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The lower third of the intertidal zone in the fresh wetlands supports 1 m tall plants with very large heart-shaped leaf blades. Some areas of the lower intertidal zone are nearly bare "mudflats" (texture varies from sandy to sandy silt, silty, or organic), which may have sparse cover of submergent species or small emergent species. In the brackish wetlands, vascular plants are scarce or absent in the lower intertidal zone because no vascular plant species thrives in this combination of salinity and prolonged tidal flooding alternating with exposure to the air. The middle third of the intertidal zone supports a diverse mixture of plants in the fresh marshes; this community often forms a narrow (one to a few meters wide) belt along pool and creek banks, and may colonize intercreek areas, where ice, muskrats, or other agents disturb soil and vegetation (Connors et al., 2000).

Cattail, purple loosestrife, or common reed (color plate 4) typically dominate the upper intertidal zone in fresh or brackish marshes. Areas at or just above MHW in fresh-tidal wetlands support vegetation dominated by herbs, shrubs, or trees; in the most brackish wetland (Piermont Marsh) there are common reed (Phragmites aus-tralis) stands as well as salt meadows with a mixture of several dense low grass-like plants. Where MHW is situated at the upland edge of fresh-tidal marshes there is a diverse assemblage of wetland and upland plants influenced by irregular brief tidalflooding; rare plants or habitat-limited species may occur (for example, winged monkeyflower [Mimulus alatus] Sharma and Kiviat, 1994, car-dinalflower [Lobelia cardinalis], and closed gentian [Gentiana andrewsii]). Vascular plants were discussed by Kiviat (1974,1978a), Ristich, Fredrick, and Buckley (1976), Buckley and Ristich (1976), Kiviat et al. (1982), DeVries and DeWitt (1987), Senerchia-Nardone, Reilly, and Holland (1986), Reschke (1990), Kiviat and Beecher (1991), Stevens (2001), and Kiviat and Stevens (2001).

Swamps. "Marshes" are dominated by herbaceous plants although the marshes of the Hudson often contain scattered shrubs or trees; "swamps" are dominated by woody plants (shrubs or trees). Tidal swamps maybe dominated by trees, shrubs, or both; each vegetation layer (tree, shrub, herb, moss) may be species rich or poor. At lower ele vations, trees tend to be small (less than 30 cm diameter-at-breast-height [dbh]) and often have damaged crowns or multiple stems, whereas larger, healthier-looking trees grow at higher elevations on creek bank levees or in other supratidal habitats. The shrub layer is about 2-3 m tall and maybe quite dense. Purpleloosestrife (Lythrum salicaria), arrow arum (Peltandra virginica), and more than 50 other species of herbs occur in tidal swamps (Westad and Kiviat, 1986; Westad, 1987). The herbs grow mostly on top of the "hummocks" or woody root crowns formed by red maple and certain shrubs; a few herbs (for example, purple loosestrife, arrow arum) occur in the "hollows" on the swamp floor, on soil between the hummocks. As the substrate el-evationincreasesfromintertidal swamp tosuprati-dal swamp, hummocks are smaller and fewer, and herbs occur more in the hollows. Bryophyte communities (mosses and liverworts) are well developed in fresh-tidal swamps (Leonardi and Kiviat, 1990; Leonardi, 1991) and these nonvascular plants often cover the hummocks, bases of tree and shrub stems, and logs.

Species diversity. Species richness (number of species) of vascular plants on 0.25 m2 sampling plots tends to increase with elevation in Hudson River tidal marshes, although this is not consistent across all elevation levels or marsh types. In 1996 data on the vegetation of fifteen reference marshes, richness was zero to six species in the subtidal shallows and lower intertidal zone, and zero to ten species in the middle and upper intertidal zones (Kiviat et al., unpublished data). Subtidal vegetation may comprise a single, dominant species such as wild celery or, in more sheltered areas such as quiet secondary tidal creeks, a mixture of several species. Even in dense water-chestnut beds there may be three kinds of duckweeds (Lemna minor, Spirodela polyrrhiza, Wolffia) among the water-chestnut leaves. In the lower intertidal zone, spat-terdock is often highly dominant; however, there can be mixtures of several species. Middle and upper intertidal zones tend to be species-rich, espe-ciallyinthe sheltered, high-organic matter marshes where muskrat or human activities, or the creek-associated disturbances, have reduced dominance by cattail (Typha). It is these seemingly haphazard assemblages of arrow arum, purple loosestrife, broadleaf arrowhead, dotted smartweed, water-hemp, cattails, and other plants, and the birds and butterflies attracted to them, that characterize the creek banks that many human visitors see from a boat.

Biomass. Peak aboveground standing crop (biomass) of vascular vegetation increases with elevation in the Hudson River marshes. Biomass is about 140 g dry mass • m-2 in submergent vegetation, 360 in water-chestnut, 150 in spatter-dock, 210 in pickerelweed, and 1,100 in cattail in fresh marshes (Kiviat and Beecher, 1991; also see Mihocko et al., 2003). In brackish marshes, biomass is roughly 1,000 g dry mass • m-2 in cattail, big cordgrass, saltmarsh cordgrass, purple loosestrife, and common reed, and 400 in spike grass, salt-meadow cordgrass, arrow arum, and pickerelweed (common reed attains the highest values) (Buckley and Ristich, 1977). These data are from dominant stands, optimum salinities, and seasonal peaks of biomass; biomass varies considerably. Peak biomass is attained about mid-July to mid-August, although some species (e.g., arrow-arum, river bulrush) may reach peak biomass early and begin to senesce and lose aboveground biomass in August.

