5
Wetland Characterization: Water, Substrate, and Biota

INTRODUCTION

Much of the controversy over wetland delineation can be reduced to a single question: which characteristics can be used to identify wetland ecosystems and distinguish them from other ecosystems? Many wetland ecosystems and their boundaries can be identified unequivocally most of the time, some present difficulties at all times, and others do so under some circumstances. This chapter provides an analysis of the properties that characterize wetlands and distinguish them from other ecosystems. The major issues to be dealt with in this chapter are hydrology; soils; vegetation; other indicators of the substrate and biological criteria; and combinations of information on water, substrate, and biota.

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). Wetlands can exist where the surface is flooded for extended periods or where there is saturation because ground water moves or stands close to the land surface.

As explained in Chapter 3, recurrent, sustained saturation of the upper part of the substrate is the most basic requirement for wetlands. The importance of hydrology in the formation and maintenance of wetlands is well accepted, but the threshold conditions that satisfy the hydrologic criterion and the methods to be used for determining the presence or absence of wetland hydrology are still in



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Wetlands: Characteristics and Boundaries 5 Wetland Characterization: Water, Substrate, and Biota INTRODUCTION Much of the controversy over wetland delineation can be reduced to a single question: which characteristics can be used to identify wetland ecosystems and distinguish them from other ecosystems? Many wetland ecosystems and their boundaries can be identified unequivocally most of the time, some present difficulties at all times, and others do so under some circumstances. This chapter provides an analysis of the properties that characterize wetlands and distinguish them from other ecosystems. The major issues to be dealt with in this chapter are hydrology; soils; vegetation; other indicators of the substrate and biological criteria; and combinations of information on water, substrate, and biota. 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). Wetlands can exist where the surface is flooded for extended periods or where there is saturation because ground water moves or stands close to the land surface. As explained in Chapter 3, recurrent, sustained saturation of the upper part of the substrate is the most basic requirement for wetlands. The importance of hydrology in the formation and maintenance of wetlands is well accepted, but the threshold conditions that satisfy the hydrologic criterion and the methods to be used for determining the presence or absence of wetland hydrology are still in

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Wetlands: Characteristics and Boundaries need of study. Several important principles have been established as a framework for hydrologic assessment of wetlands. 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. A wetland's hydroperiod integrates all aspects of its water budget (rainfall, evapotranspiration, runoff from adjacent areas, flooding, net seepage of ground water). A major technical challenge is to determine an average or characteristic hydroperiod for sites on which there are no hydrologic data, or for which hydrologic data cover only a short interval. Figure 2.3 shows hydroperiods for selected wetlands. The elevation of the water surface is shown relative to the elevation of the land surface, which is arbitrarily set at zero. As shown by Figure 2.3, water levels in some wetlands (for example, a marsh maintained by ground water, or a tidal marsh) are always above or close to the surface; In contrast, water levels in bottomland hardwood forest might come close to the surface only during specific periods of the year. Water levels in a tidal salt marsh can fluctuate dally. The water levels in a fen, which is maintained mainly by continuous ground water discharge, fluctuate the least. In many wetlands that are wet only seasonally, direct evidence of wetland hydrology might not be obvious for relatively long periods. The hydrologic boundary of a wetland is different from the hydrologic boundary of the watershed that contains it. The wetland is that locus of points in which the water balance produces enough saturation to maintain substrate and biota that are characteristic of wetlands. In contrast, the watershed that contains the wetland typically includes upland areas that share a common drainage pathway with the wetland. The wetland boundary might change over time as a complex function of factors that control the balance of terms in the water budget for the entire watershed. Climate change would be the most basic natural cause of change in the boundary of a wetland, but other factors—for example, sedimentation in channels, earthquakes, the activities of beavers, and land management practices—can alter hydrology and change the size of a wetland. Need to Evaluate Wetland Hydrology Because particular hydrologic conditions are essential requirements for wetlands, it is logical that hydrology be evaluated when wetlands are identified or delineated. This is now the case: All wetland delineation manuals require direct

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Wetlands: Characteristics and Boundaries or indirect evidence of saturation or inundation at a frequency and duration reflective of wetland hydrology. Direct evidence is often difficult to obtain, however, because indicators of hydrology are much more variable on a short time scale than are the main indicators of substrate (hydric soils) or biota (hydrophytic vegetation). This is especially true for seasonal wetlands (like the bottomland hardwood forest, Figure 2.3), which can be without flooding or saturation for several months every year. The hydrologic status of such sites cannot be evaluated from one or even from several site inspections. A thorough hydrologic analysis, including the collection of field data over a period of several months (or, in some cases, over a year or more) could be required. Fortunately, hydric soils and hydrophytic vegetation are reliable indirect indicators of wetland hydrology and can be used to infer its presence when the hydrology has not been modified. When the hydrolog of a site has been altered, soils and vegetation might not be reliable indicators, and the hydrologic status of the site must be evaluated independently. For all sites, hydrology must be evaluated at least to the extent of determining whether it has been changed. If it has, further direct hydrologic analysis is essential; if not, other indicators related to substrate (hydric soils) or biota (hydrophytic vegetation) can be used to infer hydrology, if the evidence from them is strong and consistent with such hydrologically relevant information as landscape position and surface indicators of hydrology. There are also many instances in which strong indirect indicators can be used to infer that wetland hydrology is not present, as in areas that contain extensive mammal burrows. In some cases, a direct evaluation of hydrology is necessary. Drainage ditches, dams, or channel modifications can alter the hydrology of a site to the extent that the conditions that are necessary to sustain wetland vegetation or soils no longer exist, even though the soils are still classified as hydric and relict wetland vegetation is present. The opposite also can occur. For example, natural or anthropogenic modifications can create wetland hydrology on sites where the soils cannot be classified as hydric. In some cases, evidence from soils and vegetation is so unclear that a direct evaluation of hydrology is necessary. There are two questions to answer in a direct evaluation of hydrology: Is the site saturated or inundated for a sufficient duration and frequency to demonstrate that wetland hydrology is present? Where is the boundary of the zone that satisfies the hydrologic criterion? Hydrologic Criterion The thresholds (direct indicators) for the hydrologic criterion are normally defined in terms of the frequency or duration of continuous flooding or saturation within a given distance of the surface during the growing season. The long-term threshold for hydrology of a wetland is that which, at minimum, is necessary to maintain the vegetation or other organisms of wetlands as well as characteristic physical and chemical features of wetland substrate, such as hydric soils. Unfor-

