Wetlands Characteristics and Boundaries (1995) / Chapter Skim
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5 WETLAND CHARACTERIZATION: WATER, SUBSTRATE, AND BIOTA
5 WETLAND CHARACTERIZATION: WATER, SUBSTRATE, AND BIOTA
Pages 90-148
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From page 90... ...
HYDROLOGY Wetlands are the interface for the major water reservoirs in the hydrologic cycle: surface water, ground water, atmospheric water, and, in some places, seawater. Standing water in wetlands is either the result of surface flooding or outcropping of the water table, which is the top of the saturated zone where pore pressure equals atmospheric pressure (Freeze and Cherry, 1979~.
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From page 91... ...
Nature of Wetland Hydrology The duration and frequency of saturation or inundation of a site vary according to the site's hydrogeologic setting, and they depend on regional differences in physiography and climate and on antecedent moisture conditions (Skaggs et al., 1991; Winter, 1992; Brinson, 1993a; Mausbach and Richardson, 1994~. The duration of saturation or inundation can be depicted for a wetland's hydroperiod, on a graph that shows the position of the water table or standing water in the area over time.
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From page 92... ...
Hydrologic Criterion The thresholds (direct indicators) for the hydrologic criterion are normally defined in teas of the frequency or duration of continuous flooding or saturation within a given distance of the surface during the growing season.
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From page 93... ...
Duration is important, but in fact wetland hydrology involves four related elements: saturation in relation to water table depth, duration of saturation and its relation to growing season, frequency of saturation or flooding, and critical depth of saturation. Saturation in Relation to Water Table Depth The water table is often assumed to be the boundary between saturated and unsaturated zones in soils.
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From page 94... ...
Consequently, duration thresholds are attached specifically to the growing season, which is then referenced to soil or air temperatures; saturation at other times is discounted. The implied assumptions are that plants and soil organisms are uniformly active over the growing season and uniformly inactive and that the growing season can be defined by a standard convention for regions of widely differing climate.
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From page 95... ...
For this reason, a much longer period of saturation might be required for anaerobic conditions to develop in the early spring than in summer, when soil temperatures are highest, even though both spring and summer are part of the growing season. The true critical duration would vary continuously with soil temperature if other factors, such as availability of organic matter, were constant.
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From page 96... ...
The 1987 Corps manual applies the concept to inundation or saturation of soil, rather than of nonsoil substrates. The manual uses the growing season through its adoption of the definition of hydric soils from the National Technical Committee for Hydric Soils (Chapter 4~.
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From page 97... ...
, which limits the relevance of soil temperature at this depth. Currently, the Hydric Soils List refers to the growing season as "the portion of the year when soil temperatures are above biologic zero in the upper part." This is more realistic than reference to 20 in.
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From page 98... ...
The biological zero concept as developed for wetlands leads to the conclusion that shallow permafrost soils have no growing season, which runs counter to the reality that tundra and taiga ecosystems flourish on such soils. Native plant species adapted to cool temperate, boreal, arctic, and alpine environments remain physiologically active at soil temperatures below biological zero (finer, l991b; Bedford et al., 1992~.
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From page 99... ...
suggests that in many years biological activity occurs over a period considerably longer than that defined by the average number of frost-free days. Growing season as defined by the frost-free period is particularly problematic in arctic
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From page 100... ...
. The number of years that exceed the 11-day duration threshold for saturation during the growing season is shown in Figure 5.3a as a function of hydraulic
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From page 101... ...
The graph shows the effect of hydraulic conductivity and length of growing season on the number of years that the water table depth is less than 30 cm (11.7 in.) for at least 11 consecutive days during the growing season.
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From page 102... ...
20 16 1 2 ° 8 o z 4 o WETLANDS: CHARACTERISTICS AND BOUNDARIES I: ~ \ _ ~ C 4 5 6 K, cm/hr 0 1 2 3 ~ L ~ ~ C 7 8 9 10 1 1 12 I ~ MAR3 0-NOV7 arc FEB2 8-DEC7 ° JAN 1-DEC3 1 FIGURE 5.3(b) Effect of hydraulic conductivity and growing season on the number of years that the water table depth was less than 30 cm (11.7 in.)
