such as Zizania aquatica
(wild rice) and various species of Typha (cattail) also grow, often in dense stands (Odum et
al. 1984).
2. Inland Marshes
Inland freshwater marshes are a diverse group of wetlands that, in the U.S., range in size
from quite small (<1 ha) to the size of the Everglades (currently 607,000 ha). They are found
worldwide wherever hydrologic and geologic conditions allow for their formation. Many
different kinds of freshwater marshes have been defined, and they are often named for the
dominant vegetation. In this book we divide marshes into three broad categories based on
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their landscape position: lacustrine marshes, riverine marshes, and depressional marshes.
Inland marshes that accumulate peat, commonly called fens and bogs, are discussed in
Section III.C, Peatlands.

Several thousand plant species are adapted to freshwater marshes (Cook 1996; Reed
1997) and whether the marshes are lacustrine, riverine, or depressional, the plants are often
the same. The plant communities of freshwater marshes tend to be diverse and highly pro-
ductive (Herdendorf 1987; Keeley 1988; Kantrud et al. 1989; Galatowitsch and van der Valk
FIGURE 2.4
Sagittaria latifolia (arrowhead) of the Alismataceae (water plantain family) devel-
ops floating leaves when it is immature or when the water level rises (as seen
here). It is often seen with fully emergent leaves. S. latifolia grows in both tidal
and depressional freshwater wetlands. (Photo by T. Rice.)
FIGURE 2.3
Nuphar advena (spatterdock) of the Nymphaeaceae (water lily family) has thick,
spongy roots and produces both emergent and floating leaves. Its yellow blos-
soms float or are held above the water’s surface on rigid stalks. N. advena grows
in both tidal and inland freshwater marshes in the eastern and midwestern U.S.
(Photo by H. Crowell.)
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1994). The structure of plant communities varies with climate, substrate type, flooding
regime, water depth, and nutrient availability. Some of the most common emergent plants of
freshwater marshes are monocots, many in three major families: Poaceae (grass), Cyperaceae
(sedge), and Juncaceae (rush). Other common emergent families are Typhaceae (cattail),
Sparganiaceae (bur reed), Alismataceae (water plantain), Butomaceae (flowering rush),
Araceae (arum), Pontederiaceae (pickerelweed), Iridaceae (iris), Polygonaceae (smartweed),
Lythraceae (loosestrife), Apiaceae (=Umbelliferae; parsley), and Lamiaceae (=Labiatae;
mint). Commonly encountered submerged species are in the Najadaceae (naiads),
Potamogetonaceae (pondweed), Zannichelliaceae (horned pondweed), Hydrocharitaceae
(frogbit), Ceratophyllaceae (hornwort), Ranunculaceae (buttercup), and Haloragaceae
(water milfoil). Floating species include those in the Lemnaceae (duckweed). Common float-
ing-leaved plants are in the Nymphaeaceae family (water lily; Figure 2.5). Often, a zone of
shrubs surrounds depressional wetlands. Frequently found shrub genera include Salix (wil-

low), Spiraea (meadow sweet), Rosa (rose), Cephalanthus (buttonbush), Alnus (alder), and
Cornus (dogwood).
a. Lacustrine Marshes
As defined by Cowardin and others (1979) the lacustrine system is divided into the lim-
netic, or deep water habitat, and the littoral, or edge habitat. Lacustrine wetlands include
littoral aquatic beds, dominated by submerged and floating-leaved species, and emergent
marshes slightly upland (Figure 2.6). A lake’s shape dictates whether lacustrine wetlands
can exist along its fringes. A steep-sided V- or U-shaped lake, generally formed by tectonic
forces, has less water in contact with sediments, a more abrupt drop from edge to deep
water, and generally supports little, if any, littoral vegetation. Lacustrine wetlands are
more likely to occur along shallow lake basins, often formed by glaciation (Wetzel 1983a).
The size and depth of littoral wetlands shift with changes in water level due to precipita-
tion or changes in drainage or runoff.