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Wetlands: Characteristics and Boundaries tunately, there is much uncertainty about the duration and frequency of saturation that define this threshold, especially because the threshold can be expected to vary from one region to another. Chapter 4 discusses thresholds of saturation and the critical depth for saturation (the water table depth) as they are defined in federal delineation manuals. The depth and duration thresholds proposed for the water table vary from less than 1.5 ft (46 cm) for 7 days (1989 interagency manual) to 0 ft (saturation to the surface) for 21 days (1991 proposed revisions). The 1989 interagency manual's threshold refers only to mineral soils of low permeability (<6 in. [15 cm] per hour) that are poorly drained or very poorly drained. The different thresholds specified in the federal manuals, when applied to the same sites, would correspond to widely different hydrologic regimes. Some thresholds would include sites that are well drained from an agricultural perspective (Skaggs et al., 1994); others would exclude recognized wetlands. Discussions of the hydrologic thresholds for wetlands have generally emphasized the duration of flooding or saturation. 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. In some cases, however, allowances have been made for a zone of saturation that extends above the water table because saturation can occur in the capillary fringe (Bouwer, 1978; Freeze and Cherry, 1979). The capillary fringe, or tension-saturated zone, is the region immediately above the water table in which pores are fully saturated but the pressure head is negative, indicating that the water is held in place by surface tension. The height of the capillary fringe above the water table can be determined theoretically from soil moisture retention curves. Large values for the height of the capillary fringe have been reported for soils with uniform pore size distributions. The surface layers of soils, however, usually have large pore spaces—caused by roots, burrows and other discontinuities—that empty under very little suction. For this reason, saturation caused by capillary action often extends only a small distance above the water table (a few inches). In the hydrologic assessment of wetlands, the water table depth need not be corrected for a capillary fringe unless field evidence shows that the capillary fringe is large. Wetlands sometimes can have finely grained soils that raise the zone of saturation significantly above the water table, in which case the water table is not a reliable guide to saturation. If not, the water table reasonably approximates the saturated zone for wetland soils and should be the main basis for direct assessment of the hydrology of wetlands.

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Wetlands: Characteristics and Boundaries Where the water table fluctuates, air is nearly always trapped as the water table rises (Bouwer, 1978). Even when relatively small samples of soil are inundated under laboratory conditions, air is trapped and the sample can be fully saturated only under suction. For example, Adam et al. (1969) report that 5-50% of the pore volume can contain air after initial stages of wetting. Because of the trapped air, which supplements the oxygen dissolved in the water, anaerobic conditions might not develop quickly even below the water table. With time, the air dissolves in the soil water and slowly diffuses to the atmosphere (McWhorter et al., 1973) or, in the case of oxygen, it is consumed by microbes and other organisms. The amount of air trapped as the water table rises depends on soil properties; antecedent soil water content; and whether saturation is caused by rainfall, seepage, or flooding. Duration of Saturation and the Growing Season Conventions for the direct evaluation of hydrology typically involve a numeric threshold for the number of days of continuous saturation necessary to maintain wetlands (Chapter 4). It is well recognized, that temperature affects the rate of oxygen depletion and redox depression in soils, as well as the sensitivity of plants to saturated conditions. 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. These assumptions are unrealistically simple, and they can lead to errors in evaluating hydrologic data. Effects of Soil Temperature on Development of Anaerobic Conditions Depletion of oxygen and subsequent suppression of redox potential by the conversion of oxidized substances to reduced substances is expected in any soil that is saturated for many days and that contains a significant amount of organic carbon. Recurrent depletion of oxygen and suppression of redox potential are characteristic of most wetlands and are responsible for creating and maintaining a number of the diagnostic features of wetland ecosystems. As explained in the section on soils, depletion of oxygen and suppression of redox potential are caused by the respiratory oxygen demand of roots and soil organisms. Among the soil organisms, microbes are most important. Microbes are the driving force behind extreme reduction of redox potential that is found in some wetlands. Respiration rates of plants, animals, and microbes are strongly affected by temperature. As a rule of thumb, the rate of respiration doubles in response to an increase in temperature of 10°C (Peters, 1983). Thus the respiration rates of root tissues and of soil organisms, including microbes, is strongly affected by the

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Wetlands: Characteristics and Boundaries temperature of soil. Because of the strong dependence of respiration rate on temperature, the degree of seasonal and regional variation in respiration rates is quite large. For example, the warming of a soil in the Midwest from 0°C in late winter to as much as 20°C in the last half of the summer would be expected to raise the oxygen demand of each microbial cell and each root hair in the soil by approximately fourfold. Similarly, a perennially warm soil, as might be found in Florida, shows a much higher respiratory demand on an annual basis than a perennially cold soil at very high latitude. Because the demand for oxygen in a soil is strongly dependent on temperature, the speed with which anaerobic conditions develop in a soil varies from one month to another at a given site and also from one region to another. The definition of thresholds for the duration of saturation necessary to produce anaerobic conditions must take into account the effect of temperature on respiration. This explains why fixed saturation thresholds (e.g., 14 or 21 days) are only crude estimates of the actual time that is required for anaerobic conditions to develop at a given site. As explained below, the critical threshold for saturation of soils can be defined in a more sophisticated way by two possible approaches, which can be used separately or in combination: (1) definition of saturation thresholds specific to individual regions of the U.S. (see Chapter 7), or (2) use of a ''degree-day'' concept, which would allow time to be weighted by temperature, so that the critical duration is shorter when temperature is higher and longer when temperature is lower. These two possibilities could be developed independently, but ideally would be used together. For example, the first approximation of the duration threshold would be made independently for each region on the basis of information from that region. Within the region, the degree-day concept could be used to account for variation in the threshold that might occur as a result of seasonal variation or intraregional climate differences. Soil temperatures change gradually over an annual cycle. 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 pan 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. This is shown by Figure 5.1, where average monthly temperature for St. Louis, Missouri, is plotted and bars indicate the estimated time required (the duration threshold) for reducing conditions to develop after saturation of the soil profile with water. The duration threshold varies substantially, even Within the growing season. Temperature also causes regional variation in the duration threshold of saturation. For example, the duration threshold Would vary less for San Diego, California, than it would for Saint Paul, Minnesota. In principle, the effects of temperature on the duration threshold for saturation could be estimated for any time of year in any climate if temperature were the only factor that affects critical duration. Because several other factors influ-

