Ecological Succession

"Pioneer" seedlings on a gravel bar.

Under natural conditions most habitat diversity in the riparian zone originates from and is sustained by the high frequency of flooding and erosive disturbance caused by rivers and streams. Rivers are dynamic, disturbance-driven ecosystems. Hydrologic and geomorphic processes are a major force in determining terrestrial plant distribution and diversity in riparian areas (see the Hydrology section for more information).

Riparian habitat varies widely across the cross section of the stream corridor with different species occurring at different elevations above the riverbed (see the Riparian Plants section for more information). The species that are found in the channel are usually not the same as those found on the floodplain. In active channel areas (areas which are regularly flooded), plants are adapted to high levels of flood disturbance during the winter, while often needing to tolerate the hot, dry conditions of the gravel bars during the summer. Very few species have the ability to survive in this harsh channel environment. Those that do include alder, willow, and cottonwood, as well as some of the emergent species, such as sedges. They are called pioneer species, because they colonize recently disturbed sites.

First year seedlings begin to trap sediment, and may affect hydro-geomorphic processes.

The process of ecological succession (the progressive replacement of one community by another, developing towards a more complex community structure) in the riparian zone begins when the seeds of pioneer species, such as cottonwoods and willows, float through the air in the spring just as the water level is beginning to recede. Millions of seeds land on moist gravel bars and germinate there. As the summer progresses, the roots of these tiny seedlings follow the receding water table. These perennial water-loving plants must remain connected to the water table in order to survive on the desert-like gravel bar. Those plants that survive the summer drought and winter flood cycle will grow at incredible rates – up to 15 feet per year. As they grow, the seedlings may begin to trap sediments, and can influence the movement of the stream.

As sediment deposition occurs and the bar builds in height and is laterally distanced from the stream channel, species that are less dependent upon direct access to groundwater begin to colonize the area and eventually replace the early colonizers. Increased sedimentation (stream bank building) will eventually create floodplains that will only be flooded during the highest flood events. Floodplains are at higher elevations than the active channel and are characterized by many more species and much more structural diversity than the channel zone. Floodplain plants are less adapted to flood scour, do not require as much summer moisture, and tend to have distinct layers of vegetation. The canopy layer is comprised of large trees and the understory is made up of small trees and shrubs, vines, herbs, and downed wood. Late successional floodplain riparian forests also tend to have large numbers of dead trees and snags which are important habitat for hole-nesting species of birds and other wildlife (see the Riparian Biological Communities section for information and pictures of common species).

A floodplain riparian forest in the San Lorenzo River watershed.

Streams often cut though broad alluvial valleys. In these alluvial zones the substrate is sand, gravel, and silt and the stream freely moves (meanders) back and forth over time, creating and destroying riparian habitat. The ability of the stream to move through this "meander corridor" continues the succession processes and allows for the development of diverse riparian forests. Historic accounts indicate that many main stem rivers may move substantially, for example some have moved over a mile and back across associated floodplains in a twenty year period. However, due to the high value of agricultural lands as well as the proximity of urban and other land uses, large amounts of stream movement may no longer be possible or desirable.

General References

Federal Interagency Stream Restoration Working Group (FISRWG). 1998. "Stream Corridor Restoration: Principles, Processes, and Practices." Federal Interagency Stream Restoration Working Group (FISRWG). GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN 3/PT.653. ISBN-0-934213-59-3. View on-line document.

Gregory, S.V., F.J. Swanson, W.A. McKee, and K.W. Cummins. 1991. An ecosystem perspective of riparian zones. BioScience 41(8):551.

Gregory, S.V., G.A. Lamberti, and K.M.S. Moore. 1988. "Influence of valley floor landforms on stream ecosystems. Proceedings of the California Riparian Systems Conference, September 22-24, 1988." USDA Forestry Service. General Technical Report PSW-110, 3-8 pp.

McBride, J., and J. Strahan. 1984. Fluvial Processes and Woodland Succession Along Dry Creek, Sonoma County, California. In California Riparian Systems: Ecology, Conservation, and Productive Management, edited by R. Warner and K. Hendrix. Berkeley: University of California Press.

McBride, J. and J. Strahan. 1985. Establishment and survival of woody riparian species on gravel bars of an intermittent stream. American Midland Naturalist 112(2):235-245.

Naiman, R. J., S. R. Elliott, J. M. Helfield, and T. C. O'Keefe. 1999. Biophysical interactions and the structure and dynamics of riverine ecosystems: The importance of biotic feedbacks. Hydrobiologia 410(1):79-86.

Tabacchi, E., A.-M. Planty-Tabacchi, M. J. Salinas, and H. Decamps. 1996. Landscape structure and diversity in riparian plant communities: A longitudinal comparative study. Regulated Rivers Research and Management 12:367-390.

Ward, J. V., and K. Tockner. 2001. Biodiversity: Towards a unifying theme for river ecology. Freshwater Biology 46:807-819.

Warner, R.E., and K.M. Hendrix. 1984. California riparian systems. Ecology, conservation, and productive management. Berkeley and Los Angeles, CA: University of California Press.

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