Compiled by Stillwater Sciences.
Non-native Invasive Weed Rating
Federal No rating
State CDFA Noxious Weed “B” Rating
Native Origin and Geographic Distribution
Arundo donax is indigenous to freshwaters of eastern Asia, specifically northern India and southern Nepal (Polunin and Huxley 1987). Approximately 2,000 years ago, A. donax was introduced around the Mediterranean Basin for use in erosion control, reeds for musical instruments and for construction of roofs, ceilings, fences and baskets (Perdue 1958). It has been introduced to most tropical and warm, temperate regions worldwide, including North and South America, Southern Africa and Australia, and thrives below 350 m (~1,150 ft) in elevation (Bell 1997, Brossard et al. 2000). In Southern California, A. donax was prevalent along the Los Angeles River as early as the 1820s and often planted for erosion control along streams and windbreaks (Robbins et al. 1951). Currently A. donax is rapidly invading rivers and streams in Mediterranean-type climates, including coastal watersheds and the central valley of California.
Arundo donax can be found along most rivers and streams in Ventura County. On the Santa Clara River, it grows in large stands or monocultures along floodplains and terraces, and has invaded most riparian vegetation types (Stillwater Sciences and URS 2007). It thrives in open riparian areas with abundant water and nutrients as well as any area susceptible to burning (Coffman 2007).
Arundo donax is rapidly invading riparian ecosystems along rivers in Mediterranean-climate regions worldwide. Following an era of human alterations to river systems in Southern California, it was widely dispersed throughout riparian ecosystems in the floods of 1969, established in terrace and floodplain locations, and is now thriving in riparian ecosystems throughout this region (Coffman 2007). Factors such as quantity of water, nutrients, light, and fire that are abundant in riparian ecosystems of Mediterranean-climate regions increase the competitive ability of A. donax (Coffman 2007).
Life History and Timing
Due to its clonal growth strategy, efficient use of resources, and high growth rate, A. donax is one of the most successful riparian weedy invaders in California (Rieger and Kreager 1989). As a perennial bamboo-like member of the grass family (Poaceae), A. donax spreads vegatatively via a well-developed rhizome; its seeds are known to be sterile outside its native range (Decruyenaere and Holt 2001, Khudamrongsawat et al. 2004). Arundo donax plants are uprooted and dispersed downstream during large, winter flood events characteristic of Mediterranean-type climates (Bell 1994). Portions of the rhizome or culm break off, float downstream, land on a bare, moist substrate as flood waters recede and begin growing. Fragments of the rhizome or culm as small as 2 cm2 (0.79 in2) have been shown to sprout under most soil types, depths and soil moisture conditions (Else 1996, Boose and Holt 1999, Wijte et al. 2005). Growing at an extremely high rate (up to 6.25 cm [2.46 in] per day under ideal conditions), A. donax quickly establishes on unvegetated or sparsely vegetated soil and grows to a height of greater than 6 m (~20 ft) after only a few months (Rieger and Kreager 1989, Coffman 2007). It then expands outward in area, quickly displacing indigenous shrubs, herbs and grasses, and eventually even trees.
When above ground biomass of A. donax dies back in late summer and fall, riparian areas dominated by this plant become susceptible to fire (Scott 1994). Riparian terraces invaded by A. donax adjacent to shrubland communities are most vulnerable (Coffman et al. 2004). Indigenous riparian trees, shrubs and other vegetation not as well-adapted to fire are burned along with A. donax and resprout much slower (Coffman 2007). Arundo donax grows back immediately to completely replace the open burned areas originally dominated by indigenous riparian vegetation ( Coffman 2007). In this manner, A. donax forms extensive stands or monocultures in riparian ecosystems, along floodplains and terraces of southern California’s river and stream systems.
Arundo donax grows primarily in floodplains and terraces of low-gradient river and stream systems in Southern California (DiTomaso 1998, Coffman 2007). Arundo donax successfully invades areas consisting of any soil type and once established can grow well in many soil moisture regimes (Singh et al. 1997, Boose and Holt 1999, Coffman 2007). It is most successful at colonizing open floodplains, containing elevated amounts of water and nutrients (Coffman 2007). However, it may be found on beaches, around homes, and next to hot springs where planted.
