Vegetation bordering to and growing within riparian wetlands fulfils many functions: it delivers both substrate for colonization and food resources for aquatic animals, it strips nutrients from the incoming water, and it provides raw material for the organic soils. It retards nutrient loss, filters nutrient input from the upland, reduces runoff by evapotranspiration, and buffers water-level fluctuations. Shading by tree canopies reduces light conditions for algal and macrophyte primary production and it equilibrates soil temperatures. Therefore, riparian wetlands differ completely according to their vegetation cover.
Unvegetated riparian wetlands occur at sites where establishment of higher plants is hampered by strong sediment movement (e.g., high-gradient and braided rivers), low temperatures (high elevation and polar zones), rocky surfaces, or periodical drought (desert rivers). The lack of shading and nutrient competition by higher plants favors growth of algae on the inorganic sediments, and productivity may be high, at least periodically.
High altitudes and/or elevated groundwater levels may preclude tree growth but allow the development of grass or herbal vegetation on riparian wetlands. Hillside swamp springs (helokrenes) can coalesce and form extensive marshes far above the flood level of the stream channel, so that the distinction between 'riparian' and 'common' wetland is difficult.
The tree species of forested riparian wetland are adapted to periodical or permanent waterlogging of the soils. They contribute an important input of organic carbon to the stream system. Large tree logs shape habitat structure by controlling flow and routing of water and sediment between stream channel and wetland. Tree roots increase sediment stability, sequester nutrients, and form habitats.
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