Applied Wetlands Science - Chapter 6 pot

Zentner, John “Wetland Enhancement, Restoration, and Creation” Applied Wetlands Science and Technology Editor Donald M. Kent Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 6 Wetland Enhancement, Restoration, and Creation John Zentner CONTENTS Site Selection and Analysis Topography Vegetation Association Mapping Site History and Current Status Hydrological Analysis Soil Analysis Cultural Constraints Adjacent Site Conditions The Use of Template Associations Small-Scale Experimental Construction Goal Setting Elements of a Goal Statement Goal-Setting Process Practicability Construction Design Geography Size and Shape Location Slope Adjacent Uses Hydrology Hydroperiod and Depth Water Supply Soil ©2001 CRC Press LLC Vegetation Succession Planting Design Plant Selection Stock Selection Planting Density Weed Control Cultural Issues Mosquitoes Water Quality Implementation Construction Sequencing Protective Flagging Weed Removal Salvaging Grading Planting Water Supply Fencing As Builts Maintenance Weed Control Erosion Control Herbivory Plant Care Irrigation System Maintenance Litter Removal General Maintenance Frequency Minimizing Maintenance Efforts Research Needs References Freshwater wetlands develop at elevations above open water aquatic habitats and below uplands. They are found in a wide range of hydrologic conditions, from permanently flooded (to a depth of 1 m) to seasonally saturated. Freshwater wetlands occur on a wide variety of soil types including both organic and mineral soils, as well as in nonsoil conditions. Most freshwater wetlands are either freshwater marshes or riparian woodlands. Freshwater marshes are dominated by herbaceous emergents and can be divided into three general categories reflective of hydrology (Figure 1). Wet meadows are temporarily or intermittently flooded and dominated by graminoids and Juncaceae. In the United States, seasonal marshes are seasonally flooded or saturated and dominated by Cyperaceae and Juncaceae. Perennial marshes are permanently or Figure 1 Freshwater marshes are dominated by herbaceous emergents and can be divided into three hydrological categories: perennial marsh, seasonal marsh, and wet meadow. UPLAND WET MEADOW SEASONAL MARSH PERENNIAL MARSH OPEN WATER Hydrology Dominant Plants Temporarily flooded or intermittently flooded Seasonally flooded or saturated Permanently flooded or semi-permanently flooded Cattail, bulrush, tules Sedges, rushesGrasses, rushes ©2001 CRC Press LLC ©2001 CRC Press LLC semipermanently flooded and dominated by tall emergents such as cattails ( Typha latifolia ) or bulrush ( Scirpus acutus ). Riparian woodlands are dominated by shrubs and trees and are characterized by impermanent and varying periods of inundation or root zone saturation during the growing season. Compared to freshwater marshes, riparian woodlands occur on relatively permeable and well-oxygenated substrates. As with freshwater marshes, riparian woodlands can be categorized by hydrological regime (Figure 2). High terrace woodlands are temporarily flooded and dominated by a variety of species, especially oaks ( Quercus spp.), that are typified by heavy seeds with relatively longer viability. Mid-terrace woodlands are seasonally flooded and generally dominated by green ash ( Fraxinus pennsylvanica ), sycamore ( Platanus occidentalis ), and other species with medium weight seeds. And low terrace woodlands are semipermanently flooded and generally dominated by willows ( Salix spp.), silver maple ( Acer sac- charinum ), and similar species with relatively light seeds of limited viability. These categories correspond to Categories V (higher hardwood wetlands), IV (medium hardwood wetlands), and III (lower hardwood wetlands), as described by Cowardin et al. (1979), Larson et al. (1981), and Clark and Benforado (1981). Coastal wetlands share many of the characteristics of freshwater wetlands and are generally defined as those wetlands that lie within the realm and effects, however minor, of tidal salt water. As such, coastal wetlands include saltmarsh, fresh and brackish tidal marsh, and, in tropical waters, mangrove. Saltmarsh is the most ubiquitous type of coastal wetland, occurring on all coasts where appropriate substrate and tidal regimes are present (Figure 3). Saltmarshes are distributed over a relatively broad salinity range, from the high intertidal zone to the oligohaline habitats upstream on tidal tributaries where salinity may never exceed 10 parts per thousand. In the United States, various species of cordgrass ( Spartina spp.) are dominant, with rushes ( Juncus spp.) also common. On the Pacific Coast, Spartina foliosa and pickleweed ( Salicornia virginica ) are common. On the Gulf Coast and in the southeast, other species of cordgrass ( Spartina alterniflora , S . patens ), saltgrass ( Distichlis spicata ), and black needlerush ( Juncus roemerianus ) are typical. Sawgrass ( Cladium jamaicense ) marshes, such as the Everglades, occur in more brackish water. On the Atlantic Coast and to the northeast, Spartina cyno- suroides , S . alterniflora , J . roemerianus , and some other species are common. Wax myrtle ( Myrica cerifera ) and groundsel tree ( Baccharis halimifolia ) are typical shrubs associated with salt marshes from the Gulf Coast to New England. Oligohaline (salinity of 0.5 to 5.0 ppt) and tidal freshwater marshes (salinity is less than 0.5 ppt) are herbaceous wetlands located in tidally influenced rivers or streams. The plant community exhibits a diverse mixture of true marine species and typical freshwater taxa that tolerate low salinities (Cowardin et al., 1979; Lewis, 1990). Mangrove forests are limited in distribution to subtropical and tropical zones (Figure 4). In the United States, they occur predominantly in southern Florida, sparsely along the Gulf Coast to the Laguna Madre of Texas, and extensively in Puerto Rico (Kuenzler, 1974). Many species of trees of different families are called mangroves, with black mangrove ( Avicennia germinans ), red mangrove ( Rhizo- phora mangle ), and white mangrove ( Laguncularia racemosa ) common to the Figure 2 Riparian woodlands are dominated by shrubs and trees. As with freshwater marshes, riparian woodlands can be categorized by hydrological regime: semipermanently flooded, seasonally flooded, and temporarily flooded. UPLAND HIGH TERRACE WOODLAND MID-TERRACE WOODLAND WOODLAND LOW TERRACE Hydrology Dominant Plants Temporarily flooded Seasonally flooded Oaks Green Ash, Sycamore Willows, Silver Maple Semi-Permanently flooded ©2001 CRC Press LLC ©2001 CRC Press LLC United States. There are an additional 31 species worldwide (Tomlinson, 1986). All mangroves have features in common that are adaptations for survival in the saline conditions of the intertidal zone (Tomlinson, 1986). Morphological adapta- tions include the aerial roots (pneumatophores) of black mangrove that provide for gas exchange and the viviparous, floating propagules of red mangroves. In fact, nearly all species of mangroves are viviparous, with the white mangrove being an exception. Physiological mechanisms for dealing with salt excretion or exclusion are also characteristic of mangroves. Lewis (1990) recently defined wetland enhancement, restoration, and creation. Enhancement is an increase in values afforded to a specific vegetation association by construction actions. Restoration is the recreation of a specific vegetation asso- ciation on a site where that association was once known to occur. Creation is the construction of a wetland from an upland or aquatic site. The term construction will be used to encompass enhancement, restoration, and creation in this chapter. The earliest wetland construction projects may have occurred many thousands of years ago with the manipulation and creation of freshwater and coastal wetlands to enhance rice or fish harvests. Waterfowl conservation and hunting organizations have been responsible for numerous wetland construction projects in this century. New York, especially, had numerous small freshwater marshes created in the mid- 1950s (Dane, 1959). Specific problems associated with wetlands, such as nuisance mosquitoes, have also resulted in a number of useful research and enhancement practices. More recently, interest in wetland construction is a response to regulatory requirements. Nevertheless, there is also a greater public interest and understanding Figure 3 Saltmarshes are the most widely distributed of coastal wetlands. They occur over a relatively broad salinity range from the high intertidal zone to oligohaline habitats on tidal tributaries. ©2001 CRC Press LLC of the environmental values of wetlands and a desire for the recreation of lost or diminished landscapes and values. This chapter discusses the construction of freshwater marshes, riparian wood- lands, and coastal wetlands. The construction process is described in five steps inherent to designing, building, and maintaining wetland landscapes. These steps are site analysis, goal setting, construction design, implementation, and maintenance. Implicit in these steps are two tenets. First, specification of a target vegetation association (TVA) is the primary goal. A TVA provides the habitat for any plant or wildlife populations that may be desired on the project site and, because of its structural nature, facilitates maintenance and monitoring efforts. Second, in con- struction planning there is no substitution for directed observation of the project site and the TVA. SITE SELECTION AND ANALYSIS The ideal construction site will most closely meet the requirements of the community to be constructed. Therefore, the goals of the project will have substantial bearing on final site selection. Construction projects inherently require understanding the initial cause of habitat differentiation or degradation and determining the prob- ability that the habitat can be constructed and maintained. The question of habitat displacement inevitably needs to be addressed in the site selection process as well. Site analysis is the first tangible step in a wetland construction project. The analysis may be completed during a wetland delineation effort, as the first