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Wetlands: Characteristics and Boundaries FIGURE 5.1 Mean monthly temperature of St. Louis, Missouri, and the length of continuous saturation required to develop anaerobic conditions in the root zone at various times of year, as shown by the length of solid bars (W Skaggs, unpublished data). ence the rate at which anaerobic conditions or plant responses develop, however, no simple estimate is likely to be realistic. Furthermore, current data are inadequate to define the duration threshold for saturation of wetlands over the wide range of soils, climates, and wetland types in the United States. Information is available for some regions, however, and duration thresholds can be approximated from information on the tolerance of a few sensitive upland plants to flooding and saturation. More research is needed on relationships of the duration of saturation to the development of wetland soils and vegetation, from which a more flexible, temperature-based adjustment of the duration threshold can be derived. Definitions of Growing Season and Their Application to Wetlands Cowardin et al. (1979) define the growing season as the frost-free portion of the year, but they apply the concept only to saturation or inundation of nonsoil substrates. 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). Growing season has thus

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Wetlands: Characteristics and Boundaries evolved from a minor constraint on the classification of nonsoil substrates to a major consideration in the identification of vegetated wetlands generally. The most common use of the growing season concept is in agriculture. While information on growing season for crops may be useful in evaluating wetlands, the growing season concept as applied to wetlands relates specifically to wetland organisms (especially vascular plants), and not to crops. Growing season has been defined as the period during which a given soil temperature at a specified depth is exceeded or as some function of the frost-free portion of the year. The 1987 Corps manual uses both definitions (Chapter 4). The 1988 Environmental Protection Agency (EPA) manual similarly defines growing season by its reference to biological zero (41°F, 5°C), but it does not mention frost-free days. Essentially the same approach appears in the 1989 interagency manual. A different definition appears in the 1991 proposed revisions, which define growing season as the interval from 3 weeks before to 3 weeks after killing frost, with exceptions for: "areas experiencing freezing temperatures throughout the year (e.g., montane, tundra, and boreal areas) that nevertheless support hydrophytic vegetation." The 1991 proposed revisions thus recognize errors in treatment of perennial cold regions as one of the flaws in earlier definitions of growing season (Bedford et al., 1992). In fact, the concept of biological zero, which is inherent in the use of growing season to define the metabolic activity of soils and plants, leads to numerous problems. Biological Zero The idea of biological zero is based on the notion that a limit of biological activity occurs at a specific temperature, below which, as stated in the 1987 Corps manual, "metabolic processes of soil microorganisms, plant roots, and animals are negligible." That manual places the temperature at 41°F (5°C), measured at a depth of about 20 in. (50 cm). This threshold fails for wetland communities in cold regions (Ping et al., 1992), and it might fail for some temperate communities as well (Disc, 1992). Furthermore, many wetland plants root at shallower depths (Tiner, 1991b; Bedford et al., 1992), 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. (50 cm), but "upper part'' is not defined. The use of biological zero is particularly inappropriate for defining growing season in permafrost wetlands. Pergelic Cryaquepts, which with histic Pergelic Cryaquepts (Appendix A, Soil Taxonomy) make up 65% of currently described permafrost soils in Alaska (Moore et al., 1993), have mean annual soil temperatures of with active layers (seasonally thawed zones) averaging 20 in. (50 cm) and rarely exceeding about 3 ft (1 m) in thickness (Ping et al., 1992). Temperatures in the saturated zone often are only slightly greater than 0°C during

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Wetlands: Characteristics and Boundaries the warmest weeks. For example, according to the 1989 interagency manual, Barrow, Alaska, would have a growing season of 0 days. 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 (Tiner, 1991b; Bedford et al., 1992). Below-ground parts of arctic plants grow at biological zero (McCown, 1978, in Chapin et al., 1980; Chapin and Shaver, 1985) and absorb nutrients at low temperatures (Chapin and Shaver, 1985). Roots of deciduous taiga trees respire actively at 41°F (5°C) (Lawrence and Oechel, 1983), and their ability to absorb phosphate is relatively insensitive to temperature (Chapin, 1986). Plant growth above ground also occurs at low soil temperatures, even under snow cover (subnivean). Arctic, alpine, and montane plant species grow (Billings and Bliss, 1959; Kimball et al., 1973; Kimball and Salisbury, 1974; Salisbury, 1984), flower (Bliss, 1971), or compete (Egerton and Wilson, 1993) in the subnivean environment at soil temperatures near freezing. Even in northern hardwood forest, spring-flowering herbs can develop leaves when soil temperatures are near 32°F (0°C) and there is partial snow cover (Vézina and Grandtner, 1965). Evergreen shrubs on exposed sites photosynthesize when the root zone is frozen (Webber et al., 1980), mosses and tundra graminoids photosynthesize when not covered by snow, and graminoids and the shrub Dryas initiate growth and photosynthesis within 1 day of snowmelt (Tieszen et al., 1980). Tundra plants achieve high rates of production (Chapin et al., 1980) because their photosynthetic optima are from 18° to 54°F (10° to 30°C) below those of plants in temperate regions, and they often maintain significant rates of photosynthesis to 25°F (-4°c) (Chapin and Shaver, 1985). Some tundra and taiga mosses and lichens photosynthesize at >50% of maximum rates at 32°F (0°C) or 41°F (5°C) (Tieszen et al., 1980; Chapin and Shaver, 1985; Oechel and Lawrence, 1985). Other biological activity outside the frost-free period includes photosynthesis in alpine plants, subnivean growth of at least 20 plant species (including two winter annual cereals) at snowpack temperatures (Salisbury, 1984), regreening of leaves and growth of roots in taiga plants (Kummerow et al., 1983; Tryon and Chapin, 1983), and growth of shoots in a temperate sedge (Bedford et al., 1988). Similarly, cyanobacteria associated with taiga lichens and bryophytes fix nitrogen at 38°F (3.5°C) (Alexander and Billington, 1986) and can be active for nearly 1 month after the average date of the first frost. Intact cyanobacterial crusts from subalpine habitats show no reduction in nitrogenase activity after repeated freeze-and-thaw cycles, and cyanobacterial nitrogenase activity of sedge meadow cores from a high arctic lowland are nearly 30% of maximum at 39°F (3.7°C) (Chapin et al., 1991). Soil microbes also are active in tundra and taiga wetlands when the soil

