This ecological system is very similar to both the Western Great Plains Open Freshwater Depression Wetland and the Western Great Plains Closed Depression Wetland found in wetland complexes in the central and northeastern portion of Montana. However, this system differs due to increased soil salinity, which causes these systems to become brackish. This high salinity is attributed to high evaporation and the accumulation of minerals dissolved in the water. Wetlands in this system are discharge wetlands, where water high in dissolved salts has moved from the regional groundwater system into the depression. Hydroperiods vary depending on precipitation and snowmelt, the primary source of water. Water is prevented from percolating out of the depression due to impermeable dense clay, and salt encrustations can occur on the surface with drying. Species that typify this system are salt-tolerant and halophytic graminoids such as alkali bulrush (Schoenoplectus maritimus), common three square (Schoenoplectus pungens), inland saltgrass (Distichlis spicata), Nuttall’s alkali grass (Puccinellia nuttalliana), foxtail barley (Hordeum jubatum), red swampfire (Salicornia rubra) and freshwater cordgrass (Spartina pectinata), and shrubs such as black greasewood (Sarcobatus vermiculatus). During exceptionally wet years, an increase in precipitation can dilute the salt concentration in the soils in some cases, allowing for less salt-tolerant species to occur. The distribution of this system extends into central Montana, where it occurs in the matrix of the Northwestern Great Plains Mixed Grass Prairie. However, these depressions are most concentrated to the north of the HiLine and Route 2, from the Blackfeet Reservation to the North Dakota Border. Individual occurrences can also be found across the Northwest Glaciated Plains north of the Missouri River.
Vegetation within this system is highly influenced by soil salinity and soil moisture. Salt-tolerant and halophytic species that typify this system include alkali bulrush (Schoenoplectus maritimus), common three square (Schoenoplectus pungens), inland saltgrass (Distichlis spicata), Nuttall’s alkali grass (Puccinellia nuttalliana), foxtail barley (Hordeum jubatum), red swampfire (Salicornia rubra) and freshwater cordgrass (Spartina pectinata), and shrubs such as black greasewood (Sarcobatus vermiculatus). Other species include western wheatgrass (Pascopyrum smithii) and foxtail barley (Hordeum jubatum). Plant zonation related to soil salinity is often apparent in these systems. with distinct rings occurring around the fringe of the depression. In deeper, more depressed halophytic habitats, red swampfire or prairie cordgrass will dominate with Nuttall’s alkali grass found directly upslope, followed by inland saltgrass. Shrubs such as greasewood and winterfat (Krascheninnikovia lanata) are common around the outer margins of this system. Pursh seepweed (Suaeda calceoliformis), annual goosefoot (Chenopodium species) and seaside arrowgrass (Triglochin maritima) are common forbs.
In northeastern Montana, the alkali bulrush association occurs as an emergent band around open water or as zonal vegetation around other plant associations. Water tables are often high, often remaining above the soil surface at least through late summer. Soils are poorly drained, alkaline Entisols. Alkali bulrush forms dense, monotypic stands with up to 91% cover. In some areas along the wetland edge, very minor amounts of common spikerush (Eleocharis palustris) may be present. Alkali bulrush can survive periods of total inundation up to 1 meter (3.3 feet) deep, as well as drought periods where the water table remains less than 1 meter below the soil surface. It is a vigorously rhizomatous species that colonizes and spreads when the water table is within 10 centimeters (4 inches) of the surface. Cover of alkali bulrush may be replaced by red swampfire and other associated species during drought years.
Red swampfire occurs in the drawdown zone that is flooded during the early part of the growing season but where the water table drops below soil surface by late spring or early summer. Soils in this zone usually have silty-clay to clay texture, and the soil surface is covered with salt crusts. Principle salts are sulfates and chlorides of sodium and magnesium. It is one of a very few species that can persist in these hyper-saline conditions when the water table drops below the soil surface (Dodd and Coupland,1966).
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