step in a Figure 4 Mangrove forests are found throughout the subtropics and tropics. The tree species which comprise mangrove forests have morphological and physiological adapta- tions for surviving in intertidal saline conditions. ©2001 CRC Press LLC neighborhood restoration program, or from many other perspectives. This step pro- vides the basic framework for goal setting, identification of the TVAs, and develop- ment of performance standards. The site analysis typically includes a topographic assessment, wetland vegetation association mapping and analyses, and a review of historic conditions. The analyses will also include an assessment of hydrologic, soil, and cultural conditions, and examination of any nearby templates, or examples, of TVAs. Small-scale experi- mental wetland construction efforts should be initiated at this time if possible. Generally, the initial site analysis is completed within a period of three weeks to three months, with a relatively intensive effort in defining site topography, vegetation associations, soils, and historic and cultural conditions in the first few weeks. Less extensive effort is expended in monitoring hydrology and constructing and observing experimental wetlands over the remaining period. Topography Elevation and slope are two of the more critical factors in determining the success of wetland construction projects. It is difficult to recommend an absolute planting elevation for a given species because of local and geographic variability. Synergistic effects may also alter growth at given elevations. For example, reduced salinity may permit growth of smooth cordgrass at higher and lower elevations than those typically recommended for the species. The optimum elevation can be determined empirically by observing and mea- suring the lower and upper elevation limits of a nearby natural wetland. Lewis (1990) recommends that the lowest and highest points should be disregarded, and only the middle range used for planting. If reference information is unavailable, an adequate test program should be conducted prior to initiating construction. Some degree of slope is essential for proper drainage. A gradual slope will increase the area available for planting and will dissipate hydrologic energy over a greater area, thereby reducing the possibility of erosion (Broome, 1990). Slope should be toward water sources to minimize ponding as substrates settle following site preparation. Gentle slopes do not drain as extensively as steeper ones, which is generally beneficial to most wetland vegetation. However, gentle slopes are more susceptible than steeper slopes to ponding, which can be especially problematic for riparian woodland species unless the soils are also relatively permeable. Vegetation Association Mapping Wetland vegetation associations for the construction site and surrounding area can be identified, and their extent determined, by mapping from an aerial photograph. Aerial photographs are usually available as stock or library film from an aerial survey center. These centers fly important regions every two to three years, providing a backlog or library of film that can be reproduced at a variety of scales. Requesting a half-tone mylar as well as a print of the project site is important. The mylar can be used to make blueprints for field use, and blackline prints can be reproduced and included in reports. ©2001 CRC Press LLC The borders of the vegetation associations are more precisely defined through on-site analysis after initial mapping from an aerial photograph. For marshes, ran- domly selected 1-m plots representative of each vegetation association can be sam- pled for species and cover. Randomly selected 0.25 hectare (ha) polygons (defined by tree cover and separation among woodland vegetation associations) are effective in woodlands. Table 1 provides an example of vegetation association mapping from a hypo- thetical freshwater wetland project site in California characterized by gently sloping hills surrounding a central creek. The result is a table that describes the vegetation associations and the absolute and relative cover of marsh and riparian woodland vegetation. A scan of the table and knowledge of the region reveal that three vegetation associations are present and that the site is dominated by nonnative species and species representative of disturbed areas. It is also appropriate at this time to identify and survey any upland areas that may be considered for wetland construction to ensure that valuable upland habitats are not inadvertently lost. Site History and Current Status Shisler (1990) states that if wetlands are not present there has to be a reason, especially for tidal wetlands. Determining this reason is fundamental to reviewing a site for potential wetland habitat construction. If possible, past functioning of the Table 1 Example of Vegetation Association Mapping from a Hypothetical Project Site in California 1 Vegetation Association Plant Species Area (ha) Cover (%) Wet meadow 1.4 80 Lolium perenne* 40 Hordeum hystrix* 30 Elymus triticoides 20 Elymus glaucus 10 Seasonal marsh 1.4 50 Lolium perenne* 40 Juncus balticus 30 Lotus corniculatus* 20 Polypogon monspieliensis* 10 Perennial marsh 0.9 100 Typha latifolia** 80 Scirpus acutus 20 Low-terrace woodland 0.5 100 Salix lasiolepis** 100 * Nonnative species. **Species indicative of disturbance. 