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Wetlands: Characteristics and Boundaries temperature is below biological zero. Bacteria from tundra soils respire to 20° or 19°F (-6.5° or -7°C) (Flanagan and Bunnell, 1980). Fungal biomass can increase within a temperature range of 32° to 26°F (0° to 2°C), but growth generally ceases below 27°F (-3°C) (Bunnell et al., 1980; Flanagan and Bunnell, 1980). Further evidence for microbial activity below nominal biological zero includes the presence of cold-adapted fungi in cold, acidic peat soils (Grishkan and Berman, 1993); in greater respiration rates and larger populations of some taiga microbes at 39°F (4°C) than at 68°F (20°C) (Sparrow et al., 1978); in overwinter increases in microbial biomass in near-freezing taiga soils (Zolotareva and Demkina, 1993); and in substantial early-winter carbon dioxide emissions from tundra and taiga soils (Zimov et al., 1993). The unfrozen isothermal zone (32°F [0°C]) that persists in the active layer of permafrost soils until heat loss is sufficient for phase change could provide a favorable environment for microbial activity (Zimov et al., 1993). Microbes oxidize >25% of estimated annual carbon fixation in a Wyoming subalpine meadow at winter soil temperatures of 33° to 35°F (0.5° to 1.5°C) (Sommerfeld et al., 1993). Wastewater treatment facilities, including natural and constructed wetlands subjected to wastewater discharge, also demonstrate significant microbial activity at low temperatures (S.E. Clark et al., 1970; Eckenfelder and Englande, 1970; Pick et al., 1970; Vennes and Olsson, 1970; Henry, 1974; Kent, 1987; Miller, 1989). Hydric soils develop when soil microbial activity depletes oxygen and creates reducing conditions. Methane emission from saturated tundra and taiga soils demonstrates reducing conditions and suggests that these soils can become anoxic at temperatures below biological zero (Svensson, 1983, in Svensson and Rosswall, 1984; Whalen and Reeburgh, 1992). Winter methane fluxes from Minnesota peatlands (Disc, 1992) also provide evidence for reducing conditions in cold soils. Low redox potentials (in this case, <100 mV) were documented at groundwater temperatures below 39°F (4°C) at two tundra bioremediation sites (Jorgenson and Cater, 1992; Jorgenson et al., 1993). A great deal of evidence from field and laboratory studies shows that biological activity occurs below 41°F (5°C), especially in cold regions. This casts doubt on the validity of any universal value for biological zero. Growing Season as Defined by the Frost-Free Period The use of a "mean frost-free period" poorly represents the occurrence of soil and air temperatures at which biological activity can occur in arctic, subarctic, alpine, and some temperate regions. Wide interannual variability in the number of frost-free days at locations as diverse as Iowa (Bedford et al., 1992), coastal British Columbia (Banner et al., 1986), and interior Alaska (Bowling, 1984) 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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Wetlands: Characteristics and Boundaries FIGURE 5.2 Drained wetland. tundra, where subfreezing temperatures can occur at any time of year (Savile, 1972; Chapin and Shaver, 1985). Barrow, Alaska, for example, annually averages 16 frost-free days (Sharratt, 1992), but has 91 days with a mean daily air temperature above freezing (Brown et al., 1980). Similarly, a subarctic site has been shown to have a frost-free growing season of 97 days (Slaughter and Viereck, 1986) and a thaw season of about 176 days (Dingman, 1971). Subfreezing temperatures occur daily in equatorial alpine communities (Bliss, 1971; Beck, 1987, in Kalma et al., 1992), and they occasionally occur early in the growth period in midlatitude alpine plant communities (Billings and Bliss, 1959; Holway and Ward, 1965). Interaction of Duration Threshold with Length of Growing Season The appropriate duration threshold for the saturation of soils in wetlands depends on the definition of growing season. An analysis of growing season for a specific site will illustrate this point. The hydrology of a hypothetical drained wetland (Figure 5.2) was analyzed with the simulation model DRAINMOD, using the methods described by Skaggs et al. (1994). Analyses were conducted for a sandy loam hydric soil with a constant drainable porosity of 5%, parallel drainage ditches about 4 ft (1.2 m) deep and about 330 ft (100 m) apart, and average depressional storage of about 1 in. (25 mm). The drainage intensity was varied by changing the hydraulic conductivity of the soil profile which is about 8 ft (2.4 m). The hydrology was simulated over a 40-year period (1953-1992) by use of climatological data for Plymouth, North Carolina. The growing season, based on the average last date of 28°F (-2.2°C) in the spring and average first date of 28°F (-2.2°C) in the fall, is Mar. 30 to Nov. 7, or 222 days. Thus, the hydrologic requirement for a wetland, according to the 1987 Corps manual, is 11 days (5% of 222 days). 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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Wetlands: Characteristics and Boundaries Necessity for Three Factors The variables that characterize wetlands interact and are causally related. The primary causal agent, or master variable, is water, which creates wetlands through recurrent, sustained flooding or inundation at or near the surface of the substrate. The physical and chemical characteristics of the substrate, such as hydric soils, and the characteristic biota, such as hydrophytic vegetation, are effects caused by and dependent on a hydrologic regime; they are not independent variables. The studies cited in this chapter and numerous other studies of wetlands show this to be the case. A logical extension of the causal relationships among the three factors is that effect (substrate, biota) can be used to infer cause (hydrology). For this reason, evidence supporting one criterion can be used to support another. The definition focuses on three factors, but the indicators of these need not come from three independent categories; the strength of causal relationships can be sufficient that indicators of one criterion can also be used for another. 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. Although other indicators can be used to support the presence of wetland substrate and wetland biota, hydric soils and hydrophytic vegetation are the two most common indicators. The following discussion is framed, therefore, in terms of hydric soils and hydrophytic vegetation, but the logic applies to other indicators as well. Coincidence of Characteristic Hydrology, Soils, and Vegetation Wetlands sometimes do not show strong and direct evidence of wetland hydrology, soils, and vegetation. For example, fluctuating water levels are typical of wetlands (Wharton et al., 1982; Winter, 1989; Duever, 1990; Golet et al., 1993; Mitsch and Gosselink, 1993) (Chapter 2 ). As explained in the section on hydrology, the recurring presence of water in wetlands is more or less predictable over the long term, but it can be difficult to assess over short periods. Vegetation is less variable over the short term than is hydrology, particularly for wetlands dominated by mosses or long-lived perennial herbaceous or woody plants, but variation can still present difficulties. Because perennial communities develop over several years to decades, their composition is an integrated expression of the hydrologic regime prevailing over the recent past. Although the relative abundance of individual species can change from year to year, the hydrophytic character of the overall community changes slowly in response to changes in the hydrologic regime. Annual or short-lived perennial species, however, can

