Gary P. Bell, Southern California Area Ecologist, The Nature Conservancy, Temecula Projects Office, 22115 Tenaja Road, Murrieta, CA 92562
More than 95% of the historic riparian habitat in the southern part of the state has been lost to agriculture, development, flood control, and other human-caused impacts. The greatest threat today to the remaining riparian corridors is the invasion of exotic plant species, primarily giant reed (Arundo donax. Giant reed readily invades riparian channels, especially in disturbed areas, is very competitive, difficult to control, and to the best of our knowledge does not provide either food or nesting habitat for native animals The reed does compete with native species such as willows, mulefat, and cottonwoods which provide nesting habitat for least Bell's vireo, willow flycatcher and other native species.
Ecological value of native riparian systems:
Cottonwood/willow riparian forest is a dynamic community, dependent upon periodic flooding to provide substrate, nutrients, and to cycle the community back to earlier successional stages (Figure l). Periodic floods of large magnitude, and migration of the river channel, are essential to laying down fresh alluvial deposits where seeds of mulefat, willow and cottonwood can germinate and propagules of willow can take root.
Adequate moisture and an absence of heavy flooding is particularly critical to the survival of the young trees through their first year. As these seedlings mature the river continues to deposit sediment on the floodplain. This sediment deposition builds the river terraces and, as they are elevated, other plant species colonize resulting in further diversification in the floodplain community.
When cottonwood/willow riparian scrub, which may include such species as mulefat, California grape, California blackberry ( Rubus ursinus ), and creek nettle (Urtica dioica), reaches four or five years of age, it begins to exhibit the structural diversity required by breeding least Bell's vireo. Least Bell's vireo, along with willow flycatcher, yellow-breasted chat, yellow warbler, and many other species may continue to use this diverse community for another ten to twenty years. Gradually the canopy of the maturing willows and cottonwoods begins to shade out the diverse understory of vascular plants required by these birds. Older riparian gallery forest will continue to be used by western yellow-billed cuckoo, Cooper's hawk, warbling vireo and other species, but as the stand ages the diversity of the flora and fauna within the forest declines. Annual flooding and occasional large flood events maintain this cycle of succession and therefore maintains a mosaic of diverse natural communities.
Arundo is a genus of tall perennial reed-like grasses with six species native to warmer parts of the Old World. Giant reed, Arundo donax, is the largest member of the genus and is among the largest of the grasses (Poaceae), growing to more than 25 feet tall. Giant reed is native to Europe, and is found in freshwaters in the Mediterranean region. Giant reed was purposefully introduced to California in the 1820's in the Los Angeles area as an erosion- control agent in drainage canals. Giant reed was also used as thatching for roofs of sheds, barns, and other buildings.
Giant reed is a hydrophyte, growing along lakes, streams, drains and other wet sites. It uses prodigious amounts of water to supply its incredible rate of growth. Under optimal conditions giant reed can grow more than three inches per day.
Arundo as a competitor:
Within its introduced range, giant reed is an aggressive competitor. Giant reed flowers in late summer with a large, plumelike panicle. Fortunately for California land managers the seeds produced by Arundo in this country are seldom, if ever, fertile. As such, spread, and therefore management, of giant reed is essentially an intra-basin and downstream phenomenon. This species is well adapted to the high disturbance dynamics of riparian systems as it spreads primarily vegetatively. Flood events break up clumps of Arundo and spread the pieces downstream. Fragmented stem nodes and rhizomes can take root and establish as new plant clones.
Once established this species tends to form large, continuous, clonal root masses, sometimes covering several acres, usually at the expense of native riparian vegetation which cannot compete with Arundo. Giant reed is also highly flammable throughout most of the year, and the plant appears highly adapted to extreme fire events. While fire is a natural and beneficial process in many natural communities in southern California it is a largely unnatural and pervasive threat to riparian areas. Natural wild fires usually occur during rare lightening storm events in late fall, winter, and early spring. Under these conditions the moist green vegetation of riparian areas would normally act as a fire break. Human-caused wild fires, in contrast, often occur during the dry months of the year. Dryer conditions in riparian zones at this time of year make them more vulnerable to fire damage. Because it is extremely flammable, once established within a riparian area giant reed redirects the history of a site by increasing the probability of the occurrence of wildfire, and increasing the intensity of wildfire once it does occur. Giant reed effectively changes riparian forest from a flood-defined to a fire-defined natural community (Figure 2).
