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The early studies of Shunk (1917) and Metcalf (1931) identified the major features of prairie wetlands (central or concentric peripheral zones of vegetation) and the factors that seem to control plant species composition and distribution of these zones (water depth, length of time surface water or saturated soils are present, and salinity). Most subsequent investigations (e.g., Moyle 1945; Evans and Black 1956; Walker 1959, 1965; Dix and Smeins 1967; Smeins 1967; Walker and Coupland 1970; Stewart and Kantrud 1972; Barker and Fulton 1979; Kollman and Wali 1976) have elaborated on these themes and also described the effects of changes in water levels or salinity or the impacts of cultivation, grazing, fire, mowing, and idle conditions on wetland vegetation.

Zonation

Zonation of vegetation in prairie basin wetlands was first noticed by Froebel (1870) who described a "central portion" inside "concentric circles of different species of plants."

The vegetation of nearly all prairie basin wetlands is composed of a central zone bounded peripherally by concentric zones (Figure 11) that occupy the moisture-regime gradients (Figure 2). These zones are dominated by different species or groups of species. Changes in the size, color, or morphology of the species among zones makes them a prominent feature of the vegetation. The plants found in the zones are commonly grouped and accepted as indicators of the hydrologic regime of the zones even though individual species have discrete distributions. The number of zones present in a basin increases with the degree of water permanency; that is, the length of time the zones can be expected to contain surface water. The best accepted names for these zones in the basins dealt with in this report are, in order of increasing degree of water permanency, wet meadow, shallow marsh, and deep marsh. Wet-meadow zones can be dominated by woody vegetation and not appear as meadow. The appearance of large amounts of woody vegetation around prairie wetlands is often attributed to fire suppression by European man (Bird 1961). Shallow-marsh and deep-marsh zones can be dominated by submersed plants. Wet-meadow and shallowmarsh zones are commonly cultivated and can be devoid of hydrophytes when dry or for short periods after reflooding.

Figure 11. Spatial relation of vegetational zones in Stewart and Kantrud (1971) classes of natural basin wetlands in the glaciated prairies.

Shallow marsh zones. These zones are usually dominated by coarse emergent grasses, sedges, and burreed of midheight or a few, often nonpersistent, fortes. Submersed or floating vascular plants or aquatic mosses or liverworts are fairly common, and can occur in the understory of emergents or in shallow open water. Except for the spring snow-melt period, the presence of open water in shallow marsh zones is largely a function of land use. Inundation is normally seasonal from spring to mid- or late summer. Water loss in centrally located shallow-marsh zones in central North Dakota probably averages 40 mm/day from early April until the end of June (Kantrud and Stewart 1977).

Deep marsh zones. These zones are either dominated by tall, coarse cattails or bulrushes with an understory of submersed or floating plants, or by beds of submersed vascular plants. Unconsolidated bottom devoid of vegetation is sometimes found. Water regime is usually semipermanent, with surface water present from spring through fall and frequently overwinter. Water loss in centrally located deep-marsh zones in North Dakota wetlands likely averages 25 mm/day (Kantrud and Stewart 1977).

Some characteristics of these zones are evident. In a given basin, water depth increases with zonal water permanency, but this often is not true among or between basins of different hydrological setting. For example, shallow-marsh zones in many freshwater wetlands are often deeper than deep-marsh zones in more saline wetlands. In all but the most saline wetlands, the stature of emergent vegetation increases along the gradient from temporarily to semipermanently flooded. Species richness of the emergent component of the vegetation in these zones tends to decrease with water permanence. Deviations from the "normal" zonation pattern occur regularly in the form of zonal inversions or deletions brought about by fluctuating water levels (Smeins 1967; Millar 1976). Zonal deviations can be used to interpret the recent hydrologic history of a basin wetland.