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Wetlands: Characteristics and Boundaries vary annually or even seasonally where water-level fluctuations are rapid or extreme. Soils are least variable. Unless disturbed by cultivation or dredging, soil profiles consistently exhibit morphologic properties that reflect the long-term conditions under which they formed (Buol et al., 1980; Blume and Schlichting, 1985; Bouma et al., 1990; Mausbach and Richardson, 1994). The enduring nature of hydric soils, in fact, poses a problem for delineation because hydric soils persist long after they have been drained. Vepraskas and Guertal (1992) have proposed recently, however, that some relict features can be distinguished from some contemporary ones. Contemporary redox concentrations and depletions (Appendix A) along ped surfaces and root channels must not be overlain by other redder coatings that would indicate recent drainage. Contemporary manganese nodules with sharp boundaries are probably dissolving and therefore relict. The coincidence of wetland hydrology, soils, and vegetation is likely to be irregular in the transition zone between wetland and upland (Anderson et al., 1980; Allen et al., 1989; Carter et al., 1994). 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. The balance will shift in response to changes in the frequency, duration, extent, and seasonality of surface flooding and soil saturation along the boundary. In drier years, facultative species will become more abundant and upland species can invade. In wetter years, the balance will shift toward obligate wetland plants. Because the soil changes very slowly, however, it will provide evidence of the composite hydrologic regime prevailing over years to decades. Modified Approach to Evaluating Evidence The studies cited in this chapter indicate that some modification is needed of the current approach to wetland identification and delineation. A strict requirement for independent evidence from hydrology, soils, and vegetation in identifying and delineating wetlands is often impractical, and it overlooks the strong causal relationships that unify the hydrologic regime with the other variables that characterize wetland ecosystems. It also makes delineations needlessly time-consuming when the weight of the evidence from two factors, rather than from three, is a sufficient indication of wetland status. Given the primacy of hydrology in maintaining wetlands, however, Wetlands cannot be identified where evidence clearly shows the hydrologic conditions for wetlands to be absent. Requirements for specific kinds of direct hydrologic data for delineations are not practical or necessary except when hydrology has been altered or other factors provide uncertain indications. Such requirements assume incorrectly that seasonal and interannual variability will be sufficiently small that the period of assessment will be an accurate reflection of the average condition or that long-

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Wetlands: Characteristics and Boundaries term records will be available for most wetlands. Current information about wetland soils and vegetation, along with data from numerous field studies, shows that strong causal relationships can be established between hydrology, vegetation, and soils for the wetter end of the wetland continuum. Plant communities dominated by OBL, FACW, and FAC species and lacking an abundance of UPL and FACU species develop only where the prevailing hydrologic regime is one in which flooding or saturation of the soil is very frequent or extended. Histosols (except Folists), gleyed mineral soils, most mineral soils with low chroma matrixes and mottles, and mineral soils with well-developed depletion coatings develop only where sites are very frequently flooded or saturated for extended periods of time (Brinkman, 1970; Damman, 1979; Buol et al., 1980; Clymo, 1983; Vepraskas and Guertal, 1992; Mausbach and Richardson, 1994). These characteristics demonstrate the presence of wetland hydrology. The converse is true for vegetation dominated by UPL and FACU species and by soils that lack any sign of being hydric; these characteristics should be taken as strong evidence that the hydrologic criterion cannot be satisfied. Many wetland scientists contend that some properties of hydric soils and some types of hydrophytic vegetation should be used as primary indicators of hydrology (Tiner, 1991a; 1993; Bedford et al., 1992; Golet et al., 1993; Carter et al., 1994). This is a valid principle where hydrology is unaltered. If the hydrology of a site has been altered, analysis of vegetation or soils can lead to erroneous conclusions. For such sites, the hydrologic criterion must be evaluated with field data, or modeling must be used to determine whether the site has been effectively drained. A modification of the current approach to delineation would recognize the strength of causal relationships among factors rather than treating the factors as fixed and unrelated. Such an approach would not preclude assessment of all three factors for all wetlands, but it would broaden the evidence to be used in testing the indicators to see whether the criteria are satisfied, as follows, where hydrology has not been altered: It would allow inferences about wetland hydrology to be drawn from strong evidence in nonhydrologic categories—soils, vegetation, or other indicators of the substrate and biological criteria. It would allow the thresholds for indicators to vary as a function of the strength of evidence from other indicators and the presence or absence of conflicting information. 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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Wetlands: Characteristics and Boundaries The change reflected in this modified approach to delineation lies in the stringency of evidentiary requirements, which would vary as a function of the risk of error. As the risk increases, the evidence required to make a determination also would increase. Where the risk is low, direct information on all three criteria is redundant and would not be required. The modification would apply only in the absence of hydrologic alterations. Two ways of implementing the modified approach to delineation have already been developed: primary indicators and hierarchical classification. Primary Indicators Tiner (1993) has proposed the Primary Indicators Method (PRIMET), which uses information on vegetation and soils unique to wetlands. It is intended only for use on sites where hydrology has not been significantly altered. The rationale for the method is as follows (Tiner, 1993, p. 53): Wetlands are highly varied and complex habitats subject to different hydrologic regimes, climatic conditions, soil formation processes, and geomorphologic settings across the country. Within similar geographic areas, wetlands have developed characteristics different than adjacent uplands (nonwetlands) due to the presence of water in or on top of the soil for prolonged periods during the year. The visible expression of this wetness may be evident in the plant community and/or in the underlying soil properties. Consequently, every wetland in its natural undrained condition should possess at least one distinctive feature that distinguishes it from the adjacent upland. The "primary indicators method" (PRIMET) is founded on this premise. The proposed list of primary indicators (Tiner, 1993) is short and, for the most part, conservative indicators (Table 5.3). For example, the vegetation indicators rely on OBL and FACW species or an exception morphological adaptations to frequent flooding. The soil indicators are field-verified properties known to result from prolonged seasonal high water tables such as a gleyed matrix or low chroma 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), the results of which suggest that it would be useful. Tiner (1993) also provides a method for boundary determination: The boundary is located where no primary indicators are found. For wetlands subjected to cyclical drought (temporarily flooded red maple swamps or prairie potholes), Tiner recommends use of soil indicators. In drier wetlands, soils are more reliable than is vegetation because they provide evidence of seasonal saturation and long-term hydrologic conditions. In drought-prone areas, soils are more reliable because they respond less quickly than vegetation to short-term changes in hydrology. He also recommends using soil indicators for the boundaries of wet-

