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Water Column

Depth

Scirpus maritimus and S. robustus are species of relatively shallow (less than 1 m deep) waters (Table 5). Scirpus robustus may be less tolerant of deep water, although few records are available. Scirpus maritimus seems to tolerate greater depths in freshwater or where bottoms are sandy, but these may be attributes of European subspecies (Dykyjova 1986).

Lieffers and Shay (1982b) measured 27 environmental variables in 24 Saskatchewan wetlands ranging from mixosaline to eusaline classified according to Cowardin et al. (1979). Of all the variables, mean water level correlated most with growth measures of S. maritimus. However, Lieffers and Shay (1982b) found that salinities vary colinearly with mean water levels. For example, during a year when the mean water level was 9 cm below the sediment surface in a stand of S. maritimus, the mean water conductivity was 55 mmho/cm and plant biomass was 21 g/m² dry weight. During another year, water depth was 16 cm above the sediment surface, conductivity fell to 12.3 mmho/cm, and S. maritimus biomass was 386 g/m² dry weight. Lieffers and Shay (1982b) also found that in all wetlands, stem densities peak at depths of about 25 cm, but aboveground biomass is highest when depths average 35 cm. This species withstands at least 5 years of dryness (Chapman 1974).

Clonal growth replaces seed production when water levels are high (Lieffers and Shay 1981). Scirpus maritimus plants in 40-cm-deep water may be only 5 cm taller but have nearly three times as many shoots and much greater leaf area than plants in water more than twice as deep (Dykyjova 1973). Belowground growth also decreases in deeper, fresher water (Dykyjova et al. 1972). Fiala and Kvet (1971) show how depth distribution of S. maritimus varies with exposure and probable competition: plants in exposed sites, where Typha and Phragmites are absent, grow in broad zones at depths from about 20 to 90 cm, but in sheltered locations where the two competitors dominate deeper waters, plants are restricted to narrow bands at about 40 cm depth.

In tidal wetlands in the Netherlands, the depth range of S. maritimus varies with tidal exposure (Coops and Smit 1991). Plants subjected to full tidal influence are found where water depth varies from just below to more than 1 m above MHW, whereas plants in areas with smaller tides shift toward the low waterline. The range of occurrence is smaller in wetlands that are totally separated from the sea than in tidally influenced areas. Coops and Smit (1991) found no overall relation between water depth and stem density, flowering percentage, or floral culm length at any sites but noted seedling production only in the tidally influenced areas. But when they held water at various depths above planted corms in outdoor containers, the numbers of leaves peaked at depths of 0-30 cm, belowground and aboveground biomass and culm width were greatest at depths of 30 cm, new shoot formation and flowering percentage were greatest at depths of 30-60 cm, and culm length was greatest at depths of 60 cm.

Temperature

The widespread distribution of S. maritimus suggests that this species tolerates a wide range of water temperature. In an Alberta lake inhabited by S. maritimus, water temperatures from April to August range from 0 to 32° C and closely follow air temperatures because the lake is shallow and turbid (White and Hartland-Rowe 1969). Van Wijk (1988) recorded water temperatures of 13.5-28.0° C during the growing season in a European wetland supporting S. maritimus. In Maryland, S. robustus grows where annual water temperatures range from -0.8 to 31° C; spring and summer water temperatures average 11.9° C and 25.6° C, respectively (Dietert and Shontz 1978).

Chemistry

Water salinity is a major factor in the distribution of S. maritimus and S. robustus in coastal and interior wetlands (Beeftink 1966; Stewart and Kantrud 1972; Hutchinson 1982; Newling et al. 1983; Lewis 1990; Shisler 1990). Scirpus maritimus occurs in waters containing 0.162-308 g/L salts, whereas S. robustus does not occur where salinities exceed 30 g/L (Table 6).

In most coastal areas, stands of S. maritimus usually develop best in the mesohaline portions of estuaries where freshwater flows into areas periodically immersed in seawater or the transitional areas between the sea and adjacent freshwater wetlands (Purer 1942; Filice 1954; Klavestad 1957; Beeftink 1966; Martini et al. 1979). The most well-developed stands are near the mouths of estuaries receiving substantial inflows of freshwater because salinities there are reduced (Ranwell 1972). However, S. maritimus may be absent from coastal wetlands where seawater salinity is low, such as on the northern Baltic (Siira 1970). In British Columbia, this species invades habitats when growing-season salinities of interstitial water fall to 3.5-15.5 g/L during early summer because of inflows of fresh river water (Hutchinson 1982). Obligate halophytes in full-strength seawater must develop cell-sap osmotic pressures greater than 20 bars to survive (Ranwell 1972). Arnold (1955, cited in Ranwell 1972) showed that the osmotic pressure of S. maritimus is only 14.9 bars, and therefore Ranwell considered the plant a facultative halophyte that also persists in nonsaline habitats. Dodd and Coupland (1966a) found similar low pressures in this species. Lohammar (1965) considered the plant a freshwater species in some parts of Sweden. Although plants may be less abundant, the fresher parts of an estuary can support stands of S. maritimus that are more robust and stay greener longer than in the more saline parts (Gillham 1957).