Vegetation Dynamics<> Prairie wetland vegetation changes more dramatically than most other natural vegetation in North America because of the effects of regional climatic instability on hydrologic regimes. Late winter snowfall, spring runoff, summer precipitation, and evapotranspiration are all highly variable between years and within seasons. Extended droughts as well as short series of years with much higher than average precipitation are common.

High water levels can kill up to 25% of the emergent vegetation in prairie wetlands, but when water levels fall, new bands of vegetation quickly develop on newly exposed shorelines (Walker 1959, 1965). Many emergent species such as Typha reproduce vegetatively except during low-water conditions, when reproduction by seed is common (Bedish 1964). Extremely high water levels can also kill trees and other woody vegetation that have developed in wet meadow zones, leaving a ring of dead trees that can persist for years.

Stewart and Kantrud (1971) consider the degree of interspersion of cover and open water mostly a function of water depth rather than water permanency. Studies by Weller and Spatcher (1965), Weller and Fredrickson (1974) and van der Valk and Davis (1976, 1978a) on semipermanent Iowa wetlands outline the cyclic effects of physical and biological forces on the species composition and distribution of wetland vegetation.

The stages in this vegetation cycle are shown in Figure 12 and its effects on the biota are outlined in Table 3. In the dry-marsh stage, water is absent or low in the central zone and muskrats can be absent. Vegetation on the marsh floor consists of mudflat annuals and seedlings of emergent species. The annuals can complete their life cycle and add seeds to the seed bank during this stage. With the resumption of normal precipitation, the wetland enters the regenerating marsh stage. Muskrat populations begin to recover. Mudflat annuals decompose and are eliminated, but opagules of submersed and floating species from the seed bank germinate, and within a few years the emergent species spread vegetatively and normal patterns of zonation are established. Major inputs of seeds of the emergent species to the seed bank occur at this time. If water levels continue to rise, emergent vegetation begins to die from excess water depth, senescence, disease, and insects. This is the degenerating marsh stage. Continued high water results in the lake-marsh stage dominated by submersed vegetation only, because of excess muskrat damage and the inability of seeds of emergents to germinate under deep water. Finally, drought or artificial drawdown return the wetland to the dry-marsh stage.

Figure 12. A generalized vegetation cycle in an Iowa prairie wetland (van der Valk and Davis 1978a).

Figure 13. A typical sequence of wetland phases as related to variable water conditions (Stewart and Kantrud 1971).

Much information has been gathered on the effects of salinity (concentration of total dissolved solids in the soil or water column) on prairie wetland vegetation since the early observations of Bailey (1888) and Visher (1912). Such information has been summarized recently by Kantrud et al. (1989). Areas containing large numbers of wetlands of relatively high salinity are mostly restricted to glacial outwash plains lying in the western and northwestern portion of the Prairie Pothole Region of the Dakotas.

The gradient from fresh to hypersaline water is a continuum, and any divisions are arbitrary. In this report we refer only to the salinity subclasses of Stewart and Kantrud (1971) and the water-chemistry modifiers of Cowardin et al. (1979).

Surface water in temporary and seasonal wetland basins in the Dakotas is usually fresh or slightly brackish, although a few moderately brackish seasonal basins have been observed (Stewart and Kantrud 1971). The fresh and slightly brackish subclasses of Stewart and Kantrud (1971) correspond to the fresh (<0.8 mS/cm) and lower oligosaline (0.82.0 mS/cm) water-chemistry modifiers of the Cowardin et al. (1979) classification system.

Semipermanently flooded wetland basins, and their associated wet meadow, shallow-marsh and deep-marsh zones, can be fresh, slightly brackish, moderately brackish, brackish, or subsaline. Most of these basins are slightly brackish. The moderately brackish and brackish subclasses of Stewart and Kantrud (1971) correspond to the upper oligosaline (2.0-8.0 mS/cm) and lower mesosaline (8.0-15 mS/cm) modifiers of Cowardin et al. (1979), whereas the subsaline subclass includes the range of the upper mesosaline (15-30 mS/cm) and polysaline (30-45 mS/cm) modifiers.

