This widespread wetland system occurs throughout the arid and semi-arid regions of North America. In Montana, this system is typically found in depressions surrounded by an upland matrix of mixed prairie, shrub steppe, or steppe vegetation. Natural marshes occur in and adjacent to ponds and prairie potholes, as fringes around lakes or oxbows, and along slow-flowing streams and rivers as riparian marshes. Marshes are classified as either seasonal or semipermanent based on the dominant vegetation found in the deepest portion of the wetland; vegetation is representative of the hydroperiod. A central shallow marsh zone dominated by graminoids and sedges characterizes seasonal wetlands, while semipermanent wetlands are continually inundated, with water depths up to 2 meters (6.5 feet) and a deeper central marsh zone dominated by cattails (Typha species) and bulrushes (Schoenoplectus species). Water chemistry may be alkaline or semi-alkaline, but the alkalinity is highly variable even within the same complex of wetlands. Marshes have distinctive soils that are typically mineral, but can also accumulate organic material. Soils characteristics reflect long periods of anaerobic conditions. Dominant vegetation often includes western wheatgrass (Pascopyrum smithii), Northwest Territory sedge (Carex utriculata), Nebraska sedge (Carex nebrascensis), broadleaf cattail (Typha latifolia), and hardstem bulrush (Schoenoplectus acutus). Alkaline marsh communities include western wheatgrass, fresh water cordgrass (Spartina pectinata), and seashore saltgrass (Distichlis spicata).
This system is found in environments where precipitation is approximately 25 to 50 centimeters (10 to 20 inches) per year. In Montana, this system is typically found in depressions surrounded by an upland matrix of mixed prairie, shrub steppe, steppe vegetation and forests near the mountains. Natural marshes occur in and adjacent to ponds and prairie potholes, as fringes around lakes or oxbows, and along slow-flowing streams and rivers as riparian marshes. Water chemistry may be alkaline or semi-alkaline, but is highly variable even within the same complex of wetlands. Marshes have distinctive soils that are typically mineral, but can also accumulate organic material. Soils characteristics reflect long periods of anaerobic conditions, with gleying, high organic content, and redoximorphic features. Wetland marshes are classified as either seasonal or semi-permanent based on the dominant vegetation found in the deepest portion of the wetland (Stewart and Kantrud, 1971 and LaBaugh et al., 1996). Vegetation communities occurring in these marsh systems is representative of their hydroperiod; some basins dry to bare soil after seasonal flooding, while others will have a variety of wetland types in a zoned pattern dependent on seasonal water table depths and salt concentrations (Kudray and Cooper, 2006).
Vegetation communities change according to wet-drought cycles. In seasonal ponds that dry out annually, and in semipermanent wetlands during drought years, buried seeds of both annuals and perennials germinate, covering exposed mud flats (Hansen et al., 1995). In semi-permanent marshes, the drawdown zone is typically dominated by western wheat grass (Pascopyrum smithii) near the upland edge, with Northwest Territory sedge (Carex utriculata) and Nebraska sedge (Carex nebrascensis) as the dominant sedges located down gradient, and broadleaf cattail (Typha latifolia) and hardstem bulrush (Schoenoplectus acutus) located in the deeper, central portion of the marsh. Water sedge (Carex aquatilis) is frequently co-dominant with Northwest Territory sedge. Less commonly, blister sedge (Carex vesicaria) and awned sedge (Carex atherodes) are intermixed with Northwest Territory sedge or occur as co-dominants on similar sites. Beyond the emergent vegetation, floating-leaved hydrophytes may be present in wetter sites with longer inundation periods, including water lilies (Nymphaea species), yellow pondlily (Nuphar species), buttercup (Ranunculus species) and pondweed (Potamogeton species). Other floating species may be present in shallow water, such as duckweed, (Lemna species), and submergents such as common hornwort (Ceratophyllum demersum), horned pondweed (Zannichellia palustris), mare’s tail (Hippuris vulgaris) and water milfoil (Myriophyllum species).