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Wetlands: Characteristics and Boundaries TABLE 5.3 Primary Indicators of Wetlandsa Vegetation Indicators of Wetlands V1. OBL species comprise more than 50 percent of the abundant species of the plant community. (An abundant species is a plant species with 20 percent or more areal cover in the plant community.) V2. OBL and FACW species comprise more than 50 percent of the abundant species of the plant community. V3. OBL perennial species collectively represent at least 10 percent areal cover in the plant community and are evenly distributed throughout the community and not restricted to depressional areas. V4. One abundant plant species in the community has one or more of the following morphological adaptations: pneumatophores (knees), prop roots, hypertrophied lenticels, buttressed stems or trunks, and floating leaves. (Note: Some of these features may be of limited value in tropical U.S., e.g., Hawaii.) V5. Surface encrustations of algae, usually blue-green algae, are materially present. (Note: This is a particularly useful indicator of drier wetlands in arid and semiarid regions.) V6. The presence of significant patches of peat mosses (Sphagnum spp.) along the Gulf and Atlantic Coastal Plain. (Note: This may be useful elsewhere in the temperate zone.) V7. The presence of a dominant groundcover Of peat mosses (Sphagnum spp.) in boreal and subarctic regions. Soil Indicators of Wetlands S1. Organic soils (except Folists) present. S2. Histic epipedon (e.g., organic surface layer 8-16 inches thick) present. S3. Sulfidic material (H2S odor of ''rotten eggs'') present within 12 inches of the soil surface. S4. Gleyedb (low chroma) horizon or dominant ped faces (chroma 2 or less with mottles or chroma 1 or less with or without mottles) present immediately (within 1 inch) below the surface layer (A- or E-horizon) and within 18 inches of the soil surface. S5. Nonsandy soils with a low chroma matrix (chroma of 2 or less) within 18 inches of the soil surface and one of the following present within 12 inches of the surface: a. iron and manganese concretions or nodules, or b. distinct or prominent oxidized rhizospheres along several living roots, or c. low chroma mottles. S6. Sandy soils with one of the following present: a. thin surface layer (1 inch or greater) of peat or muck where a leaf litter surface mat is present, or b. surface layer of peat or muck of any thickness where a leaf litter surface mat is absent, or c. a surface layer (A-horizon) having a low chroma matrix (chroma 1 or less and value of 3 or less) greater than 4 inches thick, or d. vertical organic streaking or blotchiness within 12 inches of the surface, or e. easily recognized (distinct or prominent) high chroma mottles occupy at least 2 percent of the low chroma subsoil matrix within 12 inches of the surface, or f. organic concretions within 12 inches of the surface, or g. easily recognized (distinct or prominent) oxidized rhizospheres along living roots within 12 inches of the surface, or h. a cemented layer (orstein) within 18 inches of the soil surface.