The general effect of increased salinity in any zone of wetland vegetation is to decrease the number of species. In polysaline wetlands, only the single submersed angiosperm Ruppia maritima occurs. A similar reduction in species is noted during drawdown conditions (Stewart and Kantrud 1971, 1972), when monodominant stands of Kochia scoparia are often found in dry polysaline basins. There also is a tendency towards succulence in plants found in more saline basins.

Salinity levels can fluctuate widely within and among seasons, particularly in smaller wetlands and those of intermediate and high salinity (Rozkowska and Rozkowski 1969, Stewart and Kantrud 1972). These changes can be accompanied by changes in vegetative species composition (Ungar et al. 1979). For example, aquatic bed dominated by Chara sp., Zannichellia palustris, and Potamogeton pectinatus often alternates with stands of Ruppia maritima in certain prairie wetlands as surface waters are respectively diluted and concentrated.

Common dominance types of emergent vegetation under various land uses and water chemistry conditions are shown for wetlands with temporary, seasonal, and semipermanent water regimes in Appendix B, along with similar information for the class aquatic bed.

It is difficult to establish meaningful salinity tolerances for individual species in their natural habitats, because of the complex of factors associated with salinity fluctuations and ecotypic variations among species (Kantrud et al. 1989). Nevertheless, we present the combined data of Smeins (1967), Disrud (1968), Sletten and Larson (1984), and H.A. Kantrud (unpubl.), as published in Kantrud et al. (1989) to estimate the salinity tolerance of many emergent and aquatic bed species in Appendix C. These are crude estimates because specific conductivities of surface waters and soil extracts from the root zone often differ greatly.

Many species have a broad range of salinity tolerance. This is a common trait among hydrophytes of the Prairie Pothole Region. Species that are tolerant of high salinities can survive low salinities (Ungar 1966). This suggests that competition or other factors play an important role in determining species composition in less saline sites (Ungar 1974).

Disturbance

Human-related disturbance is one of the most important factors affecting species composition of wetland vegetation in the northern prairies (Smeins 1967, Walker and Coupland 1968, 1970). The intensity of disturbance is often more important than the type of disturbance, and the native hydrophytes in the area are adapted to respond quickly to changing conditions and form dense stands. Common dominance types of wetland vegetation in various water regimes are related to disturbance and idle conditions in Appendix B.

Drainage and cultivation are the most extreme disturbances seen in most prairie wetlands in the Dakotas, although in some instances the basins themselves have also been destroyed by filling with earth or rocks, or used for solid-waste disposal (see Section 5.3).

Basins that are cultivated but not drained are extremely common in the Dakotas (Figure 14). Stewart and Kantrud (1973) estimated that 29% of the area and 52% of the individual wetlands in the Prairie Pothole Region of North Dakota had been cultivated for crop production in 1967, a year of excellent water conditions. These wetlands were primarily temporary and seasonal. The wet-meadow and shallow-marsh zones of many semipermanent wetlands are also regularly cultivated. During the prolonged extreme drought of the 1930s, the bottoms of many basins with semipermanently flooded and intermittently exposed water regimes were used for crop production in the Dakotas. Sedimentation is extremely common in basins located in cropland in this area, as there is no barrier to runoff water from the uplands, and the practices of summer fallow and fall plowing are widespread. In addition, the heaviest rains of the season often occur just after seeding when fields are newly cultivated.

Figure 14. Cultivated temporarily and seasonally flooded basins dot the landscape in the intensively farmed regions of the Dakotas (near Lakota, North Dakota, June 1978; photograph by Alan Sargeant).

Drawdown species invade cultivated basins very rapidly as water recedes, forming plant associations different from those found in similar basins in grassland. This is the cropland drawdown phase of Stewart and Kantrud (1971).

Basins cultivated intensively during dry periods often have no hydrophytic vegetation, or else the vegetation consists only of upland weeds, commercial crops, or their residue. This is the cropland tillage phase of Stewart and Kantrud (1971). Hydrophytes become reestablished from seed banks very quickly in these basins as water is replenished.