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From page 103... ...
This could expedite the refinement of duration thresholds. Evaluation of duration thresholds for wetlands requires long-term data on water table depth and corresponding information on soil morphology and vegetation across a range of conditions.
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From page 104... ...
. Similar reasoning applies to the interactions of duration and frequency essential for support of hydrophytic vegetation.
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From page 105... ...
The longest span of days for any given year that the water table was within 1 It (30 cm) of the surface during the growing season is shown in Figure 5.4 for each year of the 40-year simulation period (1951-1990~.
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From page 106... ...
~ FIGURE 5.5. Length of longest continuous period that the water table depth is less than 1 It (30 cm)
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From page 107... ...
. An extended period of saturation is required for anaerobic conditions to develop in soils that are infrequently saturated, especially if saturation occurs early in the growing season when soil temperatures are low.
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From page 108... ...
Water table data could then be compared with data from the reference site; the comparison would show whether the test site is wetter or drier than the reference site. A disadvantage of this approach is that reference wetlands would be needed for many wetland types and for many locations.
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From page 109... ...
on water table depth if they are used with long-term data on meteorology. Models also can be used to determine whether short-term measurements of water table and surface water elevations represent "normal" conditions.
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From page 110... ...
According to the Natural Resources Conservation Service (NRCS) , "hydric soil" is a type of "technical soil grouping" that was developed "for the application of national legislation concerned with the environment and with agricultural commodity production" (Soil Survey Staff, 1993~.
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From page 111... ...
supporting bottomland hardwood wetland vegetation was anaerobic (Eh < 300 mV) for fewer than 7 days during the growing seasons of 1984 and 1985, even though it was saturated (water table depth <1 It [30 cm]
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From page 112... ...
during the period of saturation (Soil Survey Staff, 1975; Pickering and Veneman, 1984) , but several studies have demonstrated significant microbial activity at temperatures below 41°F (5°C)
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From page 113... ...
Hydric Soils List "Hydric Soils of the United States" (USDA, 1985) also called the Hydric
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From page 114... ...
a frequently occurring water table at less than 1.0 ft from the surface for a significant period (usually more than 2 weeksJ during the growing season if permeability is equal to or greater than 6.0 in/in in all layers within 20 in, or (3) a frequently occurring water table water at less than 1.5 It from the surface for a significant period (usually more than 2 weeks)
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From page 115... ...
Second, any soil that is frequently ponded or flooded during the growing season is defined as hydric, regardless of other soil characteristics or water table depth at other times of the year. Third, all other soils are defined as hydric on the basis of a combination of soil taxonomy and water table depth.
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From page 116... ...
Soils in the SIR data base that met the 1985 NTCHS requirements for hydric soils were listed in the first edition of the Hydric Soils List (USDA, 19851. In addition to listing hydric soils, this publication listed more than 200 soil series that are poorly or very poorly drained but not considered to be hydric because their water table is too far below the surface or because they flood at times other than the growing season.
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From page 117... ...
report that Florida developed provisions requiring substantially longer duration of seasonal high water tables than that specified by the national critena. Recommendations based on research were accepted by the NTCHS, thus reducing Florida's hydric soils list by 58 series (13% of the state)
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From page 118... ...
Anaerobic conditions can alter soil properties in ways that reflect the frequency and duration of saturation with water. As early as 1964, Lyford showed that soil mottling could be used to estimate the seasonal maximum height of the water table, provided that there had been no artificial drainage.
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From page 119... ...
In such cases, hydrology or biota would better indicate wetland status. Use of Soil Surveys Soils in the United States have been surveyed and classified in a way that indicates relative wetness.
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From page 120... ...
These surveys, when combined with the county lists of hydric soils designated by the NTCHS and state soil scientists, provide the delineator with an excellent starting point for a wetland determination. Although soil surveys provide excellent background information for wetland delineation, they are subject to error, and the presence of hydric soils should be verified at the site.