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Wetlands: Characteristics and Boundaries S7. Native prairie soils with a low chroma matrix (chroma of 2 or less) within 18 inches of the soil surface and one of the following present: a. thin surface layer (at least one-quarter inch thick) of peat or muck, or b. accumulation of iron (high chroma mottles, especially oxidized rhizospheres) within 12 inches of the surface, or c. iron and manganese concretions within the surface layer (A-horizon, mollic epipedon), or d. low chroma (gray-colored) matrix or mottles present immediately below the surface layer (A-horizon, mollic epipedon) and the crushed color is chroma 2 or less. (Note: The native prairie region extends northward from Texas to the Dakotas and adjacent Canada.)   S8.Remains of aquatic invertebrates are present within 12 inches of the soil surface in nontidal pothole-like depressions.   S9.Other regionally applicable, field-verifiable soil properties associated with prolonged seasonal high water tables. a The presence of any of these characteristics in an area that has not been drained typically indicates wetland. The upper limit of wetland is determined by the point at which none of these indicators is observed. Source: Tiner, 1993, Table 2, with permission from SWS. b Gleyed colors are low chroma colors (chroma of 2 or less in aggregated soils and chroma 1 or less in soils not aggregated, plus hues bluer than 10Y) formed by excessive soil wetness; other non-gleyed low chroma soils may occur due to (1) dark-colored materials (e.g., granite and phylites), (2) human introduction of organic materials (e.g., manure) to improve soil fertility, (3) podzolization (natural soil leaching process in acid woodlands where a light-colored, often grayish, E-horizon or alluvial-horizon develops below the A-horizon; these uniform light gray colors are not due to wetness.) lands in gently sloping terrain, where the plant community often provides mixed indications. These principles are supported by the scientific literature. PRIMET does not use hydrology as an indicator. Instead, vegetation and soils are used as indicators of hydrology. Tiner (1993) correctly points out that the visual signs of hydrology, including even direct observation of water, are indicators only of hydrologic events and not of their duration and frequency. Occasional flooding does not distinctively separate wetlands from uplands, many of which flood occasionally. Hierarchical Approach Another modified approach was outlined by the federal agencies that developed the 1989 manual. A tiered or hierarchical approach treats the evidence in accordance with the probability of reaching an erroneous conclusion. For example, a soil that shows only marginal signs of being hydric would not be accorded the same diagnostic value as a soil that is clearly hydric. Obvious wetland and upland would require less evidence than would problematic sites, such as

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Wetlands: Characteristics and Boundaries those dominated by facultative species. Sites with soils and vegetation only weakly indicative of wetlands would require hydrologic evaluation. As with PRIMET, the approach would be applied only to sites without hydrologic modification. The hierarchical approach is similar to PRIMET in seeking to use the strongest evidence, and only the evidence that is necessary and sufficient, for making determinations. It differs, however, in several ways. First, it relies on combinations of indicators rather than single indicators. 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. Third, it allows the use of some off-site information, such as soil maps and aerial photographs, in combination with field verification. Fourth, it uses far fewer soil indicators than PRIMET does. Finally, it does not use plant morphology adaptations, algal crests, Sphagnum moss, or aquatic invertebrates. In this sense, it is less comprehensive than PRIMET. Future Delineation Manuals Substantial knowledge of the nation's wetlands is embodied in the federal manuals that have been used to identify and delineate them. As a class of ecosystems, wetlands are remarkably diverse. Codifying this diversity into rules for recognizing the wetlands of the nation as a whole represents a significant challenge that the authors of the federal manuals generally have met well. Use of the manuals has helped to refine understanding of the essential characteristics of wetlands and to identify various problems with the manuals. A single federal manual whose core is drawn from existing manuals should be drafted. It should encompass the knowledge gained through the use of the manuals and recent refinements in scientific understanding of wetlands. This manual should be supported by the development of regional supplements that provide detailed criteria and indicators consistent with the federal manual (Chapter 7). RECOMMENDATIONS Improvements in the scientific understanding of wetlands since 1987 and refinement of regulatory practice through experience over almost a decade of intensive wetland regulation suggest that a new federal delineation manual should be prepared for common use by all federal agencies involved in the regulation of wetlands. This new manual should draw freely from the strengths of the existing manuals, and should not be identical to any of them. The recommendations that follow are intended to aid in its preparation. In the absence of hydrologic alteration and evidence to the contrary,

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Wetlands: Characteristics and Boundaries TABLE 5.4 A List of Indicator Combinations That are Diagnostic of Wetlands According to the Federal Interagency Committee's Proposal for Use of Evidence in Wetland Determinations 1. A minimum of 5 years of direct observations of hydrology (e.g., ground water well data and tide or stream gauge records) during years of normal rainfall and correlated with long-term hydrologic records for the specific geographical area demonstrate that the area is inundated and/or saturated at a sufficient frequency and duration; or 2. Examination of aerial photographs [from] a minimum of 5 years (preferably from early spring or other wet parts of the year) and other off-site information, locally correlated with hydric soils and hydrophytic vegetation, reveals evidence of inundation and/or saturation in most years sufficient to support hydric soils and hydrophytic vegetation; or 3. 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 4. Field verified plant communities, on mapped hydric soils, with a mean prevalence index ; or 5. All field verified hydric organic soils (Histosols except Folists) dominated by facultative or wetter vegetation (P.I. <3.5); or 6. Field verified hydric mineral soils classified as a Histic subgroup of an Aquic Suborder, Sulfaquents, or Hydraquents, dominated by facultative or wetter vegetation (P.I. <3.5); or 7. Field verified organic surface layer 8-16 inches thick (or mineral histic epipedon) dominated by facultative or wetter vegetation (P.I. <3.5); or 8. Field verified gleyed subsoil immediately below the A, Ap, or E-horizon (gleyed according to the gley page of the Munsell Soil Color Book) dominated by FAC or wetter vegetation (P.I. <3.5); or 9. Field verified hydric mineral soils and one of the following plant communities where: (a) more than 50 percent of the dominant species are OBL and/or FACW species, or (b) more than 50 percent of the dominant species are FAC with OBL species present and UPL species absent, or (c) more than 50 percent of the dominant species are FAC, with OBL and/or FACW species present and FACU and/or UPL species absent, or (d) more than 50 percent of the dominant species are FAC, with OBL and FACW species more abundant (e.g., aerial coverage) than FACU and UPL species, or (e) the aerial coverage of OBL and FACW dominant species exceeds the aerial coverage of FACU and UPL dominant species, or (f) the mean prevalence index is less than 3.0; or 10. Hydric mineral soils are field verified using regional indicators of significant soil saturation, and other plant communities not listed above dominated by FAC species or having a mean prevalence index equal to 3.5, and indicators of wetland hydrology (refer to issue paper on wetland hydrology). the presence of field-verified hydric soils can be used as strong evidence of wetland hydrology. In the absence of hydrologic alteration and evidence to the contrary, vegetation dominated by OBL and FACW species, or by a combination of OBL, FACW, and FAC species can be used as strong evidence of wetland hydrology.