FIGURE 2.5
Nymphaea odorata (fragrant water lily) of the Nymphaeaceae (water lily family)
is a floating-leaved plant that grows in freshwater marshes and lake edges
throughout the eastern U.S. and southeastern Canada. (Photo by H. Crowell.)
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Lacustrine wetlands are located around the world, along the edges of both small and
large lakes. Most of the world’s lakes are small with a high ratio of lacustrine marsh area
to open water (Wetzel and Hough 1973). Along large lakes with tides, such as the
Laurentian Great Lakes of the U.S. and Canada, wetlands occur in coastal lagoons behind
barrier beaches, in tributary mouths, and as managed marshes protected by dikes
(Herdendorf 1987; Glooschenko et al. 1993). Lacustrine wetlands along the Great Lakes
tend to fall into one of three categories, according to depth: wet meadow, marsh, or aquatic.
The plant communities of these three categories are dominated, respectively, by sedges
and grasses, emergents such as Typha, and submerged species (Glooschenko et al. 1993).
b. Riverine Marshes
Riverine marshes form along rivers and streams, in the lowlands behind river levees, or in

old oxbows of rivers (Figure 2.7). While many riverine wetlands are forested, herbaceous
wetlands can be found at the edges of forests, or in newly opened areas such as beaver-
formed wetlands (van der Valk and Bliss 1971; Johnston and Naiman 1990; Johnston 1994).
The most extensive riverine marshes in the U.S. are found in the Mississippi River flood-
plain. Marshes that fringe streams or are flooded by river water are subjected to flowing
water which carries a higher mass input of sediments and nutrients and allows increased
export of waste products. Such wetlands often have higher plant productivity than still-
water wetlands (Brinson et al. 1981; Lugo et al. 1988).
c. Depressional Marshes
Depressional wetlands are lowlands, or basins, that are either hydrologically connected to
other wetlands or bodies of water, or hydrologically isolated. Depressions include former lake
basins, shallow peat-filled valleys between existing lakes, and glacially formed basins
(Kushlan 1990; Galatowitsch and van der Valk 1994). Depressional wetlands are found world-
wide, at all latitudes, and may be forested wetlands, marshes, or peatlands (forested depres-
sional wetlands and peatlands are discussed below). Examples of depressional marshes
within the U.S. and Canada include prairie potholes, playas, and vernal pools.
FIGURE 2.6
Lacustrine marsh with limnetic zone dominated by phytoplankton, a littoral aquatic
bed dominated by submerged and floating-leaved plants, and a littoral emergent
marsh.
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One extensive region of depressional wetlands is found in the prairie pothole region of
Iowa, Minnesota, North and South Dakota in the U.S., and Alberta, Saskatchewan, and
Manitoba in Canada (see Chapter 9, Case Study 9.C, Figure 9.C.1). This area encompasses
over 70 million ha of land (Kantrud et al. 1989) with millions of prairie potholes scattered
throughout (Shay and Shay 1986). Depressions within the prairie landscape were formed dur-
ing the last glaciation. As the glaciers retreated, basins were left behind where caves and tun-
nels in the ice sheet had been. The basins range in size from several meters in diameter to lakes
of several hundred hectares (Glooschenko et al. 1993). In southern Minnesota and northern

Iowa the basins are shallow depressions linked by drainage ways. In other parts of the prairie
pothole region, there are fewer surface links between the potholes (Galatowitsch and van der
Valk 1994).
Five different pothole habitat types have been identified as a function of water depth:
wet prairie, sedge meadow, shallow marsh, deep marsh, and permanent open water. Some pot-
holes are only shallow depressions and may support only wet prairie and sedge meadow,
while others are permanent ponds and lakes. Because the physical aspects of the habitat are
so diverse, potholes support a wide array of wetland vegetation. In the southern pothole
region of Iowa and southern Minnesota, nearly 350 plant species can be found and up to
one third of those may be found within a single basin (Galatowitsch and van der Valk 1994).
