All drainage classes, long periods of aeration
Somewhat poorly drained, frequent periods of high water table
Poorly drained, long periods of saturation
Figure 3 Types of soils common in swamp forests. The three categories reflect the amount of organic matter that has accumulated on the soil surface, which is in turn controlled by the soil drainage and hydrology. After Trettin CC, Jurgensen MF, Gale MR, and McLaughlin JA (1995) Soil carbon in northern forested wetlands: Impacts of silvicultural practices. In: McFee WW and Kelly JM (eds.) Carbon Forms and Functions in Forest Soils, pp. 437-461. Madison, WI: Soil Science Society of America.
The term swamp generally implies a forested wetland. However, due to the wide range of physical settings (see the previous two sections) and geographic locations ranging from the boreal to the tropical climatic zones on each continent, there are no consistent characteristics or attributes beyond the occurrence of hydrophytic vegetation. Accordingly, swamps may be dominated by either conifers or angiosperms, but a common situation would be a mixture of species and communities reflecting relatively minor differences in a microsite. For example, while a floodplain forest may be broadly characterized as a bottomland hardwood swamp, it contains a mosaic of vegetative communities which reflect small differences in hydrology and soils.
The ecological functions of swamps are significant, because of their prevalence and the wide range of conditions that they occupy. The following overview highlights some of the major ecosystem functions that are provided from swamp wetlands; specifics for a particular type of swamp are available from the regional references.
Hydrologic functions that are mediated by swamp wetlands depend on the hydrogeomorphic setting. Riverine swamps provide temporary storage for floodwaters, thereby reducing the peak flow to downstream areas. This function is physically based, with little interaction with the type of forest vegetation. However, changes in land use, especially conversion to agriculture, in the floodplain, may reduce the water storage potential, resulting in enhanced downstream conveyance of flow. The flood storage function also serves to sustain stream flow, as the waters slowly drain from the area. Swamps occurring in a depressional setting may be a source of groundwater recharge, where accumulated surface water slowly infiltrates through the subsurface sediments. In estuarine and lacustrine settings, swamps occurring at the land-water margin are important for the stability of the shoreline.
The effects of a swamp on water quality depend on the hydrogeomorphic setting. The riverine swamp affects water quality in two primary ways - by physical and biogeochemical reactions. Sediment removal is an important function of the riverine swamps; this is a process where sediment in the floodwaters settles out onto the floodplain surface. The deposited sediment provides nutrients to the swamp vegetation and it represents the removal of a contaminant from the floodwater. Floodplains with dense understory vegetation can be more effective than open forest settings in filtering sediment from the floodwaters.
The floodplain and riparian zone swamps may also remove chemical constituents from the water, particularly nitrogen and phosphorus. As a result of the anaerobic soil conditions, nitrate nitrogen, which is a common pollutant in surface and shallow-subsurface runoff, can be converted to nitrogen gas, thereby removing it from the water. The removal of phosphorus compounds typically involves reactions associated with the sediments.
Swamps are important for the diversity of habitat conditions that they provide. At the large scale, swamps comprise part of the mosaic of land types, yielding wet, vegetative conditions among uplands. At smaller scales, within a swamp, there are a multitude of habitat conditions that are largely dictated by elevation relative to the mean high water level.
The terrestrial habitats provided by swamps are diverse due to variations in vegetative composition and structure, which are largely regulated by the hydrologic conditions of the site. The habitat also changes through the development of the forest. In early successional stages, the vegetation is typically a dense combination of shrubs and trees; then, as the trees gain dominance, the shrub layers die back yielding a less dense understory.
Correspondingly, the habitat conditions for amphibians, birds, reptiles, and mammals change as the stand evolves. The swamp forests are particularly important habitat for birds, especially migratory song birds.
Swamps also provide important aquatic habitat for fish, birds, and amphibians. Organic matter produced in the swamp is an important energy source for aquatic organisms, including those living in water bodies within the swamp and also larger receiving bodies such as lakes, rivers, and oceans. In floodplains, the floating debris and logs provide physical structures that are an important component of the aquatic habitat.
In many areas, swamps have been converted into agricultural use, through the use of drainage systems and clearing of the forest vegetation. The merits of restoring the converted wetlands back to swamp forests include the reestablishment of flood water storage, in the case of floodplains, and the development of wildlife habitat. The restoration of swamp forests is complicated by the myriad of soil and hydrologic conditions that one may encounter, and the effects of past management practices which necessitate the restoration may also exacerbate the situation. However, with proper consideration of the hydrologic setting and matching species to the soil and water regimes, functional restoration is feasible. The typical sequence of restoring swamp forests is to reestablish the wetland hydrology by blocking drainage ditches, and planting appropriate tree and understory species.
Swamps provide both direct and indirect values to society. Direct values include raw materials, such as timber and food stocks. Indirect values include floodwater storage, water supply, water quality, recreation, esthetics, wildlife diversity, and biodiversity. The valuation will depend on inherent characteristics of the resource that are largely constrained by the biogeographic zone and location within a watershed, societal norms, and economic conditions.
See also: Water Cycle Management; Watershed Management.
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