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From page 121... ...
These plants are called hydrophytes, and the plant communities are described as being dominated by hydrophytic vegetation. Communities composed of these plant species have been used
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From page 122... ...
, developed classification schemes in which they recognized terrestrial, aquatic, and wetland plants, and grouped the latter two types as hydrophytes (Sculthorpe, 1967~. According to this convention, terrestrial species tolerate neither flooding nor soil saturation during the growing season, aquatic species tolerate flooding but not dewatering, wetland species tolerate both (Boule, 1994~.
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From page 123... ...
pecies that have demonstrated an ability (presumably because of morphological andlor physiological adaptations andIor reproductive strategies) to achieve maturity and reproduce in an environment where all or portions of the soil within the root zone become, periodically or continuously, saturated or inundated during the growing season.
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From page 124... ...
Each of the 6,728 plant species currently on the Hydrophyte List has a separate indicator for each region in which it occurs. The indicator assignment can vary from region to region because of ecotypic variation within species.
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From page 125... ...
Ecotypes of FACU species are particularly problematic. About 21% of the species on the Hydrophyte List are FACU species.
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From page 126... ...
Because most wetland ecotypes cannot be distinguished easily from the overall population, however, FAC and FACU species will continue to pose problems in delineation. Determining Predominance of Hydrophytic Vegetation Many techniques have been used in characterizing plant communities (Mueller-Dombois and Ellenberg, 1974; Greig-Smith, 1983)
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From page 127... ...
The most abundant species are called the dominant species (Greig-Smith, 1983~. The 50% rule is one way of applying dominance measures to the classification of plant communities.
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From page 128... ...
Abundance of UPL species in association with FAC species, however, would indicate that the site is not wetland. Conversely, the vegetation of a FAC-dominated site where OBL or FACW species are distributed throughout the site, but constitute only 20% of the individuals, is strongly indicative of wetland.
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From page 129... ...
Studies that compare the use of the prevalence index and the 50% rule on the same sites are needed for a range of wetland types. Evaluation of Thresholds The thresholds used by the federal manuals for separating wetlands from other ecosystems on the basis of hydrophytic vegetation are 50% for the dominance measure and 3.0 for the prevalence index.
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From page 130... ...
Although the very poorly drained (hydric) soils had relative cover exceeding 50% for OBL plus FACW species, the communities of poorly drained soils failed to exceed the 50% threshold even though the soils, which were hydric, indicated wetland.
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From page 131... ...
Numerous wetlands are dominated by FAC or FACU species, either on a long-term basis or as part of natural changes associated with climatic cycles (Niering, 1953; Curtis, 1959; Weller and Spatcher, 1965; van der Valk and Davis, 1978; Ledig and Little, 1979; Huenneke, 1982; Sharitz and Gibbons, 1982; Schalles and Shure, 1989; Golet et al., 1993; Carter et al., 1994~. Examples include wetlands that are inundated or saturated frequently or for extended periods of time: red maple (FAC)
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From page 132... ...
(1993) found that exclusion of FAC species did not clarify their analyses of red maple swamps and adjacent upland forests.
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From page 133... ...
If the FACU and UPL species are dominant and OBL or FACW species are absent or of very low abundance, the vegetation strongly indicates that the area is not saturated frequently or for long durations. Conversely, dominance by OBL, FACW, or FAC species, if UPL species are absent or of very low abundance, is strong evidence that an area is saturated very frequently or for very long periods of time.
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From page 134... ...
