Riparian Forests and Woodlands
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Rita DePuydt
California Native Plant Society

Riparian vegetation refers to the vegetation that grows along the shores of freshwater rivers and lakes. Riparian comes from the Latin ripa, meaning a stream or river bank. Along many parts of California's lowland rivers are found mature forests and woodlands of deciduous, broadleaf hardwoods similar to the forests of eastern North America.

About twenty million years ago, the north temperate zone was wetter with less seasonal fluctuation. During this period, North America was covered by a nearly continuous forest belt of broadleaf tree species mixed with several conifers. As mountains rose, casting their rain shadows, and the climate warmed and dried, the broadleaf forests in the west were ultimately reduced to the riparian belts along rivers and streams. Broad coverage remained in eastern North America where the climate remained colder and wetter. In the west, the riparian trees sank down deep roots, tapping into the permanent water table, enabling them to compensate for dry summers. Most of the trees in California's riparian forests are winter deciduous, losing their leaves in winter like their eastern relatives, despite mild winter temperatures. Bordering either side of a stream is a floodplain through which the stream meanders over periods ranging from several years to centuries. As a river meanders, the bank on one side erodes while sediments accumulate downstream on the opposite side. Few plant species can tolerate continuous flooding. In a flowing stream, vegetation zones spatially replace each other. A shrub zone, dominated by species of willow and Baccharis (locally called mulefat), develops slightly above and back from the continuously-flooded area. These plants are able to spread by seed and rhizomes, rapidly invading exposed sand or gravel bars. If the stands are protected from flooding for 15 to 20 years, a willow woodland eventually emerges. Still higher and further back in the floodplain, a riparian forest develops with an overstory that averages 80 feet tall that can be quite dense. This represents a mature riparian habitat. This zone is dominated by cottonwoods in association with oak, white alder, willows, California bay-laurel, sycamore, and walnut depending on location. White alder is found in abundance along perennial streams.

The cottonwood zone of the riparian forest has a complex architecture. Below the trees are layers of shrubs, herbs and vines. More species of birds nest in this forest than in any other California plant community. In addition, 25% of California's land mammals depend on the riparian habitat. Twenty-one of these are facing threats of extinction due to habitat loss.

Forests of valley oak or coast live oak, in association with sycamore, are located in the drier, outer floodplain. Coast live oak is found along Central and Southern California coastal rivers, while valley oak dominates this zone in the Central Valley. Here the trees grow in an open, parklike setting. Oak and sycamore grow best where the water table is about 35 feet below the surface and flooding is only occasional. In contrast, flooding in the cottonwood zone is frequent and the water table is only 10-20 feet deep. In the past, a riparian forest extended away from the major rivers as far as three miles on either side.

Riparian forests are among the most productive of natural ecosystems. An intact riparian zone acts as a filter between streams and the adjacent environment. The riparian zone prevents agricultural fertilizer and animal wastes from seeping into streams and ground water. It reduces sedimentation in stream beds, thus protecting spawning beds. Streambank vegetation lessens erosion and controls the release of nutrients to the aquatic environment. Overhanging canopies prevent water from cooling and thereby losing its dissolved oxygen. Riparian vegetation also provides habitat for invertebrates that are a source of food for aquatic and terrestrial life. A healthy riparian cover is the starting point of sound watershed management. In California, many of the riparian forests have been replaced by orchards or have been damaged to some extent by grazing. For instance, in the Central Valley, less than 10% of the original riparian cover remains. The rich, alluvial soils of the floodplains are ideal agricultural land. Grazing of stock in riparian areas results in the removal of palatable plants, eating and trampling of seedlings, invasion of non-palatable weed species, and the degradation of stream banks. In Southern California, only 3 to 5% of the pre-settlement riparian forest remains, the rest having been converted primarily to farming or urban uses.

A second major impact has been the construction of dams and reservoirs which have altered the levels of many streams and inundated valleys once occupied by riparian communities.

A third impact relates to urbanization where flood control becomes paramount. Efforts to control flooding include removal of riparian vegetation to speed the movement of floodwaters, dredging, channelization, and enclosing the stream in concrete. The rate of conversion of agricultural land to urban land has increased dramatically in the past several decades. In the five years from 1977 to 1982, the counties of Los Angeles, Orange, San Bernardino and Riverside urbanized 100,000 agricultural acres; San Diego County converted 60,000 acres; while 65,000 acres were converted to urban uses in the San Joaquin Valley.

Most of the low-level areas of California contain only remnants of a once-glorious landscape. One of these remnants is found in the riparian forest along the Santa Clara River from east of Piru (in Ventura County) to Bouquet Creek near the city of Santa Clarita (in Los Angeles County). This nearly ten-mile stretch of riparian habitat contains a continuous belt of high- to moderate-quality cottonwood willow forest and cottonwood willow woodlands. This sector contains the L. A. County Significant Ecological Area (SEA) #23.

These extensive belts of cottonwood and willow provide important habitat for the endangered least Bell's vireo and other sensitive bird species including the southwestern willow flycatcher, western yellow-billed cuckoo, yellow warbler, and yellow-breasted chat. An aquatic species also lives in this section of the river, the federal and state listed unarmoured three-spined stickleback. This tiny fish (less than three inches) lives in the quiet water of weedy pools and backwater where the water stays below 75 degrees F. Unfortunately, this aesthetically pleasing and biologically valuable habitat is on private property, five miles of which is slated for one of the largest developments in Southern California. The Newhall Ranch Project has a plan for the construction of a new city of 70,000 people that encompasses the low areas along the Santa Clara River and the upland areas in the adjacent Santa Susana Mountains. The other segment of this riparian forest and woodland lies adjacent to the rapidly-growing Santa Clarita urban area.

On a hopeful note, the Nature Conservancy and other groups and agencies have undertaken the destination of riparian habitat along some of California's rivers. The overall importance of these plant communities is now more fully recognized. Protection of riparian habitat along the Santa Clara River is a primary objective of the Friends of the Santa Clara River. It is just too important to lose!

REFERENCES

ItemCalifornia's Changing Landscapes: Diversity and Conservation of California Vegetation, Michael Barbour, et al, 1993, California Native Plant Society
ItemCalifornia Vegetation, V. L. Holland and D. Keil, 1990. California Polytechnic State University


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