In California, A. donax is known to increase the risk of flooding, create fire hazards, out-compete indigenous species for scarce water resources, and reduce the value of riparian habitat for wildlife (Bell 1994, Bell 1997, DiTomaso 1998). Wildfires ignited by man at unnatural and dangerous times of the year burn rapidly through riparian corridors infested with A. donax and may help spread fires across watersheds (Coffman 2007). The federally endangered least Bell’s vireo (Vireo bellii pusillus) and other riparian birds require structural diversity provided by riparian scrub and mature forest communities for breeding (Zembal 1990, Bell 1994, Bell 1997). When natural riparian vegetation types are replaced by thick stands of A. donax, bird species abundance and other native wildlife have been found to decline (Bell 1994, Bell 1997, Herrera and Dudley 2003, Kisner 2004, Labinger and Greaves 2001). Labinger and Greaves (2001) observed over the course of their study (1994–2000) that while dense thickets of A. donax supported very low bird diversity:
… a low to moderate mixture of giant reed [Arundo donax] with native willow woodland supported high bird diversity in some areas [such as near the Freeman Diversion]…. In such areas giant reed was also used for nesting, as noted by at least 17 nests of least Bell’s vireo, one nest of southwestern willow flycatcher, and several other species such as Anna’s hummingbird, bushtit, and common yellowthroat.
Greenhouse and field experiments suggest that A. donax does best in open areas, with high soil moisture and nutrients (Coffman 2007). However, Salix laevigata (red willow) is a strong competitor in these areas as well as in shaded areas with low soil moisture. Baccharis salicifolia (mule fat) outcompetes A. donax in shaded areas with high soil moisture and open, dry areas (Coffman 2007). Addition of nitrogen fertilizer under high moisture conditions significantly increased both A. donax and S. laevigata biomass, but had little effect on B. salicofolia or Populus balsamifera (black cottonwood) biomass (Coffman 2007). Due to its higher post-fire growth rate and immediate growth response compared to natives, fire appears to contribute to the A. donax invasion process especially in riparian terraces (Scott 1994, Coffman 2007).
Anthropogenic Watershed Disturbances that Promote Invasion
Human alterations associated with urbanization of watersheds in California in addition to the natural flood regime have created ideal conditions for A. donax invasion. Ever expanding residential and agricultural development in coastal Southern California has led to increased water availability and nutrient loading of riparian ecosystems. Consequently, open areas along floodplains formed by floods and clearing of terraces for development create an ideal location for weedy species like A. donax to establish. Mature riparian forests continue to be removed to make room for agriculture, golf courses, and residential and commercial development. Fire is more frequent in riparian corridors due to anthropogenic ignition during the dry summer and fall months when A. donax infested areas provide a large amount of dry fuel (Scott 1994, Coffman 2007).
Management strategies for the control and removal of A. donax should be based on location and size of the infestation. The first priority management strategy recommended is removal of A. donax from riparian terrace habitats where infested areas are easily accessible and require less maintenance than along floodplains (Coffman et al. 2004). Arundo donax should be removed first from riparian terraces located adjacent to fire-prone shrubland plant communities where it poses a fire hazard (Coffman et al. 2004). Removal of A. donax infestations on riparian terraces with high soil moisture and nutrient availability will be most difficult and will likely require active revegetation with native plants. Watershed removal plans need to be developed to eradicate A. donax from floodplains. Unless A. donax is removed from floodplains on a watershed-scale working from the headwaters downstream, it is likely to recolonize removal areas after flood events. Both riparian terrace and floodplain areas may require revegetation with native plants to insure continued success of A. donax eradication, prevention of other weed infestations, and restoration of functional riparian ecosystems.
Both mechanical and hand clearing techniques may be used to remove A. donax. Mechanical clearing methods include mulching or total excavation of all above-ground and below-ground biomass. Hand clearing methods include either painting of A. donax stumps with herbicide (glyphosate) after cutting or foliar applications of herbicide (Sonoma Ecology Center and California State University 1999). Research on biocontrol agents for A. donax is underway on the Santa Clara River (T. Dudley, pers. comm.).
Several A. donax removal programs have been implemented or planned along the Santa Clara River recently. The first large A. donax removal project in riparian ecosystems of the Santa Clara River was conducted on a riparian terrace at the Valley View Ranch in Santa Paula through the Partners for Fish and Wildlife program. The landowner entered into an agreement with the U.S. Fish and Wildlife Service to remove all A. donax within the approximately 20-acre property between 2000 and 2002 (S. Hedrick, pers. comm.; D. Pritchett, pers. comm.). Several removal techniques were implemented including: total removal of all biomass using heavy equipment, mulching with hammer-flail equipment that left biomass in place as mulch, and foliar spraying by hand in less accessible areas. Routine foliar herbicide spraying maintenance has been on-going from 2000 to present on this property and most of the A. donax has been successfully removed.