Playas are ephemeral freshwater ponds that vary in area from a few square meters to
hundreds of hectares and in depth from a few centimeters to a few meters (MacKay et al.
1992). Over 25,000 playas are scattered throughout an 8.2 million-ha region on the high
plains of New Mexico and northern Texas. Some playas may have been formed by prairie
wind erosion 10,000 to 15,000 years ago (Bolen et al. 1989). This theory is supported by the
presence of large lee-side dunes adjacent to some playa basins. The dunes’ volume approx-
imates the volume of material removed from the playas. Not all playas have dunes, and
some may have been formed where geologic joints provided paths of weakness for surface
drainage and water accumulation (Zartman and Fish 1992).
Playas support a high diversity of wetland plants. Many are the same wetland emer-
gents found in prairie potholes, such as species of Typha, Scirpus (bulrush), and Polygonum
(smartweed). Often, Potamogeton species (pondweed) dominate the open water areas
while Echinochloa crus-galli (barnyard grass) and Leptochloa filiformis (red sprangletop)
FIGURE 2.7
Riverine marsh along the upper Mississippi River, Wisconsin. (Photo by
H. Crowell.)
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grow in drier zones, called wet meadows. During dry years the vegetation is more char-
acteristic of the surrounding grasslands. Many playas are cultivated or modified for other

agricultural purposes, such as catchments for feedyard runoff, disposal areas, irrigation
pits, and sewage treatment facilities (Bolen et al. 1989; Pezzolesi et al. 1998).
California’s vernal pools are ephemeral wetlands, flooded during winter and spring
and dry throughout the summer. The Mediterranean-type climate is unpredictable and in
some years the pools do not fill. Vernal pools are what remains of the inland sea that once
covered the Central Valley of California. As California’s coastal mountains began to rise
about 10,000 years ago, the inland sea receded and became several freshwater lakes. These,
in turn, were fragmented into vernal pools that are generally less than one half hectare in
size. Only 3 to 7% of this habitat remains, so many of the endemic plant species of vernal
pools are threatened or endangered (see Chapter 1, Table 1.3). Some genera such as
Pogogyne and Orcuttia are found almost exclusively in vernal pools (Baskin 1994).
B. Forested Wetlands
Forested wetlands are dominated by woody vegetation of all sizes. Forested wetlands can
include trees over 50 m tall, dwarf trees 1 m in height in areas of environmental stress, or
shrubs (Lugo 1990). They are found from boreal regions to the tropics, along coastlines,
and in alpine regions, and they cover an estimated 250 to 330 million ha worldwide (Lugo
et al. 1988; Lugo 1990). The U.S. (including Alaska) has more than 26 million ha of forested
wetlands (Dahl and Johnson 1991; Hall et al. 1994). Like marshes, forested wetlands can be
further categorized by hydrology and dominant plant species. We have divided this sec-
tion into two major types: coastal and inland forested wetlands.
1. Coastal Forested Wetlands: Mangrove Swamps
Mangrove swamps are the only forested wetlands found along coastlines. The term man-
grove refers to both the forest type and the trees that grow there. Mangrove is the common
name of some 50 to 79 plant species, the most common of which are in the genera
Avicennia, Bruguiera, Ceriops, Laguncularia, Lumnitzera, Kandelia, Rhizophora, and
Sonneratia. Different numbers of mangrove species are sometimes reported because the
term mangrove is variably interpreted. All mangrove species share an ability to survive in
shallow and fluctuating salt water. Their primary physiological adaptations to the stress of
salinity include the ability to exclude or excrete salt from their tissues and sprout aerial
roots that aid in gas exchange (Tomlinson 1986). The highest diversity of mangrove species

is found in Asia along Indian and Pacific Ocean coastlines. In the western hemisphere,
there are eight species, and only three of these are common: Rhizophora mangle (red man-
grove; Figure 2.8), Avicennia germinans (black mangrove; Figure 2.9), and Laguncularia
racemosa (white mangrove; Lugo and Snedaker 1974; Tomlinson 1986; Rutzler and Feller
1996). Other than mangrove trees, few vascular species grow in western hemisphere man-
grove swamps. The lack of understory may be due to the high number of natural stresses
in the mangrove environment: salinity, high sulfide concentrations in the soil, root zone
anoxia, low light due to shading, and crab predation of seeds and propagules (Janzen 1985;
Corlett 1986; Lugo 1986; Snedaker and Lahmann 1988; Alongi 1998).