Another effect of cultivation is to roughen the bottom surface with ridges, furrows, clods, and wheel tracks. This creates a variety of microhabitats in which field weeds as well as emergent and submersed hydrophytes and drawdown species live in close proximity or patterned stands.

Prairie wetlands evolved under a regime of grazing by native ungulates; grazing by domestic livestock today remains as one of the most important commercial uses of these basins. Many basins located near feedlots or corrals around ranches or farmsteads are grazed all year, whereas those in outlying pastures are usually grazed during spring, summer, and fall. In pastures where artificial watering facilities are not provided and basins are few, basin wetlands can receive an inordinate amount of trampling and grazing pressure. Cattle are the most frequent grazers of Dakota wetlands, but sheep and horses are not uncommon.

Dominant plant species in grazed prairie wetlands usually differ greatly from those in areas put to other uses (Kantrud et al. 1989). Adaptations of wetland plants to grazing include nodal rooting and unpalatability (Walker and Coupland 1968). Unless unusually severe, grazing of wetlands results in greater species diversity, more complex distributional patterns, and sharper boundaries between zones (Bakker and Ruyter 1981). General effects of moderate to heavy grazing in prairie wetlands also include a decrease in overall height of emergents and an increase in aquatic bed. Schultz (1987) showed that prescribed grazing of monodominant Typha glauca stands by cattle in South Dakota wetlands resulted in a decrease in live stems, dead stems, depth of residual litter, and litter coverage. Associated changes included increased coverage of floating plants (Lemna and Riccia), higher water temperature, greater invertebrate abundance and biomass, and increased use by breeding waterfowl. Livestock trampling can affect the height and density of wetland vegetation more than consumption (Hilliard 1974). Severe grazing can decrease primary production (Reimold et al. 1975), increase water turbidity (Logan 1975), and eliminate all vascular plants (Bassett 1980).

Plants that are increased by or are tolerant of domestic livestock grazing in wetlands in or near the Prairie Pothole Region have been listed by Evans et al. (1952), Smith (1953), Smeins (1965), Dix and Smeins (1967), Walker and Coupland (1968), Stewart and Kantrud (1972) and Millar (1973). The relation of grazing to wetland vegetation and the use of such habitats by breeding waterfowl and their broods has been reviewed by Kantrud (1986a).

Removal of perennial emergent vegetation for use as livestock food or bedding is a common practice in the Prairie Pothole Region. Mowing is usually restricted to temporary and seasonal wetland basins and the wet-meadow and shallow-marsh zones of semipermanent wetlands, but during droughts even the coarse emergent species of deep-marsh zones are sometimes harvested. Wetlands thus treated appear as open water after spring runoff, but regrowth of emergents is rapid except under extremely high water levels, so by late spring a dense canopy of vegetation is usually present. Smeins (1967), Walker and Coupland (1968, 1970), and Stewart and Kantrud (1972) suggested that certain native hydrophytes were favored by mowing. However, J.B. Millar (unpubl. data, Canadian Wildlife Service) saw no detectable changes in the dominant emergent species in Saskatchewan wetlands after 25 years of mowing. Two introduced species, Alopecurus arundinacea and Phalaris arundinacea, are often planted in prairie wetlands for use as hay or forage crops.

Burning is not a common practice on privately owned wetlands in the Dakotas. A small proportion of the basins are burned each fall incidental to burning road rights-of-way for snow control; other basins are burned to make fall tillage easier or to decrease the amount of snow trapped in them so they can be more easily cultivated the following spring. Burning is being increasingly employed as a means to control excessive emergent vegetation to increase waterfowl production on National Wildlife Refuges and state-owned lands, yet little is known about the environmental effects of fire in prairie wetlands, and research on prescribed burning for wildlife production has often been urged (Ward 1968; Weller 1978; Kantrud 1986a).