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Wetlands: Characteristics and Boundaries Procedures should be developed for evaluating hydrology at sites that have been hydrologically altered or from which soil, vegetation, or other important indicators of site hydrology have been removed. Both the growing season and the hydrologic threshold for duration of saturation should be revised by region. Use of the growing-season concept in any form should be reevaluated for subarctic, arctic, and alpine regions as well as for the southwestern and tropical parts of the United States. The growing-season concept should be replaced with a more flexible and defensible, temperature-based adjustment of the duration threshold and region-ally-based criteria which would account for hydrology, biota, temperature, and substrate differences among regions. If direct hydrologic evaluation is needed, as in the case of altered sites or when evidence from substrate and biota is not conclusive, the evaluation should be based on water table data or on evidence of anoxia. The relevant zone for evaluation is the upper plant-rooting zone—the upper 1 ft (30 cm)—and not the soil surface. Pending the development of more sophisticated approaches and of regional guidelines, and in the absence of evidence to the contrary, the duration threshold for saturation can be taken as 14 days over the growing season in most years (on average, at a frequency greater than one out of two years). Indirect hydrologic indicators of flooding, such as water marks on trees, should not be used to determine the long-term hydrologic status of a site. Mathematical modeling can be used in analyzing hydrologic alterations and in relating short-term hydrologic measurements to long-term hydrologic conditions. Seasonal and interannual variation of weather must be considered in any direct evaluation of hydrology. Research should be undertaken on the frequency, duration, recurrence interval, and seasonality of inundation or saturation required for the maintenance of specific regional wetland classes. Guidelines should be developed for assessment of hydrologic alteration. The Hydric Soils List is useful in the identification of wetlands; its continued development should be supported by NRCS. Regional technical committees on hydric soils should be established for all U.S. states and territories. Each committee should report to NTCHS. NTCHS should consider developing a system for assigning hydric soils to fidelity categories. NTCHS should develop a list in which each soil is considered individually, rather than as a part of a taxonomic or soil drainage group. This would eliminate the need for the Hydric Soils List to reference water table depth. Soil surveys and the Hydric Soils List serve as primary reference materials for delineations, but field delineations involving soils should be based on field indicators such as soil color and morphology.

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Wetlands: Characteristics and Boundaries The Regional Field Indicators of Saturated Hydric Soils developed by NRCS should be evaluated for use in delineation. Field indicators of hydric soils should be evaluated for reliability; procedures are needed for revision of field indicators in response to field studies. NRCS should use its National Resources Inventory and other information (such as regional research projects) to determine the correlation of soil types, water table depth, redox potential, and vegetation. If the hydrology of a site has been altered, evidence from soils or vegetation must be used only with support from hydrologic analysis, including the characteristic frequency, duration, and depth of saturation. Assessment of problem soils (red or oxidized soils), or marginally hydric soils must be made by individuals experienced in identifying hydric soils. Lands on which hydrology has not been altered and on which hydric soils are present are wetlands and should be delineated as such, unless hydrologic and vegetation data do not support this conclusion. The absence of hydric soils, however, does not always indicate upland; analysis of hydrology and biota are needed for such lands. Scientific understanding of wetland soils and of correlations between plant distribution and wetland soils should be improved through research and monitoring. The Hydrophyte List is technically sound and should continue to serve as the basis for assigning species to wetland fidelity categories (obligate, facultative, etc.). Its continued improvement and revision should be supported by its sponsoring agency. The indexes for predominance of hydrophytic vegetation clearly separate hydrophytic from nonhydrophytic vegetation only when index values deviate substantially from the threshold; lands with hydrophyte dominance near 50% or a prevalence index near 3.0 cannot be assessed confidently without strong reliance on other indicators. An array of strong indicators that do not require use of formal indexes should be constructed and used in the field. Examples include the following, which should be applied only in the absence of significant hydrologic alteration: Vegetation dominated by obligate and facultative-wet species will satisfy the biological criterion. Vegetation dominated by obligate, facultative-wet, and facultative with no abundant upland or facultative-upland species will satisfy the biological criterion. Vegetation dominated by facultative or facultative-upland species will satisfy the biologic criterion if it occurs on field-verified hydric soils with strong morphological indicators. If soils are not clearly hydric, hydrologic data will be essential. In the absence of hydrologic alteration or other evidence to the contrary, vegetation dominated by obligate and facultative-wet species, but with no abun-

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Wetlands: Characteristics and Boundaries dant upland or facultative-upland species, indicates that wetland hydrology is present, unless soils are nonhydric, in which case hydrologic information is needed. Conversely, vegetation dominated by upland, facultative-upland, and facultative species and with no abundant obligate or facultative-wet species should be considered nonhydrophytic and should indicate a nonwetland area, unless soils are hydric, in which case hydrologic information is needed. Boundary determinations involving vegetation analysis should be confirmed by analysis of substrate. Delineation manuals should specify that the list of indicators that support the biological criterion can include organisms other than vascular plants. Where biological indicators other than vegetation are used, quantitative thresholds should be developed if possible to allow standardization of the indicator for a particular region or for a particular type of wetland. The application of evidence to the assessment of wetlands should be modified. All three defining factors (water, substrate, biota) must be evaluated, even though in some cases evidence can be inferential. If hydrologic information is unavailable, and if hydrology has not been modified, the presence of wetland hydrology can be evaluated from information on substrate, if this information is definitive (for example, by the presence of hydric soils); from vegetation, when the hydrophytic nature of the vegetation is unequivocal; or from other indicators for which a strong relationship to recurrent, sustained saturation can be established. If neither substrate nor vegetation provides clear evidence, delineation will require hydrologic data. All evidence must be carefully weighed, however, when a delineation is made. Both the Primary Indicators Method (PRIMET) and the hierarchical approach are conceptually sound and should be studied for use in identifying and delineating wetlands. Federal agencies that regulate wetlands should hire regulatory staff that makes up a balanced mixture of expertise in plant ecology, hydrology, and soil science. Primary data on hydric soils, hydrophytic vegetation, and hydrology of wetlands should be assembled and analyzed statistically. The results should be published, and review panels for the Hydrophyte List and Hydric Soils List should use these analyses in revising the lists. Reference wetlands should be identified for long-term study of the relationships between water, substrate, and biota.