Mangroves are restricted to areas where the mean water temperature is above 23 to
24˚C (Tomlinson 1986; Rutzler and Feller, 1996; Figure 2.10) and they cover 17 to 24 million
ha worldwide (Field 1995; Ramsar Convention Bureau 2000). Mangroves occupy the same
niche in the tropics that salt marshes occupy to the north and south. Outside of the trop-
ics, frost limits their establishment and causes periodic diebacks (Kangas and Lugo 1990).
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Mangrove forests are among the most threatened of wetland types as they are harvested
for wood products, used as waste and garbage dumps, and cleared for tourism and other
types of development. In many countries few, if any, protective measures for mangroves
are in place (Farnsworth and Ellison 1997; Li and Lee 1997; Walters 1997).
In many mangrove wetlands, hurricanes play a major role in the size and structure of
the community (Pool et al. 1977). The mangrove forests of Puerto Rico, the Bahamas, Cuba,
FIGURE 2.8
Rhizophora mangle (red mangrove) growing at water’s
edge with prop roots extending into the sediments at the
Everglades National Park, Florida. (Photo by
H. Crowell.)
FIGURE 2.9
Avicennia germinans (black mangrove) with aerial roots called pneumatophores
growing above the water’s surface. Pneumatophores aid in root gas exchange

(Everglades National Park, Florida). (Photo by H. Crowell.)
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and South Florida are frequently hit by destructive hurricanes. In other regions such as
parts of Central America and northern South America, mangroves tend to be much larger
trees, probably due to the absence of hurricanes (Odum and McIvor 1990; Lugo 1997). The
most extensive mangrove swamps with the largest trees occur where there is significant
tidal amplitude, high rainfall, heavy terrestrial runoff, and no (or rare) hurricanes, such as
on the west coast of Panama (Odum and McIvor 1990), the east coast of Costa Rica (Pool
et al. 1977), the River Niger of Western Africa (Denny 1993), the coastline of French Guiana
(Fromard et al. 1998), or the western Yucatan Peninsula (Figure 2.11). In the absence of eco-
logical constraints such as high salinity or low annual rainfall, mangrove forests near the
equator tend to develop significantly greater biomass than in the northern or southern
extent of their range (Fromard et al. 1998).
The mangrove swamps of the U.S. are in Florida, Texas, Hawaii, the U.S. Virgin Islands,
and Puerto Rico (Lugo 1997). In Florida and Central America, the three common mangrove
species occupy distinct zones (Figure 2.12). In the area closest to the sea is Rhizophora man-
gle. Just inland from the zone of R. mangle is Avicennia germinans, and immediately to the
interior is Laguncularia racemosa. Conocarpus erectus (buttonwood or grey mangrove) is
sometimes found between the mangroves and upland ecosystems, or among the L. race-
mosa trees. Each species occupies a different zone because of different adaptations. R. man-
gle grows a web of stilt-like prop roots that support the tree in standing water and protect
FIGURE 2.10
(a) The worldwide distribution of mangroves with the approximate northern and southern lim-
its for all species. The eastern and western groups do not overlap except for a possible extension
(arrow) in the western Pacific (Rhizophora samoensis). (b) Histogram of the approximate species
number per 15º of longitude, showing the species richness of the eastern group. (From
Tomlinson, P.B. 1986. The Botany of Mangroves. London. Cambridge University Press. Map
reprinted and histogram redrawn with permission.)
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FIGURE 2.11
Rhizophora mangle (red mangrove) growing on the west side
of the Yucatan Peninsula, reaching greater than average
height due in part to a lack of hurricanes (Celustun,
Mexico). (Photo by H. Crowell.)