There is a strong correlation between hydrophytic vegetation and hydric soils TABLE 5.1 Soil-Vegetation Correlation Reports Commissioned by FWS Wetland type Reference Rhode Island red maple swamps Riparian zone of Butte Sink in the Sacramento Valley, California Selected wetlands and uplands of northcentral Florida Pocosins of Croatan National Forest Riparian zones of the Gila and San Francisco Rivers, California San Francisco Bay Estuary, California Sandhills and Rainwater Basin wetlands of Nebraska Coastal Mississippi wetlands Prairie potholes of Beadle and Dauel Counties, South Dakota Riparian and emergent wetlands, Lyons County, Nevada Connecticut River Floodplain, western Massachusetts Arctic Foothills, Alaska Allen et al., 1989 Baad, 1988 Best et al., 1990 Christensen et al., 1988 Dick-Peddie et al., 1987 Eicher, 1988 Erickson and Leslie, 1987 Erickson and Leslie, 1988 Hubbard et al., 1988 Nachlinger, 1988 Veneman and Tiner, 1990 Walker et al., 1989
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From page 135... ...
(1993) found in one instance that hydrologic data contradicted information on vegetation and soils.
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From page 136... ...
In the absence of hydrologic modification and where there is no evidence to the contrary, boundaries can be set on the basis of field-verified hydric soils and vegetation dominated by OBL and FACW combined with FAC and FACU species in the absence of UPL species. Dominance by UPL, FACU, and FAC species would provide strong evidence that the vegetation is not hydrophytic.
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From page 137... ...
The use of multiple factors was adopted as a system of checks and balances intended to prevent misidentifications where soils or vegetation were relicts of former hydrologic conditions and in areas dominated by facultative plant species. The use of three factors is now the basis for wetland identification and delineation by federal agencies (Chapters 3, 4~.
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From page 138... ...
For example, if hydrologic conditions have not been altered, vegetation dominated by OBL and FACW species of plants provides evidence of wetland hydrology because of the strength of the relationship between development of this type of vegetation and frequent or prolonged flooding or saturation of the soil. Conversely vegetation dominated by FACU and UPL species shows that the hydrologic criterion is not satisfied.
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From page 139... ...
In this zone, at the limit of the wetland, the water level fluctuations within the plant rooting zone can be the most extreme. Plant species composition at any given time will reflect a shifting competitive balance between species that are more and less tolerant of soil saturation or flooding.
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From page 140... ...
For example, vegetation dominated by FAC species that occur on a nonhydric soil and lacking an abundance of OBL or FACW species would not exceed the threshold for the vegetation indicator, and vegetation dominated by FAC species on a strongly redoximorphic mineral soil and lacking an abundance of UPL or FACU species would exceed the threshold for vegetation. Variables other than hydric soils and hydrophytic vegetation could satisfy the substrate and biological criteria if strong causal relationships could be established between specific thresholds of these variables and recurrent, sustained flooding or saturation at or near the surface of the substrate.
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From page 141... ...
The soil indicators are field-verified properties known to result from prolonged seasonal high water tables such as a gleyed matrix or low chrome ped faces immediately below the surface layer. One indicator, surface encrustations of algae, has not been tested but seems reasonable; data on another, remains of aquatic invertebrates, are available only from a single set of studies (Euliss et al., 1993)
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From page 142... ...
(An abundant species is a plant species with 20 percent or more areal cover in the plant community.)
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From page 143... ...
S9.Other regionally applicable, field-verifiable soil properties associated with prolonged seasonal high water tables. aThe presence of any of these characteristics in an area that has not been drained typically indicates wetland.
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From page 144... ...
Diagnostic combinations, which occupy the top tier of the hierarchy, are summarized in Table 5.4. Second, many of the indicators require the calculation of a prevalence index, which is much more time-consuming than is using a measure of dominance.
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From page 145... ...
6. saturation in most years sufficient to support hydric soils and hydrophytic vegetation; or Field verified plant communities, occurring on mapped hydric soils, with OBL species or OBL and FACW species representing more than 50 percent of the dominant species and no FACU and UPL species as dominants; or Field verified plant communities, on mapped hydric soils, with a mean prevalence index <2.0; or 5.
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From page 146... ...
This would eliminate the need for the Hydric Soils List to reference water table depth.
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From page 147... ...
to determine the correlation of soil types, water table depth, redox potential, and vegetation.
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From page 148... ...
The results should be published, and review panels for the Hydrophyte List and Hydric Soils List should use these analyses in revising the lists.
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