Riparian Communities

Riparian communities are not biomes. Though they could be considered isolated ribbons of deciduous forest, they are better viewed as a unique habitat type. They occur within any biome wherever there is perennial water near the surface. The term riparian specifi-cally refers to the zones along the banks of rivers; however, it is also applied to the shoreline communities along slow or nonflowing waters such as marshes and lakes.

The drier the surrounding habitat, the more distinct is the riparian zone. In the desert or grassland a flowing stream supports a conspicuous oasis with forests and wildlife that would not otherwise occur in the area. The available water also augments populations of more arid-adapted species in the adjacent habitat.

Biomes and Communities

Riparian zones are so different at different latitudes and elevations that they should be thought of as several communities with similar physical characteristics, primarily their dependence on perennial water. Montane streams support alder and aspen, while at lower elevations there are cottonwoods and sycamores. In tropical deciduous forests a riparian zone may be visually indistinguishable during the wet season because the overall appearance of stream-bank and hillside trees is similar, though the species may be different. But in the dry season most of the slope vegetation is deciduous, while tropical riparian species are typically evergreen.

Some ecologists broaden the concept of riparian communities to include the banks of dry washes in deserts. A wash in the Lower Colorado River Valley with its woodland of palo verdes (Cercidium spp.), ironwoods (Olneya tesota), and desert willows (Chilopsis linearis) is clearly distinct from the surrounding creosote bush flats. These dry washes occupy less than five percent of the area of this subdivision of the Sonoran Desert, but support ninety percent of its bird life. This concentration of life is the result of the greater availability of water, even though the wash may carry surface water for only a few hours a year. Desert drainageways should be labeled "dry riparian" or "desert riparian" to avoid confusion with wetter habitats that have surface water all or most of the year.

Dry riparian habitats share most of their defining characteristics with traditional "wet" riparian habitats. They are chronically disturbed, unstable sites where water and nutrients are harvested and concentrated from larger areas (watersheds). Finally, they are corridors for dispersal of plants (seeds) and animals.