FIGURE 2.12
The zonation pattern of the common mangrove species of South Florida. Rhizophora mangle (red mangrove)
grows closest to the sea. Avicennia germinans (black mangrove) grows inland from R. mangle, and Laguncularia
racemosa (white mangrove) as well as Conocarpus erectus (buttonwood or grey mangrove; sometimes not
included in the zonation pattern) inhabit the area inland from the zone of A. germinans. (Drawing by
B. Zalokar.)
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it from tides. A. germinans sends up hundreds of pencil-shaped roots called pneu-
matophores that aid in gas exchange (Tomlinson 1986). L. racemosa outcompetes the first
two in less saline and better drained locations. On the western side of the continent, in Baja
California, Mexico, dwarf mangrove forests are found in highly saline environments, but
with the same species and zonation pattern as in Florida (Toledo et al. 1995).
Zonation may vary in some mangrove swamps due to local differences in substrate
type, salinity, wave energy, and the effects of rising sea level (Odum and McIvor 1990). For
example, A. germinans has the greatest tolerance for salt of the three dominant western
hemisphere mangrove species. It tends to dominate in mangrove swamps where salt accu-
mulates due to infrequent tidal flushing and little freshwater input (Tomlinson 1986;
Brown 1990; Rutzler and Feller 1996). In the eastern hemisphere, more mangrove species
co-exist, and therefore a greater variety of associations are formed. For example, in
Australia there are 11 common mangrove species that are found in 30 combinations
(Finlayson and Von Oertzen 1993).
2. Inland Forested Wetlands
Inland forested wetlands are referred to as either basin wetlands or riverine wetlands,

according to their location in the landscape and their sources of water (Lugo et al. 1988).
Riverine forests are heavily influenced by flooding and sediment transport in the adjacent
river or stream. Due to these energy influxes, freshwater riverine forests tend to have
higher productivity and greater species richness than basin forested wetlands. Forested
wetlands along a single river can change in structure and productivity as the river changes
from headwaters to mouth. Riverine wetlands are highly dynamic ecosystems, since a sin-
gle large flood can change the floodplain topography sufficiently to alter or eliminate the
forest habitat (Brinson 1990).
The terms used here, floodplain, bottomland, riparian, and riverine, all refer to land along
a river or stream. Bottomland and floodplain both refer to the low areas adjacent to a river or
stream. The term bottomland is most often used to refer to forested wetlands in the southern
U.S. Riparian areas include the lowlands as well as the levees adjacent to rivers and this
term is used for any such area worldwide. The literature dealing with western rivers usu-
ally uses the term riparian. The term riverine can refer to the U.S. FWS classification system
of wetlands (Cowardin et al. 1979), which restricts riverine wetlands to those found in the
stream channel and the immediate bank or levee. Here, riverine should be interpreted in the
sense given by Lugo and others (1988) to describe wetlands that are influenced by rivers or
streams (i.e., floodplain and riparian areas as well as wetlands within the stream channel).
With an emphasis on North America, we discuss three major riverine forested wetland
types here: bottomland hardwoods of the southern U.S., northeastern floodplains, and
western riparian zones. Cypress wetlands occur in both riverine and basin habitats, and
both are described below. Some inland basin forested wetlands are peatlands, which are
discussed in the following section.
a. Southern Bottomland Hardwood
Many wide floodplains of the southern U.S. have a variable topography characterized by
depressions and ridges behind the levees that border the stream channel. The topography
is determined by past meanderings of the current stream across the floodplain. The first
area behind the levee, sometimes called the backswamp or backwater, is prone to frequent
and longer periods of flooding than those behind subsequent ridges. Low elevation wet
sites lie upland from the backswamp and may only be flooded during the winter or spring.

Behind these low sites, slightly higher elevation forests tend to be only periodically
flooded and therefore better drained (Hodges 1997).
