WETLAND PLANTS: BIOLOGY AND ECOLOGY - CHAPTER 2 pdf

Wetland Plant Habitats Wetland plants grow in a variety of climates, from the tropics to polar regions — whereverthe water table is high enough, or the standing water is shallow enough,

Wetland Plant Communities

I Wetland Plant Habitats

Wetland plants grow in a variety of climates, from the tropics to polar regions — whereverthe 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 face All rooted wetland plants are adapted to at least periodically saturated substrateswhere soil oxygen levels are low to non-existent

sur-The terms for different types of wetlands help to pinpoint the differences between land communities and can be defined, at least in part, by the type of vegetation that grows

wet-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 plantsespecially adapted to wet conditions Wetland plants, in turn, support high densities offish, invertebrates, amphibians, reptiles, mammals, and birds Wetland conditions such asshallow water, high plant productivity, and anaerobic substrates provide a suitable envi-ronment for important physical, biological, and chemical processes Because of theseprocesses, wetlands play a vital role in global nutrient and element cycles Wetlands alsoprovide 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 Davis1993)

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 tify commonalties among these vastly different ecosystems

iden-A Ecological Definition

The determining factor in the wetland environment is water To a great extent, hydrologydetermines soil chemistry, topography, and vegetation All wetlands have water inputsthat exceed losses, at least seasonally It is difficult to say exactly how much water an areamust 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 wetlandsdoes not exist, mostly because the line between wet and dry environments is not easilydrawn Moisture levels vary along a continuum that shifts in time and space Wetlandsmay have standing water throughout the year, or only during a portion of the year Thoseinfluenced 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 tosustain 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 theU.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, andeach 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 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 Agencydefine wetlands as:

wet- 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 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 BiologicalIndicators) 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 ortides, but water may also be below the soil surface in a wetland Secondary indicators of

con-water level may also be used to establish wetland hydrology, such as con-water marks, driftlines, debris lodged in trees or elsewhere, and layers of sediment that form a crust on thesoil surface Hydric soils develop under low oxygen conditions that bring about diagnos-tic soil colors, textures, or odors Other soil indicators include partially decomposed plantmaterial in the soil profile (as in peatlands) or decomposing plant litter at the surface of thesoil 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 WildlifeService (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 NationalWetlands Inventory)

3 International Definition

In 1971, an international convention on wetlands was held in Ramsar, Iran by theInternational Union for the Conservation of Nature and Natural Resources (IUCN) Aninternational treaty on wetlands, the Convention on Wetlands of International ImportanceEspecially as Waterfowl Habitat, also known as the Ramsar Convention, was signed there

It “provided the framework for international cooperation for the conservation and wiseuse 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 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.

tem-In addition, wetlands “may incorporate riparian and coastal zones adjacent to the lands, and islands or bodies of marine water deeper than six meters at low tide lyingwithin the wetlands.”

wet-The Ramsar Convention definition of wetlands is broader than the U.S Army Corps ofEngineers’ 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 with1,016 sites on the Ramsar List for a total area of over 72.8 million ha (Ramsar ConventionBureau 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

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 presentgenerations while maintaining its potential to meet the needs and aspirations of futuregenerations” (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 thewetland 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 watersupplies, floodwater conveyance and storage, and shoreline and erosion protection

TABLE 2.1

Functions and Values Commonly Attributed to Wetlands

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.

a Groundwater Supply

Groundwater may move into a wetland via springs or seeps (groundwater discharge) andwater 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 thesurrounding landscape When the water table rises due to rainfall, groundwater movesinto the cypress pond In dry periods, the water movement is reversed, and the aquifer isrecharged (Ewel 1990a)

b Flood Control

Wetlands can temporarily store excess water and release it slowly over time, thus ing the impact of floods Intact and undeveloped riparian wetlands can prevent damagingfloods along rivers (Sather and Smith 1984) Depressional wetlands such as cypress ponds

buffer-or prairie potholes have the capacity to receive and stbuffer-ore at least twice as much water as asite 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 theenergy of waves and currents Wetlands along shorelines are dynamic systems, generallyreaching equilibrium between accretion and erosion of substrate Structures used forshoreline protection, such as bulkheads or jetties, can destroy the shoreline habitat byinterrupting this equilibrium These structures can also channel sediment into navigablewaterways, 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 stabilizesediments 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 shallowwater (which maximizes the sediment-to-water interface), high primary productivity, thepresence of both aerobic and anaerobic sediments, and the accumulation of litter (Mitschand Gosselink 2000) These conditions often lead to a natural cleansing of the water thatflows into wetlands Incoming suspended solids settle from the water column due to thereduced 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 throughmicrobial processes (e.g., nitrification followed by denitrification; Faulkner andRichardson 1989) which require the presence of both aerobic and anaerobic substrates.Plant uptake and plant tissue accumulation can also remove nitrogen and phosphorusfrom 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 producedunder wetland conditions (see Chapter 3, Section III.A.1, Reduced Forms of Elements)