1 The table provides absolute and relative information for use in site analysis and target vegetation association selection. [...]... approximately the mean annual flood line The mid-terrace zone occurs from the mean annual flood line to about the 1 0- to 15-year storm line, and the high-terrace woodland ©2001 CRC Press LLC occurs from the mid-terrace zone to the 100-year flood line Therefore, to circumvent blockage of flood waters by dense low-terrace riparian growth while still providing for wetlands, broad low terraces could be designed... 100-year storm level Vegetation association High impedance Moderate impedance Low impedance Channel bottom slope On-site Upstream Downstream Erosion and erosion potential On-site Upstream Downstream Sedimentation and sedimentation potential On-site Upstream Downstream Soil Soil properties have generally received little consideration in the design and construction of wetlands Exceptions include wetlands. .. inundation for wetlands abutting a river or lake Basin models are used to model wetlands isolated from riverine or lacustrine systems For tidal wetlands, a variety of models have been used to describe tidal flows In any case, tidal ranges ©2001 CRC Press LLC and important elevations (mean high water, etc.) must be identified; these are generally derived from local tidal gauges and other records Slope wetlands. .. nature of the vegetation Alternatively, the low-terrace zone could be restricted, and terraces would be constructed just above the mean annual flood line where mid-terrace riparian woodland, in friable soils, or marshes in indurated soils can be maintained (Figures 6 and 7) Additional hydrological factors, including water velocity, sediment loading, and erosion potential, will also be important Table 3 is... well, seasonal low water, mean annual flood level, and the 100-year flood level are the most pertinent water surface elevations Seasonal low water level defines the upper limit of perennial marsh and the lower limit of low-terrace riparian woodland Mean annual flow approximates the upper limit of seasonal marsh and low-terrace woodland The 100-year flood level is an important cultural limit, and flows above... need for planting ©2001 CRC Press LLC Figure 6 To avoid blockage of flood waters by dense, low riparian growth, the low-terrace zone can be restricted and terraces can be constructed above the mean annual flood line ©2001 CRC Press LLC Figure 7 Another method for avoiding blockage of flood waters while providing for wetlands is to construct seasonal marsh in low-terrace areas ©2001 CRC Press LLC Table 3... with greater affinity for water That is, a low-terrace woodland impedes flood flows more than a mid-terrace association, which in turn impedes flow more than a high-terrace association In a study of riparian woodland establishment patterns at several wetland creation projects in the Central Valley of California, Zentner and Zentner (1992) observed that the low-terrace woodland in this region occurs from... low-terrace woodland species may establish on thin cobble layers above mud soils, a useful technique for stabilizing rock weirs in wetlands However, mid- and high-terrace woodland species require a greater depth of permeable soil as noted above In some instances, soil analyses of a potential construction site may show that wetland establishment is not feasible under natural conditions Godley and Callahan... be observed weekly during the analysis to define the depth of ponding and the surface approach of groundwater However, short-term monitoring may not reflect typical groundwater levels Long-term groundwater level data may be available from local water districts or farmers For coastal wetlands, hydrologic factors that need to be evaluated during the site selection process include drainage features, such... of several alternative trees, all better adapted to the project site Small-Scale Experimental Construction When time is available, small-scale experimental projects or pilot studies should be undertaken to identify soil and hydrology constraints and to refine planning alternatives Appropriate studies include determining the water-holding capacity of unmodified soils, determining germination and salvage . Enhancement, Restoration, and Creation” Applied Wetlands Science and Technology Editor Donald M. Kent Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 6 Wetland Enhancement, Restoration, and. values of wetlands and a desire for the recreation of lost or diminished landscapes and values. This chapter discusses the construction of freshwater marshes, riparian wood- lands, and coastal wetlands. . defined through on-site analysis after initial mapping from an aerial photograph. For marshes, ran- domly selected 1-m plots representative of each vegetation association can be sam- pled for species