Riparian Plants

Riparian corridors encompass an elevational continuum, which includes the active channel (the summer low flow / wetted channel, as well as the streambed and streambanks), and the floodplain. The frequency of flood disturbance events determines community structure within riparian corridors, inlcuding invasive non-native species. Shown below is a cross sectional diagram of a riparian corridor illustrating community development in relation to disturbance from flood events. Disturbance is highest in the active channel and decreases with increasing elevation above the channel. The left bank represents a steeper slope and a smaller floodplain. The right bank has a more gradual elevation gain and a correspondingly larger riparian area. The width of the active channel is dependent upon hydrology, geomorphology, and land use history. Overall species composition differs between the active channel and the floodplain, but some species occur in both areas. To see pictures and descriptions of individual riparian plant species, go to the Plant Species Gallery.

Plants of the Active Channel

Active channel areas are zones of high disturbance. These areas are regularly flooded, often with substantial velocity and scour. The plants that are common in this zone – willows, cottonwood, and alders – are adapted to a high disturbance regime in the winter, and have adapted specialized responses (such as adventitious roots) to withstand extreme environmental conditions. During the summer, they must quickly tap into the water table and follow it as it drops. This area tends to be characterized by relatively few species, which may occur in very high numbers. Some plant species occur in both active channel and floodplain areas. Below are some riparian plants commonly found in active channel areas. Note that some plant species occur in both active channel and floodplain areas.

Plants of Floodplain Areas

Floodplain areas are less subject to flooding than active channel areas. The plants that occur on the floodplain are less adapted to flood disturbance and require less water than active channel plants. While the active channel is characterized by just a few species, floodplain areas tend to have a great number of different species, and substantial structure (for example, canopy layer, shrub layer, vine layer, and herb layer). Combined, the plant communities found in active channel and floodplain areas create a complex riparian forest structure that provides many ecosystem functions (see the Ecological Succession section for more information). Below are some riparian plants commonly found in floodplain areas. Some plant species occur in both active channel and floodplain areas.

Invasive Non-native Plants

Invasive non-native species are a serious threat to riparian plant communities. They invade streamside areas, out-compete native plants, and can dominate the riparian zone. They do not tend to provide quality habitat for native fish, wildlife, or bird species. Species such as tamarisk and giant reed use large amounts of water and may increase the flammability of the normally non-flammable riparian zone. Because they threaten native plant communities and the animals that depend upon them, these invasive species should not be planted near streamside habitats. Below are some of the invasive non-native plant species found in active channel and floodplain areas or in either individually.

Additional Resources

Visit the Plant Species Gallery on this Web site for detailed descriptions and pictures of trees, small trees and shrubs, vines, emergents, other herbaceous plants, and invasive non-native species common to riparian habitats in San Mateo and Santa Cruz Counties. For additional information about native plants in San Mateo and Santa Cruz Counties contact:

• Yerba Buena Chapter of the Native Plant Society
• Blazing Star Santa Clara Valley Chapter of the Native Plant Society
• Santa Cruz Native Plant Society

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