Biology of Arundo donax (Giant reed, giant cane)

Where it comes from: Arundo was probably introduced into California in the early 1800's (Bell 1993). Although it is often reported as a native of the Mediterranean region, it was probably introduced to this area many years ago, and originally came from eastern Asia (Fornell 1990).

The plant: Arundo is a tall, perennial, cane-like grass that resembles bamboo (Renvoize 1980). The main stems of Arundo reach a height of 3 to 10 meters, a diameter of 1 to 4 centimeters, and commonly branch during the second year of growth. The stems and leaves may remain green throughout the year, but generally they turn brownish during winter months. Flowers are borne on a large plume-like terminal panicle and seeds are wind dispersed. However, plants in southern California seem to rarely flower (J. Giessow, pers. obs.). It is reported that Arundo does not produce viable seed in most areas where it has been introduced (Perdue 1958). The root system of Arundo consists of fleshy, compact masses of rhizomes (underground stems) from which arise tough, fibrous roots that penetrate deeply into the soil. Once established, Arundo forms large, dense clonal rhizome masses that produce a high density of stems.

How it spreads: The main method of spread of Arundo is by the growth of dispersed rhizome fragments (Else 1996, Bell 1993). Floods provide the mechanical disturbance necessary to break off rhizome sections from the parent plant, and also act as the dispersing agent by carrying the pieces downstream. There are no records of Arundo seedlings in southern California and it does not seem to flower very often. It is uncertain why the seed is not viable in some areas, although Bhanwra (1988) found that seed set was poor in India due to the failure of meiosis in the majority of ovules.

Why it is 'bad': Arundo is an undesirable plant because it can take over large areas due to its aggressive growth and ability to survive in a variety of environmental conditions. Once Arundo is established is presents a fire hazard, a flood hazard, and it degrades natural resources.

FIRE: The high density and tall height of Arundo stems creates a structure that produces a hot fire. The vegetation is flammable, even when green. Because Arundo tends to fill in all areas that it grows in, fire is spread to neighboring structures of vegetation types. The height of Arundo can spread fire into tree canopies.

FLOOD: During flooding events Arundo stems and rhizomes break off and are carried downstream. This is the main way that Arundo reproduces. Due to the large amount of Arundo present in some rivers and drainages, the biomass of stems and rhizomes that is carried downstream during floods can be quite large. This biomass collects on the bridges, drainage pipes, and other flood control structures, causing compromised functioning, costly cleanup, and structure failure in some instances. The presence of Arundo in drainage ditches, water canals etc. on public or private lands requires continual maintenance to keep areas clear and functioning.

NATURAL RESOURCES:

It is estimated that 95% of the native riparian habitat in Southern California has already been destroyed (Bell 1993). The little that is remaining is a precious resource for the public and for the native flora and fauna. Unfortunately much of the remaining habitat is infested with the invasive non-native plant Arundo donax (giant reed). Arundo does not provide food and habitat resources for native wildlife that are equivalent to the native vegetation. Arundo can form thick stands that exclude native vegetation and allow almost no understory vegetation due to its dense growth (per. obs.).

Although it has often been stated that Arundo provides no benefit for native wildlife, Arundo has in fact been found to be used by some wildlife. However, Arundo still provides little value for native wildlife in comparison to native vegetation, especially when it forms large, monotypic stands. Wildlife such as woodrats and coyotes, and many bird species have been found using Arundo for cover and nesting (Greaves). Two endangered bird species, Least Bell’s vireo and the southwestern willow flycatcher, have been found to use Arundo as a nest host. Least Bell’s vireos have been found nesting on Arundo along the Santa Clara River and the San Luis Rey River. On the Santa Clara River from 1994 to 1999 approximately 5% of the vireo nests were recorded on Arundo (Greaves, pers. comm.), and on the San Luis Rey River from 1988 to 2000 there were approximately 0.5% on Arundo (5 out of a total of 906 nests) (Kus, pers. comm.). Although Arundo may provide a nest site or nest concealment, the entire territory of these birds encompasses areas with native vegetation. More data is needed to fully understand the use of Arundo by native wildlife in comparison to the native habitat, and the degree of Arundo usage in proportion to its abundance. Data is also needed on the use of Arundo by arthropods, the main food source for many bird species.

While Arundo cannot simply be dismissed as having no use for native wildlife, it clearly does not provide the complexity and diversity of native habitat. Furthermore, it is important not to lose site of the larger picture – the functioning of the entire riparian system. A riparian system that is dominated by Arundo is simply not going to function in the same manner as a native system, either ecologically or hydrologically. Arundo alters large-scale processes such as erosion, sedimentation, flooding, and fire which affect the entire ‘life cycle’ of the riparian system and all the creatures within it (whether they be plants, birds, fish, mammals, or insects). The natural flood regime and riparian succession after these events is altered by the presence of Arundo. The use of Arundo by some native wildlife indicates that Arundo control efforts should consider both present and future ecological impacts. Such considerations may include carrying out control work when native wildlife are not breeding or are not present. Replanting after Arundo removal may be needed in areas that are not expected to re-establish on their own within a reasonable period of time (such as areas that have a compromised/altered hydrological regime). This may be particularly important in drier riparian areas, ‘marginal’ riparian habitat, or ‘man-made’ riparian areas (e.g. irrigation ditches, berms) that may be outside of the zone of flooding and probably would not re-establish native vegetation without intervention. While these areas may be considered ‘degraded’ riparian habitat, they may still be very important to native wildlife and provide an important connection between the main riparian areas and upland habitat.

If we do not proceed with Arundo control then our valuable riparian systems will continue to be degraded by the ever expanding Arundo. These control efforts are very important to the long-term viability of all the species in the riparian systems, but should be carried out in a manner that does not unnecessarily impact the native wildlife, especially endangered species.

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