C Trettin, USDA, Forest Service, Charleston, SC, USA Published by Elsevier B.V.
Ecosystem Services and Values Further Reading
General Properties of a Swamp Ecological Functions
Swamp is a general term that is defined as ''spongy land, low ground filled with water, soft wet ground''
(Webster, 1983), hence its association with a wide variety of terrestrial ecosystems. Typically, a swamp is considered a forested wetland. A wetland is a type of terrestrial ecosystem that has a hydrologic regime where the soil is saturated near the surface during the growing season; the soil has hydric properties,
expressing characteristics of anaerobic conditions; and the dominant vegetation is hydrophytic with adaptations for living in the wet soils. In the case of a swamp, the forest species are adapted to the wet soil conditions. Without geographic context, there is little functional information conveyed by the term swamp other than the prevalence of wetland conditions and dense forest vegetation (Figure 1).
The following discussion is designed to convey the common hydrologic settings, soil conditions, and vegetative communities that occur within the common usage of the term swamp. References focusing on swamp forests should be consulted for specific geographic regions.
General Properties of a Swamp
The hydrologic setting controls the form and function of the wetland because of the dependence on excess water to mediate biological and geochemical reactions. There are four general settings that may be used to characterize the swamp hydrology (Figure 2). The riverine or floodplain setting is the most commonly associated hydrogeomorphic setting for swamps. It is characterized by periodic flooding from the river or stream, and it may also receive runoff from adjoining uplands. The periodicity, and flood depth and duration are the key factors that affect the type of forest communities present in the swamp. Depressional wetlands occur where there are surface depressions which receive water from the surrounding uplands, directly from precipitation, and in certain instances, they may also intersect a shallow water table. Lacustrine and estuarine fringe wetlands receive their water primarily from an open-water body; runoff from adjoining uplands and precipitation also contribute to the water balance. The common hydrologic
Figure 2 Influence of geomorphic position on hydrology. The arrows show the dominant direction of water flow for the four dominant types of geomorphic positions that are characteristic for swamps. After Vasander H (1996) Peatlands in Finland, 64pp. Helsinki: Finnish Peatland Society.
attribute of swamps in each of those settings is the presence of water above the soil surface, but the period of inundation varies widely. While it is common for swamps to have flooded conditions for periods ranging from days to months on an annual basis, it is not uncommon for there to be multiyear intervals between flood events. The factors that affect the flooding regime include timing and amount of precipitation, groundwater level, land use in the watershed contributing to the swamp, and evapotranspiration.
Swamp soils cover the full range of texture classes and degrees of organic matter accumulation (Figure 3). The wet mineral soils are characteristic of riverine and depressional settings. The histic mineral soils have a moderately thick accumulation of surface organic matter (<40 cm) reflecting prolonged periods of saturation and little scouring action if located in a floodplain, hence they may be found in any of the four hydrologic settings. The histosols or peat soils have a thick layer (>40 cm) of organic matter accumulation, representing the long periods of saturation on an annual basis. These soils typically occur in depressional settings and are not common in floodplains due to the periodic scouring that occurs during flood events.
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