Fires were common in prairie wetland vegetation in the early 19th century, as evidenced by the accounts of early traders and travelers. For example, in 1803 Henry and Thompson (Coves 1965) recorded fire rushing through n low places covered with reeds and rushes." In 1858 or 1859 Boller (1972) saw a large conflagration spread for many miles after being set by American Indians in "dry rushes in the prairie bottoms." Denig (1961), writing about his experiences during 1833-54, noted that fire would sweep over ice through wetland vegetation.

General references (Kozlowski and Ahlgren 1974; Wright and Bailey 1982) indicate that burning of marsh vegetation releases nutrients, opens the canopy and detrital layer, and allows for increased insolation and resultant earlier warming of bottom soils. Biological productivity usually increases following fire, even though plant species composition can be altered. Species composition usually changes little when perennial species with meristems at or below ground level are burned during their dormant period.

The few studies and incidental observations of fire effects on prairie wetlands (reviewed in Kantrud 1986a) do not currently provide wetland managers with sufficient quantitative information to formulate burn prescriptions. Most experimental burn-, often in combination with crushing, mowing, water level manipulations, or herbicide trials, have been directed toward control of cattails (Typha spp.)

Many prairie wetland basins now lie idle in former pastures converted to cropland or in retired cropland or wildlife management areas. Even many of the larger semipermanent wetlands are not used for grazing because in recent decades livestock numbers have generally decreased in the eastern Dakotas. In addition, the dense stands of coarse emergents, particularly Typha spp., that have developed in these basins are relatively unattractive to livestock.

Biologists have often attributed decreased use of prairie wetlands by aquatic birds to decreased habitat heterogeneity caused by a reduction in the natural ecological processes of grazing, fire, and water-level fluctuations. In the absence of such processes, autogenic succession tends to build dense stands of emergent hydrophytes in many wetlands (Walker 1959: Jahn and Moyle 1964; Whitman 1976). In the prairies, the usual result is domination by tall robust grasses, sedges, cattails, or woody plants (Kantrud 1986a).

All but the more saline or steepsided prairie wetland basins are susceptible to the establishment of dense stands of emergents because of low-gradient shorelines, small differences in soils or organic-matter content within basins, and the ability of many species to survive under a wide range of water conditions (Hammond 1961, Walker and Coupland 1968). Dense emergents can be controlled by water-level manipulation, but few natural basins in the prairie region have water control structures or sufficient depth below natural spill elevation to make such structures effective if installed.

Drastic environmental changes occur when deep-marsh zones are allowed to go idle and stands of tall emergent hydrophytes become dominant. Reduced insolation of the water column and bottom substrate and increases in litter can reduce or eliminate other species of plants in the understory (Bennett 1938; Buttery and Lambert 1965; Spence and Chrystal 1970; Vogl 1973). Submerged plants, in particular, require water of sufficient depth to reproduce (Anderson 1978; Courcelles and Bedard 1978), and the buildup of litter and organic material from emergent species can reduce water depth or eliminate shallow-water areas (Ward 1942, 1968; Walker 1959; Hammond 1961; Beule 1979).

Buildup of litter and its shading effect also can result in lower soil or water temperature and lower rates of plant decomposition (Willson 1966; Godshalk and Wetzel 1978). Various emergent species can decompose at different rates as the result of differences in species composition of macroinvertebrate populations (Danell and Sjoberg 1979). Thus, the development of monotypic stands of emergents can effectively remove some of the variation in decomposer organisms that could act to maintain or increase vegetative heterogeneity.

Species Composition and Abundance

Common dominance types of emergent wetland vegetation are shown for temporarily, seasonally, and semipermanently flooded moisture regimes in Appendix B. Dominance types in aquatic bed in seasonally and semipermanently flooded moisture regimes are shown in Appendix C. These tables also relate species composition to land use and water chemistry. Additional species, mostly of lesser importance as vegetative cover, are listed in Stewart and Kantrud (1971, 1972).