Riparian land

Riparian land refers to terrain that is adjacent to rivers and streams and is subject to periodic or occasional flooding . The plant species that grow in riparian areas are adapted to tolerate conditions of periodically waterlogged soils. Riparian lands are generally linear in shape and may occur as narrow strips of streambank vegetation in dry regions of the American Southwest or as large expanses of bottomland hardwood forests in the wetter Southeast. The ecosystems of riparian areas are generally called riparian wetlands . In the western United States, riparian vegetation generally includes willows, cottonwoods, saltcedar, tamarisk, and mesquite, depending on the degree of dryness.

The riparian zone of bottomland hardwood forests can be differentiated into several zones based on the frequency of flooding and degree of wetness of the soils. Proceeding away from the channel of the river, the zones may be described as follows: intermittently exposed, semipermanently flooded, seasonally flooded, temporarily flooded, and intermittently flooded. The vegetation of each zone is adapted to survive and thrive under the conditions of flooding peculiar to that zone. Due to the irregularities in topography and the formation of streambank levees that are normal to any landscape, it is rare that these zones always occur in the same predictable sequence.

Intermittently exposed zones have standing water present throughout the year and the vegetation grows in saturated soil throughout the growing season. Bald cypress and water tupelo are typical trees of this zone and have adaptations such as stilt roots and anaerobic root respiration to cope with the permanently flooded conditions. Semipermanently flooded zones have standing water or saturated soils through most of the year, and flooding duration may last more than six to eight weeks of the growing season. Black willow and silver maple are abundant tree species of this zone. Seasonally flooded zones are areas where flooding is usually present for three to six weeks of the growing season. A number of hardwood tree species thrive in this zone, including green ash, American elm, sweetgum, and laurel oak. Temporarily flooded zones have saturated soils for one to three weeks of the growing season. For the rest of the year, the water table will be well below the soil surface. Many oaks, such as swamp chestnut oak and water oak, as well as hickories may be found here. Intermittently flooded zones are areas where soil saturation is rarely present and flooding occurs with no predictable frequency. This is an area that may actually be difficult to distinguish from the adjacent uplands. Many transitional and upland species such as eastern red cedar, beech, sassafras, and hop-hornbeam are common.

For all riparian wetlands in the field, these zones of moisture and vegetation gradients occur in an overlapping, intergrading fashion, and the plant species are distributed in varying degrees throughout the riparian zone. Riparian wetlands are valued for their specialized plantlife and wildlife values. They are recognized as interfaces where uplands and aquatic areas meet to form intermediate ecosystems that are themselves unique in their diversity, productivity, and function. They also provide significant economic benefits by minimizing flood and erosion damage.

[Usha Vedagiri ]

RESOURCES

BOOKS

Mitsch, W. J., and J. G. Gosselink. Wetlands. New York: Van Nostrand Reinhold, 1986.