The capacity of wetlands to purify water is one of the most important societal valueswetlands 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 lands are used worldwide to treat wastewater from industrial, agricultural, and domesticsources (Kadlec and Knight 1996; see Chapter 9, Section II, Treatment Wetlands)

wet-3 Habitat

a Wildlife and Fish Habitat

Because many wetlands are highly productive ecosystems, they support a large number offish and wildlife species Some animals, such as many fish, reptiles, and amphibians,depend exclusively on wetland habitats Others utilize wetlands for only short periods oftheir life cycles (breeding, resting grounds) and some use wetlands as a source of food andwater 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 ities provide an economic value to the wildlife function of wetlands Many animals such

activ-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 thefish and shellfish of wetlands

b Plant Habitat

Wetland plant communities are among the most highly productive ecosystems in theworld (Mitsch and Gosselink 2000) The production of biomass and the export of organiccarbon 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 agricultureand development (Wisheu and Keddy 1994) Although many wetland plants are listed bythe U.S Fish and Wildlife Service as rare or endangered, wetland management plans rarelymention the conservation of rare species (Lovett-Doust and Lovett-Doust 1995; seeChapter 1, Table 1.3)

III Broad Types of Wetland Plant Communities

One of the challenges wetland ecologists face is classifying wetlands so that plant 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 mustimpose subjective boundaries on types The sources and amounts of water vary over a widerange even within the same type of wetland In addition, wetlands are found along succes-sional gradients, further complicating their classification Nonetheless, classification of

commu-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, tries, or states, and some for certain types of wetlands, such as peatlands (Shaw andFredine 1956; Taylor 1959; Bellamy 1968; Stewart and Kantrud 1971; Golet and Larson1974; Cowardin et al 1979; Beadle 1981; Zoltai 1983) Internationally, a number of nationshave classified and inventoried their wetlands, including Canada, Greece, Indonesia, andSouth Africa Some of these countries have used the Ramsar definition as a starting pointand adapted it to local conditions For example, Canada’s classification system has fivewetland classes and 70 wetland forms, half of which are types of northern peatlands.Indonesia has classified wetlands into six mangrove forest types and eight freshwaterforested wetland types (Scott and Jones 1995)

coun-In the U.S., the first well-known official wetland classification was published by theU.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 onregional wetland characteristics

In order to better define and inventory the wetlands of the U.S., the U.S Fish andWildlife Service developed a classification of wetlands and deepwater habitats based onthe 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), andthose 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 settingwithin the landscape (i.e., riverine, depressional, lacustrine fringe), its water source, andthe internal movement of water within the wetland, known as its hydrodynamics As aclassification system, the HGM approach emphasizes the topographic setting and thehydrology 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 andlandscape position that also control wetland hydrology

Alternatively, an approach based on the hydrogeologic setting (HGS) refers to the tors, both regional and local, that drive wetland hydrology and chemistry It places anemphasis on the surface and subsurface features of the landscape that cause water flowinto 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, soilthickness and permeability, and the stratigraphy, composition, and hydraulic conductivity

fac-of the underlying geologic materials He used these parameters to classify sites into one fac-of

24 “type settings” based on unique combinations of physiography and climate Thisframework has a landscape basis and has been proposed for use in classifying wetlands forresearch into their diversity and ecological functions

For the purposes of this book, we describe broad types of systems where wetlandplants grow We have categorized wetlands into three major wetland plant communities:

(1) marshes, where herbaceous species dominate; (2) forested wetlands, where trees or

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 ourdescription 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 formarshes 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 wetprairie (after the community type; Kushlan 1990) Coastal marshes and inland marshes arediscussed in more detail below

1 Coastal Marshes

a Salt Marshes

Salt marshes occur in coastal areas and are usually protected from direct wave action bybarrier 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 northNorfolk 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 areable to outcompete marsh plants (Kangas and Lugo 1990), although salt marshes do per-sist inland from mangroves in tropical (northern) Australia (Finlayson and Von Oertzen1993) and alongside mangroves in some coastal areas of Mexico (Olmsted 1993) Saltmarshes occur as far north as the subarctic and are particularly extensive around theHudson and James Bays of Canada (approximately 300,000 km2; 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.)

The plant communities of salt marshes are subjected to daily and seasonal water levelfluctuations due to tides, and to variations in freshwater inputs from overland runoff Inaddition, 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 inthe soil pore water that are sometimes higher than that of seawater (i.e., 35 ppt) due to thedeposition of salt and evaporation (Wijte and Gallagher 1996a)

In North America, some of the major remaining areas of salt marshes are on the Atlanticcoast 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 waveenergy is low (0 to 2 m amplitude; Roberts and Robertson 1986) South of this region, saltmarshes 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 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

hydrologi-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 theBay of Fundy marshes

3 Coastal Plain marshes: These marshes extend from New Jersey south along theAtlantic and along the Gulf of Mexico coast to Texas The tidal range is smallerand the inflow of silt from the coastal plain is high Included among these are theMississippi 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.)