Seasonal marshes are typically dominated by western wheat grass (Pascopyrum smithii), beaked sedge (Carex utriculata), inflated sedge (Carex vesicaria), Nebraska sedge (Carex nebrascensis), creeping spikerush (Eleocharis palustris), Baltic rush (Juncus balticus) and cattail (Typha latifolia or angustifolia). During wetter years, annuals disappear and marshes become dominated by emergent perennials. Common perennial forbs include common willow herb (Epilobium ciliatum), marsh cinquefoil (Potentilla palustris), Gmelin’s buttercup (Ranunculus gmelinii), greater creeping spearwort (Ranunculus flammula), hemlock water parsnip (Sium suave), willow dock (Rumex salicifolius), field mint (Mentha arvensis), leafy aster (Symphyotrichum foliaceum) and broadleaf arrowhead (Sagittaria latifolia). Fern allies such as water horsetail (Equisetum fluviatile) and field horsetail (Equisetum arvense) often form significant cover within seasonal marshes. Grasses common to marshes include small floating mannagrass (Glyceria borealis), tufted hairgrass (Deschampsia caespitosa), and bluejoint reedgrass (Calamagrostis canadensis).
Seasonal and semi-permanent marshes with more alkaline water chemistry are commonly found throughout central and eastern Montana. Typical species include hardstem bulrush, cattail, common threesquare (Schoenoplectus pungens), alkali bulrush (Schoenoplectus maritimus) and inland saltgrass (Distichlis spicata), red swampfire (Salicornia rubra) and prairie cordgrass (Spartina pectinata) in adjacent drawdown zones. These marsh communities are brackish and support species adapted to saline and alkaline water and soil conditions, similar to Western Great Plains Saline Depression systems.
Typically, riverine marshes subjected to unaltered, seasonal water flow and annual flooding are characterized by zonal vegetation determined by water depth with stands of bulrush (Schoenoplectus species), softstem bulrush (Schoenplectus tabernaemontani), and cattail in deeper water, and manna grass (Glyceria species), water sedge, inflated sedge, water horsetail and common spikerush in shallower water zones. Riverine marshes can be influenced by beaver activity and human caused influences that can change the structure and species richness of these plant communities. Beaver activity can increase species richness and diversify community structure by altering water flow, depth, and organic sediment accumulation.
Wet-drought year climatic cycles in Montana, often in 10 to 20 year cycles, influence the ecological communities in these systems (Hansen et al., 1995). During this climatic cycle, wetlands go through a dry marsh, regenerating marsh, degenerating marsh and a lake phase that is regulated by periodic drought and deluge (Mitsch and Gosselink, 2000). During drought periods, seeds from annuals and perennials germinate and cover exposed mud flats, but when precipitation floods the depressions, the annuals drown and the perennials survive, regenerating the marsh. Over a series of years, perennials dominate and submersed and floating-leaved hydrophytes return. After a few years of the regenerating phase, emergent vegetation begins to decline and eventually the marsh reverts to an open water system. Muskrats may play an important role in the decline of emergent vegetation in some of these systems. During drought, the drawdown to mudflats is necessary so that emergent vegetation can become reestablished. Flooding, drawdown and the eventual exposure of mud flats drive the water-level vegetation cycle. In saline soil marshes, increase in precipitation during exceptionally wet years can dilute the salt concentration in the soils, allowing for less salt-tolerant species to occur.
Species richness can vary considerably among individual examples and is especially influenced by adjacent land use. Agriculture and forestry operations, when adjacent, may cause nutrient and herbicide runoff.
Changes will occur in the plant communities due to climatic conditions and/or management activities. Draining, ditching or conversion to agriculture and pastureland can alter the hydrology of the system. Moderate to heavy grazing practices can greatly decrease cover of beaked sedge, and cause soil compaction. Invasive and exotic species such as reed canarygrass (Phalaris arundinacea), common reed (Phragmites australis) and Canadian thistle (Cirsium arvense) become established in areas of heavy grazing or other disturbances. Diversion or lateration of seasonal flooding in riverine systems can change the species composition and succesional direction of riverine marsh communities.
In marsh systems where water has been drained or altered, the original hydrology of the system must be restored. If water levels are restored, re-growth and re-colonization from dormant rhizomatous root systems of common marsh species can occur within a few years. Cattle grazing must be eliminated or controlled to allow regrowth, recolonization and resprouting from existing root systems. Many of the characteristic species found in marsh systems are rhizomatous, thus exhibit excellent erosion control properties. In some cases, if hydric soils are heavily altered due to pugging or compaction, addition of organic material may be needed to facilitate vegetation recolonization.
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