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Southern bottomland hardwood forests are found in both major and minor watersheds
from the Atlantic coast to eastern Texas and Oklahoma and as far north as the Ohio and
Wabash Rivers. Some of the largest floodplains in which bottomland forests grow are
along the lower Mississippi and its tributaries, the Arkansas, Red, Ouachita, Yazoo, and St.
Francis Rivers. Other rivers that flow toward the Gulf of Mexico as well as those that drain
the southern and middle Atlantic coast also support bottomland hardwood forested wet-
lands (Brinson 1990).
The structure of bottomland forest communities is determined, to a great extent, by the
hydroperiod. The first depression behind the levee may support Taxodium distichum (bald
cypress) and Nyssa aquatica (water tupelo) forests, both adapted to prolonged flooding
and frequently found in areas flooded 10 to 12 months per year (Conner and Day 1976;
Visser and Sasser 1995; Kellison and Young 1997). Depressions farther inland and riverine
forests with less frequent and shorter inundation are dominated by Quercus (oak), Acer
(maple), and Salix (willow) species, as well as Platanus occidentalis (sycamore),
Liquidambar styraciflua (sweetgum), Nyssa sylvatica (black tupelo or swamp black gum),
and Liriodendron tulipifera (yellow poplar; Conner and Day 1976; Brinson 1990; Kellison
and Young 1997; Hodges 1997). A number of shrubs coexist with the trees in bottomland
hardwood swamps, such as Cornus (dogwood) and Ilex (holly) species as well as Sambucus
canadensis (elderberry) and Cephalanthus occidentalis (buttonbush; Conner and Day 1976;
Conner et al. 1981). Species composition and tree size reflect the fact that bottomland
forests have been and continue to be extensively harvested (Brinson 1990; Kellison and
Young 1997).
b. Northeastern Floodplain
The northeastern floodplain forests lie north of the range of Taxodium distichum (bald
cypress) in Maryland and Virginia and south of the peatlands of northern Michigan,
Wisconsin, upper New England, and Canada. They extend east from the upper Mississippi

River valley to the Atlantic (Figure 2.13). Some of the largest watersheds are the Ohio,
Susquehanna, Shenandoah, Delaware, and upper Mississippi Rivers (Brinson 1990). With
the exception of streams in the Appalachian mountains, northeastern floodplains tend to
FIGURE 2.13
A northeastern floodplain forest in a backwater of the Kalamazoo River in
western Michigan. (Photo by H. Crowell.)
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be wide, with several ridges and low lying areas, and they are subject to periodic flooding
and sediment deposition. The natural surroundings of northeastern floodplain forests are
upland deciduous forests and they share many of the same species. Like riparian areas
elsewhere, the northeastern forests have been heavily impacted by development, forestry,
channelization of streams, and draining for agriculture. Almost all of the states of this
region have lost over half of their original wetland area, with forested wetlands the most
heavily impacted (Dahl and Johnson 1991).
Maples, particularly Acer rubrum (red maple), along with Ulmus americana (American
elm), Fagus grandifolia (American beech), and Platanus occidentalis, are a few of the trees
that are frequently found in eastern floodplain forests (Brinson 1990). In the western por-
tion of this region, along the Wabash River in Indiana, the genera found in the lowest ele-
vations are Populus (cottonwood) and Salix (willow). On slightly higher ground, Acer
rubrum, Aesculus glabra (Ohio buckeye), Cercis canadensis (Eastern redbud), F. grandifolia,
and U. americana are the dominant species (Lindsey et al. 1961). In Wisconsin, Acer saccha-
rinum (silver maple) and Fraxinus pennsylvanica (green ash) dominate floodplain forests
(Dunn and Stearns 1987). In Ohio, many forested floodplains are dominated by Populus
deltoides (cottonwood), A. saccharinum, A. negundo (box elder), P. occidentalis, and Salix
species. In the unglaciated southeastern portion of Ohio, Betula nigra (river birch), A.
negundo, A. rubrum, and A. saccharinum form the most common floodplain association
(Anderson 1982).