Inland populations of S. maritimus thrive in mesosaline (Cowardin et al. 1979) waters, where salinities usually are about 8-30 g/L (Metcalf 1931; Stewart and Kantrud 1972). However, stands occur in polysaline, eusaline, and even hypersaline waters where salinities sometimes exceed 300 g/L (Hammer and Heseltine 1988). During periods of low water, when evaporation raises salinities from a few grams per liter to around 10 g/L, S. maritimus frequently occurs in prairie wetlands normally dominated by Typha spp. and S. acutus (Krapu and Duebbert 1974; personal observation). Husak and Hejny (1978) also noted that in relatively fresh Czechoslovakian wetlands, S. maritimus communities become well established only in years when water levels are low. Millar (1969) opined that S. maritimus requires high salinity to survive in prairie wetlands, but because experiments show that the species flourishes in freshwater, he attributed its survival to the intolerance of other plants to saline environments. Nevertheless, the scarcity of S. maritimus in fresher prairie wetlands suggests an inability to compete with glycophytes there (Ungar 1974; Hammer and Heseltine 1988).

Scirpus robustus, which seldom occurs far from coastal waters, is a freshwater plant, according to Hackney and Cruz (1982). Others consider the plant a member of the oligohaline (0.5-5.0 g/L) or polyhaline (18-30 g/L) plant communities (Lewis 1990; Shisler 1990). Its optimum habitat seems to be wetlands with water salinities of about 3-10 g/L (Table 5). Scirpus robustus is less salt tolerant than most common salt marsh species. Stands develop best between uplands and intertidal marsh where there is some freshwater runoff (Johnson and Raup 1947; Bourn and Cottam 1950; Shellhammer et al. 1982; Newling et al. 1983; Zedler and Beare 1986). Impoundments may be the sources of these freshwaters (Whitman and Cole 1987). The occurrence of S. robustus in brackish to saline sites may involve ecological displacement rather than an optimum response to the environment (Pearcy et al. 1982). Plants sometimes occupy areas between stands of more salt-tolerant species (Ustin et al. 1982).

Palmisano (1970) used nutrient solutions and artificial seawater to show that only about 50% of S. robustus plants survive at salinities of 17 g/L; the upper limit of growth is about 21 g/L. His experiments with nutrient solutions and sodium chloride showed that growth is reduced 50% at 10 g/L and that growth almost ceases at 30 g/L. He also showed that culm growth is more restricted than root growth at higher salinity levels. The mean water salinity in natural stands in Palmisano's (1970) study area was 1.06 g/L.

Water salinity affects various aspects of the reproduction and growth of S. maritimus and S. robustus. Indoor greenhouse experiments of Kaushik (1963), who added calcium chloride and sodium chloride (1:2 ratio) and nutrients to tap water for his test solutions, showed that achene germination of S. maritimus decreases as salinities increase to about 15.4 g/L. At higher salinities, achenes probably cannot absorb enough water to germinate. Injury of achenes by salinity is in direct proportion to salt concentrations and length of treatment. With young plants, increased salinity reduces length, fresh weight, and number of leaves per plant as well as length of roots and root hairs. However, mortality (greater than about 20%) of young plants is not significant until salinities increase to about 5.8 g/L. In adult plants, stem diameter, height, and weight are reduced by salinities above this level. Kaushik (1963) also measured several reproductive variables and mortality of adult plants with the same test solutions. Unlike the growth measures, salinities of about 5.8 g/L stimulate spikelet and achene production. Mortality of adult S. maritimus begins when salinities reach about 11.5 g/L. Two-thirds of the plants die when salinities are raised to about 15.4 g/L for 8.3 days.

Corms also germinate best in freshwater (Mercado et al. 1971) but can withstand high salinities (Lieffers and Shay 1982b). Indoor tests by Mercado et al. (1971) revealed germination inhibition in S. maritimus, corms held 9 days in a 23.4 g/L sodium chloride solution. However, 20% of the corms held for 9 days in solutions as saline as 29.2 g/L germinate after transfer to freshwater for 8 days. Mercado et al. (1971) also showed that plants grown in 4-10 g/L sodium chloride solutions produce larger numbers and weights of corms than plants grown in dilute solutions. Scirpus maritimus shows less belowground growth in deeper, fresher water (Dykyjova et al. 1972).