Basal area (the aggregate, cross-sectional area of tree stems per unit area of ground at 1.4 m above the soil) is a good indicator of the dominance of trees. Three Hudson River tidal swamps (Mill Creek, Cruger Island Neck, and Mudder Kill) supported 8-34 m2 • ha-1 basal area, low to moderately high values (Kiviat, unpublished data). Like herbaceous plant biomass, tree basal area also increases with elevation from the upper intertidal zone to the supratidal zone. Species richness of trees, shrubs, andherbs often increases with elevationin swamps as well. The swamps studied had five to ten tree species (with stems equal to or greater than 2.5 cm dbh) and 70-467 tree stems • ha-1. Many trees (8-28 percent) had multiple stems, indicating ice damage or other stress.

Lower "plants." The algae are important organisms of the mud and plant surfaces in the intertidal zone, as well as of the plankton. Sixty-three genera of phy-toplankton were sampled in a small, recently restored marsh at Camp Smith near Peekskill (Ristich et al., 1976). The generic composition of the phy-toplankton in the marsh was considered similar to that in the main river. Bryophytes (mosses and liverworts) are prominent, even lush, on woody stem bases and exposed roots, purple loosestrife bases, logs, duckblinds, pilings, and lumber in the upper intertidal zone and (especially) the supratidal zone of the swamps and to a lesser extent marshes. A single floristic study has been conducted on tidal swamp bryophytes (Leonardi and Kiviat, 1989, 1990; Leonardi, 1991). Lichens occur on live and dead woody materials in the tidal wetlands (especially on bases, stems, and branches of swamp trees and shrubs), as well as on rocks in the upper intertidal zone and supratidal zone (Feeley-Connor, 1978;Royte, 1985). Lichens appear neither diverse nor abundant in the tidal wetlands, probably due to air (and water?) pollution.

Vegetation patterns in space and time. Vegetation displays spatial patterns and also changes over time. Temporal change in low salinity tidal wetlands, absent major disturbance, generally includes increase in stature, biomass, persistence aboveground in winter, resistance to decay, and abundance of woody species. Vegetation change is driven mainly by increase in substrate elevation from tidal deposition of sediment and in situ production of organic matter. In Tivoli North Bay, the sequence of change generally passes from SAV to spatterdock (and pickerelweed) to mixed middle intertidal species to narrowleaf cattail to woody species (or common reed). This pattern is not necessarily representative of other Hudson River wetlands, due to effects of sea level rise, variable rates of sediment deposition, different dominant plant species, and a variety of disturbances such as scouring by floods and ice. Common reed presumably speeds the buildup of the marsh surface (Rooth, Stevenson, and Cornwell, 2003) but how long Hudson River reed stands are stable, and whether they are invaded by woody plants, is unknown.

In 1991 and 1997, plant communities at the time of peak aboveground biomass (late July -early September) were mapped with true color stereo aerial photography. Simplified cover types of four emergent marshes are shown in Figure 20.3. With the partial exception of Stockport Flats, the

Figure 20.3. Emergent vegetation of four major tidal marshes at the Hudson River National Estuar-ine Research Reserve sites in summer 1997. The "other graminoid" class includes: narrowleaf cattail and sweetflag in Tivoli Bays; narrowleaf cattail, sweetflag, and bulrushes (Scirpus spp.) at Stockport Flats; narrowleaf cattail and bulrushes at Iona; and narrowleaf cattail, saltmarsh cordgrass, saltmeadow cordgrass, saltgrass, and bulrushes at Piermont (Hudson River National Estuarine Research Reserve, unpublished data).

Figure 20.3. Emergent vegetation of four major tidal marshes at the Hudson River National Estuar-ine Research Reserve sites in summer 1997. The "other graminoid" class includes: narrowleaf cattail and sweetflag in Tivoli Bays; narrowleaf cattail, sweetflag, and bulrushes (Scirpus spp.) at Stockport Flats; narrowleaf cattail and bulrushes at Iona; and narrowleaf cattail, saltmarsh cordgrass, saltmeadow cordgrass, saltgrass, and bulrushes at Piermont (Hudson River National Estuarine Research Reserve, unpublished data).

marshes are dominated by graminoid vegetation. The vegetated lower intertidal zone, dominated by emergent broadleaf plants (spatterdock and pick-erelweed), makes up a significant portion of Tivoli Bays and Stockport Flats. Wildrice is also abundant in the lower to middle intertidal zone at Stockport Flats.

Common reed occurs at all four marshes and is the dominant cover type at Piermont and Iona. Reed has been spreading at the Reserve sites for manyyears (Winogrond and Kiviat, 1997). At Iona and Tivoli the spread was nearly exponential in 1991 and continued at a high rate to at least 1997 (Fig. 20.4). Although purple loosestrife is prominent in Tivoli North Bay, it occurs mainly in mixed stands with narrowleaf cattail. Purple loosestrife has declined in some marshes during the past three decades (Kiviat, personal observation).

Qualitative comparisons of the descriptions of sites in the 1930s and 1940s (Muenscher, 1935, 1937; Foley and Taber, 1951) with today's vegetation suggests a large increase in common reed and water-chestnut, and decreases in river bulrush, wild rice, and "muskgrass" or "stonewort" (Charophyta). Wild rice was rare in the early 1970s and has increased since, perhaps due to the mid-1970s cessation of herbicide use for water-chestnut control (Kiviat, personal observation). Wild celery has become much more abundant relative to the "invasive" Eurasian watermilfoil in the Tivoli Bays area during the same period (Kiviat, personal observation).

Figure 20.4. Decrease in cattail and and increase in common reed cover at Iona Island Marsh, 1991 to 1997 (Hudson River National Estuarine Research Reserve, unpublished data).

Table 20.4. Fauna of Hudson River tidal wetlands

Group

References

Findings

Mammals

Kiviat, 1978b, 1980a, 1994,

Low diversity, few resident species;

0 0

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