c. Western Riparian Zones
In the arid regions of the western U.S., riverine areas are often the only areas moist enough

to provide suitable habitat for tree and shrub species. Because western riparian zones are
oases of water, food, and cover, they tend to have a high concentration of wildlife, particu-
larly birds (Knopf and Samson 1994). The habitat along a single river may shift from wet to
mesic to xeric, and the plant communities change accordingly. Western riparian wetlands
tend to be sparsely vegetated and have far fewer species than eastern forested wetlands. For
example, forested wetlands near the South Platte River in Colorado have only two tree
species, while a wetland forest at the same latitude in New Jersey has 24 (Brinson 1990).
Western riparian zones are located in the southwestern U.S. and the plains grasslands
of the central states. Major rivers in the southwest that support riparian zones are the San
Joaquin, Sacramento, Salt-Gila, and Rio Grande-Pecos. In the Plains States in central to
western U.S., some of the major watersheds are the Missouri, Platte, upper Arkansas, and
Canadian (Brinson 1990). The riparian zones of these rivers tend to be narrower than in the
east because the streams have steeper gradients, which leads to more severe flooding of
surrounding areas (Mitsch and Gosselink 2000). Less soil development, less deposition of
silt, and less stream meandering have occurred in the west than in the east, so the flood-
plains tend to have few if any ridges and backswamps. Only about 1% of the west is ripar-
ian habitat and most riparian areas in the western U.S. (in southern California over 95%)
have been destroyed by development, agriculture, grazing, logging, or manipulation of
water resources, such as damming (Faber et al. 1989; Brinson 1990; Knopf and Samson
1994). Western riparian areas may be “the most modified land type in the western U.S.”
(Wilen and Tiner 1993).
Typical trees in central U.S. arid riparian zones are species of Populus, Salix, and Fraxinus
(ash), as well as Platanus wrightii (sycamore) and Acer negundo (Keammerer et al. 1975;
Faber et al. 1989). In the southwest, a number of riparian areas are dominated by Prosopis
species (mesquite) and Populus, as well as the invasive Elaeagnus angustifolia (Russian olive)
and Tamarix (saltcedar). The most frequently encountered species of saltcedar, Tamarix
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ramosissima and T. chinensis, have displaced native riparian tree and shrub species through-
out the western U.S. (Figure 2.14; Brinson 1990; Busch and Smith 1995).

d. Cypress Swamps
Forested wetlands dominated by Taxodium species (cypress) are found throughout the
southeastern U.S. There are two types of cypress trees, commonly called bald cypress and
pond cypress. There is some disagreement about whether or not these are separate species,
called Taxodium distichum and Taxodium ascendens, respectively, or two varieties: Taxodium
distichum var. distichum and Taxodium distichum var. nutans (Ewel 1990a). We refer to them
as T. distichum and T. ascendens, after Reed (1997). The two trees are generally found in dif-
ferent habitats: bald cypress is found in flowing water swamps and pond cypress grows in
stillwater swamps. However, bald and pond cypress do grow together in some areas. Bald
cypress grows where water flows through a deeper area, as along a channel, and pond
cypress may grow in the same depression, but in shallower areas, away from the main
water flow (Ewel 1990b).
Cypress wetlands are found in Florida and in the Atlantic coastal zone as far north as
Delaware. They also grow in the Gulf of Mexico coastal zone into Louisiana and Texas and
north as far as southern Illinois (Ewel 1990a). The largest remaining cypress forest is in Big
Cypress National Preserve, north of the Everglades in Florida. About one third of the
620,000-ha preserve is dominated by cypress (U.S. National Park Service 1997). Many
cypress wetlands were harvested by early settlers and harvesting continued through the
early 1900s. Cypress wood was considered desirable for building, boats, bridges, and
docks because of its resistance to decay. It can be a very long-lived species and reports of
harvests at the beginning of the 1900s indicated that many trees were 400 to 600 years old,
with some as old as 915 years (Mattoon 1915, as reported by Visser and Sasser 1995). Today
one can occasionally find a 1000-year-old cypress tree in protected areas (Figure 2.15).