2Rhizomes respond quickly after fire, sending up new sprouts and quickly outgrowing any native species which might have otherwise taken root in a burned-over site. Fire events thus tend to help push riparian stands in the direction of pure Arundo donax. This usually results in significant stands of giant reed with little additional plant species diversity.
Giant reed as habitat - NOT!:
Establishment and success of giant reed within a riparian corridor thus results in a decline in the diversity of native riparian plant species - reed supplants native habitat. All evidence indicates that giant reed does not provide either food or habitat for native species of wildlife. Areas largely taken over by giant reed are therefore depauperate of wildlife. This also means that native flora and fauna do not offer any significant control mechanisms for giant reed. It is uncertain what the natural controlling mechanisms for this species are in the Old World.
Recent studies by SAWPA have indicated that reed also lacks the structure necessary to provide significant shading of bank-edge river habitats, resulting in warmer water than would be found with a native gallery forest of willows. As a result, riverine areas dominated by giant reed tend to have warmer water temperatures and lower diversity of aquatic animals, including fishes. In the Santa Ana River system this lack of streambank structure and shading has been implicated in the reduction of rare native stream fishes including the arroyo chub (Gila orcuttdi), three-spired stickleback (casterosteus aculatus), speckled dace (Rhinichthys osculus), and Santa Ana sucker (catostomus santaanae).
Other studies have indicated that in addition to higher water temperatures, this lack of stream-side canopy structure may result in increased pH in the shallower sections of the river due to high algal photosynthetic activity. In turn, high pH facilitates the conversion of total ammonia to the toxic unionized ammonia form which further degrades water quality for aquatic species and for downstream users.
By virtue of its growth characteristics, adaptations to disturbance, especially fire, its lack of natural predators and competitors in North America, and its unsuitability as food or habitat for native wildlife, giant reed has established itself as one of the primary threats to native riparian habitats in the western United States.
Chadwick and Associates. 1992. Santa Ana River use attainability analysis. Volume 2: Aquatic biology, habitat & toxicity analysis. Santa Ana Watershed Project Authority, Riverside, CA.
Franzreb, K. 1989. Ecology and conservation of the endangered least Bell's vireo. U.S. Fish and Wildlife Service, Biological Report 89(1). 17 pp.
Gregory, S.V., F.J. Swanson, W.A. McKee, and K.W. Cummins. 1991. An ecosystem perspective of riparian zones. Bioscience 41:540551.
Pike, J. and L.R.Hays. 1993. Final Report: The status and management of the Least Bell's Vireo within the Prado Basin, California, 1986-1992. Prepared for The Nature Conservancy, Temecula, CA.
Holland, R.F. 1986. Preliminary descriptions of the terrestrial natural communities of California. Non-game Heritage Division, California Department of Fish and Game, Sacramento, CA. 156 pp.
Hoover, D.A. 1990. Unnatural plant communities in the Santa Monica Mountains. Fremontia 18:26-29.
Hoshovsky, M. 1989. Arundo donax. Element Stewardship Abstract. The Nature Conservancy, San Francisco, CA. 10 pp.
Munz, P.A. 1974. A flora of southern California. Univ. of California Press, Berkeley, 1086 pp.
Richter, B.D. and H.E. Richter. 1992. Development of groundwater and ecological models for protecting a southwestern riparian system. Proceedings, First International Symposium on Groundwater Ecology, Tampa, Florida.
Robbins, W.W. 1940. Alien plants growing without cultivation in California. Cal. Agric. Exp. Sta., Bull 685, 128 pp.
Schoenherr, A.A. 1992. A natural history of California. California Natural History Guides, No. 56. University of California Press, Berkeley, CA. 772 pp.
Zembal, R. 1990. Riparian habitat and breeding birds along the Santa Margarita and Santa Ana Rivers of southern California. Pp. 98-114 In: A.A.Schoenherr [ed.] Endangered plant communities of southern California. Southern California Botanists, Special Publ. No. 3.
Zembal, R., K.J. Kramer and R.J. Bransfield. 1985. Survey of vegetation and vertebrate fauna in the Prado Basin and the Santa Ana River Canyon, California. U.S. fish and Wildlife Service, Laguna Niguel field Office.