Dietert and Shontz (1978) found that 6-week-old cultured seedlings of S. robustus irrigated with freshwater grow much taller than those irrigated with 10 g/L salinity water and that 20 g/L water retards all growth. During this study, achenes were collected in an area where natural salinity was 11.4 g/L (spring) and 12.3 g/L (summer). Later culture experiments showed that even low levels of salinity inhibit growth of this species, whereas photosynthesis is inhibited only at moderate to high salinities (Pearcy and Ustin 1984). Salt tolerance of S. robustus is greater during the germination stage than during later stages of development (George 1980).

Little is known about the effects of specific ions on the two bulrushes. In otherwise suitable environments, total ionic content rather than concentrations of particular ions seems to determine whether S. maritimus occurs in prairie wetlands (Stewart and Kantrud 1972; Table 7). In wetlands in Saskatchewan and in other areas of the Great Plains, populations of S. maritimus thrive where sodium and magnesium ions reach high concentrations (Lieffers and Shay 1982b).

Wetlands that support the two bulrushes vary greatly in water chemistry (Table 8). Scirpus maritimus sometimes occurs in wetlands high in sewage effluents or nutrients (Chavan and Sabnis 1959; Kvet and Ondok 1973) and can withstand heavy contamination by oil (Stebbings 1970). Palmisano (1967) found no significant correlations between the distribution of S. robustus and alkalinity, hardness, or concentrations of magnesium, chloride, potassium, phosphorus, and calcium in Louisiana wetlands. Wilkinson (1970) found that this species grows best where water pH is less than 6.5, even though sediment pH may be much lower (to 3.1).

Fluctuations

Periodic occurrence of water or fluctuating water levels often characterize habitat of S. maritimus (Dodd et al. 1964; Hammer et al. 1975; Haslam et al. 1975; Hejny and Husak 1978; Husak and Hejny 1978). Bottom elevation, which in tidal wetlands controls the amount of aerial exposure required for the plant to attain high biomass, influences distribution (Hutchinson 1982; Karagatzides and Hutchinson 1991). In coastal British Columbia, S. maritimus occurs from the edge of the tidal flats far into the high marsh. This corresponds to an area that is submersed no more than about 60% of the time under normal tide conditions; biomass relative to other emerged plants is greatest in the zone submersed about 30% of the time (Hutchinson 1982). Plants that withstand wave action and flooding at each high tide tend to be much shorter than plants at slightly higher elevations (Stirrett 1954). About 80% of the aboveground production of S. maritimus may be carried away by tides and either deposited at higher elevations or removed to the estuary (Eilers 1975).

Populations of S. maritimus in inland waters also thrive under fluctuating water regimes. In the prairies, the frequency of occurrence of this bulrush sometimes increases greatly with gradual declines in water levels (Walker 1965). The rates of S. maritimus achene ger- mination are higher in bottom cores from a Manitoba wetland subjected to artificial moist-sediment conditions than in cores flooded with 2-3 cm of water Pederson 1983). Lieffers and Shay (1981, 1982a,b) show that populatioada in prairie Canada thrive where seasonal water levels fluctuate greatly, often a meter or more in the range 70 cm above to 40 cm below the sediment surface. This range is greater than that of several other emergents with which S. maritimus often associates in this area (Shay and Shay 1986). The abundance of S. maritimus declines under stable or rising water regimes, especially in less saline wetlands (Walker 1965; Millar 1973).

In Utah, stands of S. maritimus quickly establish when very shallow water flows across barren salt flats for 2-3 years (Nelson 1954). There, newly flooded salt flats dominated by Salicornia also develop persistent stands of S. maritimus and other emergents where depths of surface waters fluctuate up to 20 cm (Kadlec and Smith 1989).

Scirpus robustus is naturally adapted to fluctuating water levels. Philipp and Brown (1965) include S. robustus in a group of plants at least partially submerged with each daily tidal cycle. Sasser (1977) found this bulrush mostly in areas of Louisiana salt marshes that flood about 4,000-5,000 h/year during about 125-150 floodings/year, but concluded that the plant's occurrence was not strongly related to either the frequency or duration of flooding. In tidal wetlands, the duration of sediment submergence may affect the local distribution of this species more than depth of submergence, amounts and concentrations of sediment salts, water and organic matter content of the sediments, or seasonal changes in water salinity (Mall 1969). Scirpus robustus seems to grow best where tidal amplitudes are less than 1 m (Palmisano and Newsom 1968; Flowers 1973), although this may be modified by wind speed and direction and by flow from adjacent rivers (Hackney and Cruz 1982). (See the sections on propagation and management for more information on the effects of water-level fluctuations on this species.)