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Macrophytes

Scirpus maritimus and S. robustus often occur in pure stands (Keith 1961; Hyer 1963; Palmisano 1970; Eilers 1975; Adam 1981; Shay and Shay 1986) that occupy discrete elevational zones (Verhoeven 1980; Jacobson and Jacobson 1989). These bulrush species have physiological mechanisms that enable them to prosper in moderately saline environments and survive when salinities increase past the point tolerated by many other vascular hydrophytes. Laboratory cultures show high germination and growth in freshwater, especially by S. robustus, but the ability of both species to compete in nature with glycophytes in freshwater seems low. Compared with some of their potentially important competitors, S. maritimus and S. robustus probably survive better in nutrient-poor sediments or exposed sites. The two bulrushes are shallow-water plants, intolerant of flooding for extended periods but adapted to survive low water levels, periodic exposure of bottom substrates, and drought. Achenes of both bulrushes disperse widely, allowing pioneering by sexual reproduction on bare substrates when moisture conditions permit. Nevertheless, compared to many other emergent hydrophytes, these bulrushes are relatively short, and competition, especially for light, seems to be an important factor in their spatial and temporal distribution.

Despite these limitations, S. maritimus and S. robustus commonly coexist with many other emergent hydrophytes. For example, as many as 72 other species of vascular plants may occur on small (generally 2 × 2 m, but some larger) plots with S. maritimus in British salt marshes (Adam 1981). Scirpus maritimus has a much wider distribution, a more extensive literature, and consequently a much longer list of associated species (Appendix A) than S. robustus (Appendix B). Although the information in these appendixes does not reflect a random sample of the habitat of the two bulrushes, species or genera showing high frequency of association with them may be important competitors in managed wetlands. However, most species listed in these tables are not serious competitors. Examples of the latter are ground-layer species or vernal species that mature earlier than S. maritimus (Handoo and Kaul 1982). Also, neither positive nor negative associations imply anything about competition, which is difficult to prove even in controlled environments.

Based on sources in Appendix A, two groups of plants are the most frequent associates of S. maritimus. The first includes much taller, often long-lived perennial graminoids—especially Phragmites australis, Typha latifolia, S. acutus, and S. tabernaemontani—that often shade out S. maritimus. Although some of these plants have a tendency toward halophytism, their height alone probably provides a competitive advantage over S. maritimus in deeper, more permanently flooded wetlands (Nelson 1954; Stewart and Kantrud 1972; Shay and Shay 1986; Kantrud et al. 1989). The number of emergent species associated with S. maritimus decreases with increasing water depth (Reinink and Van der Toorn 1975). Small increases in water depth for most of the year enable the more aggressive Phragmites australis and Typha latifolia to shade out stands of S. maritimus in Czechoslovakian fish ponds (Fiala et al. 1968; Fiala and Kvet 1971). Competition for sunlight probably allows Phragmites australis to replace S. maritimus in the absence of grazing by horses in the French Camargue (Duncan and D'Herbes 1982). Similarly, idle, ungrazed conditions may cause stands of Typha spp. and S. acutus to replace S. maritimus in many northern Great Plains wetlands (Kantrud et al. 1989). Conversely, S. maritimus may invade wetlands when Typha is reduced (Nelson and Dietz 1966).

The second group of frequent associates of S. maritimus includes Triglochin maritima, Eleocharis palustris, Atriplex patula, S. americanus, Hordeum jubatum, and several species of Juncus. These plants are similar to S. maritimus in height and show strong halophytic tendencies. Increases or decreases in the competitive advantage of these species over S. maritimus are to be expected with subtle changes in salinity, elevation, disturbance and inundation regime, substrate texture, and many other factors (Ranwell 1961; Hyer 1963; Dodd and Coupland 1966b; Walker and Coupland 1970; Eilers 1975; Hutchinson 1982; Ewing 1983; Vince and Snow 1984; Shay and Shay 1986; Price et al. 1988). Bolen (1964) noted invasion of S. acutus, S. olneyi, Distichlis spicata, and Eleocharis rostellata in a relict community of S. maritimus surviving under abnormally low soil salinity and presumed the eventual elimination of the community. The hybrid Spartina townsendii, discovered in England in 1870, can eliminate Scirpus maritimus (Oliver 1925). Similarly, stands of the introduced Spartina patens in Oregon wetlands are expected to replace the native Deschampsia cespitosa-Scirpus maritimus community (Frenkel and Boss 1988). Conversely, introduced or adventive populations of S. maritimus sometimes replace native emergent species in Australia (Congdon 1981).

Stands of S. maritimus also frequently intersperse with submersed and floating angiosperms, especially with Lemna spp., Potamogeton pectinatus, Ranunculus spp., Ruppia spp., Spirodela polyrrhiza, and Zannichellia palustris (Metcalf 1931; Jensen 1940; Walker and Coupland 1970; Stewart and Kantrud 1971; Chapman 1974; Molinier and Tallon 1974, cited in Podlejski 1981; Riley and McKay 1980; Britton and Podlejski 1981; Lieffers 1981; Rejmankova 1978; Grillas and Duncan 1986). Several of these species, particularly Ruppia spp. and Zannichellia palustris, often fill in gaps in stands of Scirpus maritimus created by mowing, grazing, or ice scour (Tyler 1969).

Frequent associates of the coastal-dwelling S. robustus are the salt-tolerant graminoids Distichlis spicata, Spartina alterniflora, S. cynosuroides, S. patens, and Scirpus olneyi (Appendix B). All except Distichlis spicata are similar in height to or taller than S. robustus. Of the major emergent hydrophytes in coastal Louisiana wetlands, Distichlis spicata shows the strongest association with S. robustus, but its major competitor is Spartina patens (Palmisano 1970). This deep-rooted species tolerates a greater range of salinities and water levels than Scirpus robustus. Thus, standard management for increasing amounts of S. robustus may not be suitable in wetlands dominated by Spartina patens (Palmisano 1970). The most common submerged and floating associates of this bulrush are Ruppia maritima, Myriophyllum spicatum, and Lemna minor (Palmisano 1970; Chabreck 1972; Kelley and Porcher 1986).

Chapman (1974) provided information on ecological succession in salt marshes worldwide as elevations increase or decrease through accretion or subsidence. He considered maritime salt marshes essentially developmental, with accumulations of soil and organic matter leading to drier, less saline wetlands shoreward. Under this concept, maritime wetland communities with Scirpus maritimus as a dominant usually develop at intermediate elevations and salinities on mudflats or where the submergents Ruppia, Zostera, or Zannichellia, or the emergent Spartina were formerly dominant. In turn, communities dominated by Phragmites, Carex, or Juncus usually replace the communities where Scirpus maritimus dominates (Johannessen 1961a; Van Donselarr et al. 1961; Ranwell 1964; Flowers 1973; Chapman 1974; Jefferson 1974). For inland (Nebraska) wetlands, Ungar et al. (1969) outlined a hypothetical succession based on reduced salinity and lowered water levels. They suggested that a S. maritimus-dominated community could succeed communities of submersed (Ruppia) or emersed (Salicornia) species. Communities dominated by Hordeum or Iva species or Distichlis spicata could then replace the Scirpus maritimus community when water levels or salinities fall, or the sites could revert to bare salt pans in a cyclical pattern if salinity and moisture levels stabilize.