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 est part of the marsh, closest to the sea The short form (10 to 80 cm) grows inland fromthere (Valiela et al 1978; Anderson and Treshow 1980; Niering and Warren 1980) Morestressful conditions in the inland area of the low marsh, such as nitrogen limitation (Valielaand Teal 1974; Gallagher 1975), high salinity (Anderson and Treshow 1980), and low soiloxygen levels (Howes et al 1981), may cause the height difference (see Chapter 4, Case

low-Study 4.A, Factors Controlling the Growth Form of Spartina alterniflora)

Salt marshes provide a striking example of plant species zonation in response to ronmental variation, with different species occurring at different marsh elevations Eachspecies’ 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-

envi-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 monospecificwithin 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 theeast, 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 wideflat river deltas and estuaries The majority of Pacific coast salt marshes that did exist havebeen 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 SanDiego (Zedler 1977), in northern San Francisco Bay (Mahall and Park 1976), Tomales Baynorth of San Francisco (Chambers et al 1994), Nehalem Bay in northern Oregon (Eilers1979), Puget Sound in Washington (Burg et al 1980), at the head of fjords and on the QueenCharlotte Islands in British Columbia (Glooschenko et al 1993), and in Cook Inlet nearAnchorage, Alaska (Vince and Snow 1984)

The plant communities of western salt marshes tend to be more diverse than Atlanticcoast 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 salinelocations (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 ofSpain, 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 alsofringe northern Africa along the Tunisian, Moroccan, and Algerian coasts (Britton andCrivelli 1993) The seaward portions of European salt marshes are often tidal mudflats,

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 saltmarshes are flooded twice daily, many European marshes are only partially flooded, withtheir 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 intoestuaries or oceans Their position within the landscape places them at the interfacebetween the upstream sources of fresh water and the downstream sources of tides Theyoccur worldwide, wherever these conditions are met Tidal freshwater wetlands cover anestimated 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 areabout 164,000 ha with over half (89,000 ha) in New Jersey, and most of the remaining in theChesapeake 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, tidalfreshwater wetlands harbor diverse communities of plants and animals Since salinity andsulfur 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 formssuch as shrubs, floating plants, grasses, and forbs, rather than the monotypic stands ofgrasses 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 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

rag-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 sizefrom quite small (<1 ha) to the size of the Everglades (currently 607,000 ha) They are foundworldwide wherever hydrologic and geologic conditions allow for their formation Manydifferent kinds of freshwater marshes have been defined, and they are often named for thedominant vegetation In this book we divide marshes into three broad categories based on

their landscape position: lacustrine marshes, riverine marshes, and depressional marshes.Inland marshes that accumulate peat, commonly called fens and bogs, are discussed inSection III.C, Peatlands.

Several thousand plant species are adapted to freshwater marshes (Cook 1996; Reed1997) and whether the marshes are lacustrine, riverine, or depressional, the plants are oftenthe 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.)

1994) The structure of plant communities varies with climate, substrate type, floodingregime, water depth, and nutrient availability Some of the most common emergent plants offreshwater 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 low), Spiraea (meadow sweet), Rosa (rose), Cephalanthus (buttonbush), Alnus (alder), and Cornus (dogwood)

(wil-a Lacustrine Marshes

As defined by Cowardin and others (1979) the lacustrine system is divided into the netic, or deep water habitat, and the littoral, or edge habitat Lacustrine wetlands includelittoral aquatic beds, dominated by submerged and floating-leaved species, and emergentmarshes slightly upland (Figure 2.6) A lake’s shape dictates whether lacustrine wetlandscan exist along its fringes A steep-sided V- or U-shaped lake, generally formed by tectonicforces, has less water in contact with sediments, a more abrupt drop from edge to deepwater, and generally supports little, if any, littoral vegetation Lacustrine wetlands aremore 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

lim-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.)

Lacustrine wetlands are located around the world, along the edges of both small andlarge 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 theLaurentian Great Lakes of the U.S and Canada, wetlands occur in coastal lagoons behindbarrier 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 inold oxbows of rivers (Figure 2.7) While many riverine wetlands are forested, herbaceouswetlands 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 flowingwater which carries a higher mass input of sediments and nutrients and allows increasedexport 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 toother wetlands or bodies of water, or hydrologically isolated Depressions include former lakebasins, 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 marsheswithin 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.

One extensive region of depressional wetlands is found in the prairie pothole region ofIowa, Minnesota, North and South Dakota in the U.S., and Alberta, Saskatchewan, andManitoba in Canada (see Chapter 9, Case Study 9.C, Figure 9.C.1) This area encompassesover 70 million ha of land (Kantrud et al 1989) with millions of prairie potholes scatteredthroughout (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 northernIowa the basins are shallow depressions linked by drainage ways In other parts of the prairiepothole region, there are fewer surface links between the potholes (Galatowitsch and van derValk 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 potholeregion of Iowa and southern Minnesota, nearly 350 plant species can be found and up toone 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 tohundreds 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 highplains of New Mexico and northern Texas Some playas may have been formed by prairiewind erosion 10,000 to 15,000 years ago (Bolen et al 1989) This theory is supported by thepresence 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, andsome may have been formed where geologic joints provided paths of weakness for surfacedrainage 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.)