This Great Plains system occurs in lowland depressions and along lake borders with open basins and a permanent water source through most of the year. This system is distinguished from the Western Great Plains Closed Depression Wetlands by having a larger watershed and/or significant connection to the groundwater table. Soils are typically Mollisols, Entisols or occasionally Histosols. Soil pH varies from neutral to slightly alkaline. In Montana, this system is especially well represented along major and secondary tributaries of the Milk, Marias and Two Medicine rivers in the northwestern Great Plains glaciated pothole region. Throughout Montana, most sites within this system are found at elevations of 664-2,027 meters (2,180-6,650 feet). Species diversity can be high in some occurrences. These wetlands usually contain emergent graminoids such as cattails (Typha species), sedges (Carex species), spikerushes (Eleocharis species), rushes (Juncus species) and bulrushes (Schoenoplectus species), as well as floating vegetation such as pondweeds (Potamogeton species), arrowhead (Sagittaria species), or common hornwort (Ceratophyllum demersum). At montane elevations, these systems can be moderately complex with a variety of species and communities. Increased grazing pressure in and adjacent to these systems will change the plant communities that are present. In semi-permanent systems, the drawdown zone is typically dominated by beaked sedge (Carex utriculata) water sedge (Carex aquatilis), and Nebraska sedge (Carex nebrascensis). In seasonal ponds that draw down annually, and in semipermanent wetlands during drought years, buried seeds of both annuals and perennials will germinate in exposed mud flats.
Open depression wetlands are found throughout the Northwestern Glaciated Great Plains region of Montana. They form in lowlands, and along lake borders and stream margins. They generally have more open basins, a large watershed, and a permanent water source throughout most of the year, except during exceptional drought years. This system differs from closed depressional wetlands by having a larger watershed and/or significant connection to the groundwater table (Cook and Hauer 2007). In Montana, most sites within this system are found at elevations of 664-2,027 meters (2,180-6,650 feet). Soils are typically Mollisols, Entisols or occasionally Histosols. Soil pH varies from neutral to slightly alkaline.
Open depression wetlands
often have submerged aquatic plants in the open water zone including common
hornwort (Ceratophyllum demersum),
short spikewater milfoil (Myriophyllum
sibiricum), and horned pondweed (Zannichellia palustris)
as well as floating-leaved plants including
pondweeds (Stuckenia and Potamogeton species), white water crowfoot (Ranunculus
aquatilis) and arrowheads (Sagittariaspecies). The central marsh zone is typically dominated
by hardstem bulrush (Schoenoplectus
acutus), but softstem
bulrush (Schoenoplectus tabernaemontani),
common threesquare (Schoenoplectus pungens) and
alkali bulrush (Schoenoplectus maritimus), often co-dominate. Also found in the marsh
zone are cattails (Typha species), water
knotweed (Polygonum amphibium),
and hemlock water parsnip (Sium suave). The seasonally flooded zones are typically
dominated by graminoids including common spikerush (Eleocharis palustris), needle
spikerush (Eleocharis acicularis), American sloughgrass
wheat sedge (Carex atherodes),
foxtail barley (Hordeum jubatum), shortawn foxtail (Alopecurus aequalis), and water foxtail (Alopecurus geniculatus). Open depressional
systems are often bordered by wet prairie zones characterized by species such
as slimstem reedgrass (Calamagrostis stricta),
clustered field sedge (Carex praegracilis), bluejoint (Calamagrostis canadensis)
and fowl bluegrass (Poa palustris).
Open depressions with more alkaline or saline water and soil chemistry will
typically be bordered by species such as saltgrass (Distichlis spicata),
western wheatgrass (Pascopyrum smithii), and freshwater cordgrass
(Spartina pectinata). Sites that have been moderately grazed often
have an increase in Baltic rush (Juncus balticus), knotted rush (Juncus
nodosus), foxtail barley (Hordeum
jubatum), American sloughgrass
and western wheatgrass (Pascopyrum smithii).
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