The Nature Conservancy (TNC) removed A. donax from their 98-acre property in the lower Santa Clara River just upstream of the Santa Paula 12th Street bridge in September–October 2002 (E. Kelley, pers. comm.). Large stands of A. donax (above- and below-ground biomass) were removed mechanically using a bobcat, and A. donax interspersed within native riparian vegetation was treated with foliar spraying of herbicide. Resprouts were treated in January 2003 using the foliar spraying technique. In fall 2005, TNC completed a 5-acre A. donax removal project on their property upstream of Highway 101, hand removing A. donax post-flood debris and burning the debris piles (S. Matsumoto, pers. comm.). Monitoring and maintenance plans are underway. Visual inspections were conducted during winter 2005–2006, shoots that emerged in early spring 2006 were sprayed with herbicide, and any resprouts that remain in fall 2006 will be sprayed. Smaller removal projects have been implemented in conjunction with restoration efforts and infrastructure projects such as the Hedrick Ranch Nature Area restoration project (1998–2006; S. Hedrick, pers. comm.) and the Santa Paula airport emergency repairs in 2005 (C. Burns, pers. comm.).
The Ventura County Resource Conservation District (VCRCD) has completed the Upper Santa Clara River Watershed Arundo/Tamarisk Removal Plan (SCARP) and is initiating a similar plan for the lower watershed with the goal of implementing a long-term (20-years) removal plan for A. donax and Tamarix spp. (VCRCD 2006; N. Cabanting, pers. comm..). This plan includes a programmatic California Environmental Quality Act (CEQA) and National Environmental Policy Act (NEPA) document and related documentation for the implementation, maintenance, and monitoring of A. donax and Tamarix spp. removal projects within riparian ecosystems (500-year floodplain) of the upper Santa Clara River watershed. The goal of this comprehensive document is to streamline the permitting process for any agency or organization to perform A. donax and Tamarix spp. removal projects of any size within upper Santa Clara Watershed. From September 2005 to March 2006, the California Conservation Corps and the Los Angeles Agricultural Commissioner (for VCRCD) removed all A. donax from approximately 75 acres (15 acres of solid A. donax) of their 297-acre demonstration project at the confluence of the South Fork of the Santa Clara River and San Francisquito Creek in the City of Santa Clarita. A combination of cut/paint (1–2 acres), foliar spray (2–3 acres), and cut and spray regrowth (12–15 acres) methods were used in this removal project. The VCRCD plans to monitor and maintain this removal area for the next 4–5 years and expand the area of removal each year. According to visual inspection in May 2006, the cut/paint method performed the best with little regrowth (N. Cabanting, pers. comm.). Both the other two methods worked, but adjacent rhizomes had some new regrowth.
Despite the recent research, pilot removal projects, and monitoring programs focused on A. donax, many key uncertainties still exist regarding interactions between A. donax and native riparian species, the effects of A. donax on ecosystem services and functions, and effective control and monitoring of infestations. Outstanding questions regarding A. donax ecology and management on the Santa Clara River are categorized and summarized below.
Interactions with Native Riparian Plant Species:
- How much water is used (transpired) by A. donax compared to native riparian species under various environmental conditions and seasons?
- What is the optimal nutrient uptake rate (and range) and efficiency of N, P and K for A. donax compared to native riparian plant species?
- How does root structure, function and interactions with the soil vary between A. donax and native riparian plants species?
- How do A. donax and native riparian plant species differ in the source (location) of water they used?
Effects on Ecosystem Services and Functions:
- How does leaf litter and denitrification contribute to nutrient cycling in A. donax stands versus native riparian vegetation?
- What is the relationship between biodiversity and riparian ecosystem function in streams like the Santa Clara River?
Control and Monitoring:
- What is the effectiveness of various biocontrol agents on A. donax in the Santa Clara River?
- What is the most effective remote sensing approach for mapping and methods for monitoring A. donax removal and spread?
- What is the long-term effectiveness of herbicide application on suppressing resprouting?
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