Many of the backswamps closest to southeastern rivers are habitats for bald cypress,
often in association with Nyssa aquatica (water tupelo). These riparian cypress stands are
considered within the category of southern bottomland hardwoods (Brinson 1990; Ewel
1990a; Visser and Sasser 1995). Monospecific stands of bald cypress can be found in some
FIGURE 2.14
Tamarix ramosissima (saltcedar), an invasive species of western riparian zones,
growing along the Colorado River in Arizona. (Photo by K. Irick Moffett.)

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southern floodplains and along the edges of lakes (Ewel 1990b), or in wetlands known as
cypress strands, which are linear swamps with slowly flowing water (Ewel and
Wickenheiser 1988). The water level of all cypress swamps fluctuates throughout the year.
In Florida, for example, most cypress swamps are inundated from 5 to 9 months each year.
Drawndown periods are vital to cypress swamps because cypress seeds will not germinate
under standing water. Inundation also reduces competition from plants that grow more
quickly under drier conditions (Mitsch and Ewel 1979; Ewel 1990a).
Pond cypress is often found in swamps with little or no water throughflow; these are
called cypress ponds, domes, or heads. The term dome refers to the shape of these forests seen
from a distance: the largest trees are in the center and the smaller ones are around the edges
of the pond (Brown 1990). Cypress ponds often have standing water for more than
6 months per year and there is generally no streamflow into or out of them. They vary in
size from about 1 to 10 ha (Ewel and Wickenheiser 1988). Pond cypress can also grow in
poorly drained savannas where their growth is stunted. In this environment they are
referred to as dwarf cypress (Ewel 1990a).
C. Peatlands
Peatlands are wetlands in which dead plant matter (peat) accumulates due to slow decom-
position. While many wetlands accumulate organic litter, peatlands can be distinguished
by a deep accumulation (generally >30 cm; Glaser 1987) and by the fact that they are found
in areas with short growing seasons. Peat can be any decaying vegetative matter, but very
often it is dominated by moss, usually one of the Sphagnum species (of which there are at
least 185 worldwide; Figure 2.16; Crum 1992). Peat forms a variety of domed or raised
FIGURE 2.15
Taxodium distichum (bald cypress) estimated to be
1000 years old in the Francis Beidler Forest, South
Carolina. (Photo by J. Cronk.)
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shapes as it accumulates. Peat decomposes slowly because of cold temperatures and low
levels of the oxygen needed by decomposers.
Peatlands have been classified into two main types based upon the source of water.
Fens are fed by groundwater that carries minerals from the surrounding soil, and are some-
times called minerotrophic after their water source. The calcium concentration and the pH
of fens tend to be relatively high. Bogs receive mostly rainwater (ombrotrophic) and tend to
be much poorer in nutrients and minerals and have a lower pH (Bellamy 1968; Moore and
Bellamy 1974; Wassen et al. 1990). Within the categories of bogs and fens, there is a contin-
uum of water and substrate chemistry brought about by the different sources of water.
Peatlands have been categorized according to the pH of the interstitial water (Figure 2.17).
The more acidic (pH < 5) have been called Sphagnum bogs, extreme poor fens, or simply bogs.
At higher pH values where calcium carbonate inputs buffer the acidic water, the vegeta-
tion is often dominated by sedges, and the term fen is used. More detailed categories of
fens according to pH are intermediate fen (pH 5.2 to 6.4), transitional rich fen (pH 5.8 to
FIGURE 2.16
An example of a Sphagnum moss, S. papillosum. This moss usually grows in wet
acidic conditions with some groundwater input (Crum 1992) and was found
in exactly such conditions at Miner Lake in southwestern Michigan. (Photo by
H. Crowell.)
FIGURE 2.17
Three classifications of peatlands according to pH value (names and their pH ranges are from [a] Sjors 1950,
[b] Bellamy 1968, and [c] Crum 1992).
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