The invasion by the alien shrub saltcedar (principally Tamarix ramosissima) is perhaps the worst ecological disaster ever to befall the riparian areas of the western United States. Saltcedar was first introduced from central Asia in 1837 and was widely planted in the west as an ornamental and for streambank stabilization. It soon became naturalized, and by the early 1900's was recognized as a minor pest along some streams. Its populations exploded during the 1930's and during the next 20 years, it occupied most of the stream bottoms and lake shores from the Mexican border to Montana and from the central Great Plains to California. By the 1950's, it occupied large areas in almost pure stands of impenetrable thickets, having crowded out nearly all of the native riparian vegetation. By 1970, saltcedar occupied over one million acres of this most critical ecosystem in the semi-arid and desert areas of the west (Robinson 1965; Horton and Campbell 1974).
The genus Tamarix, with ca. 54 species, originated in Asia, with a primary center of origin in Pakistan, Iran, Afghanistan and southern Turkmenistan to western China, and a secondary center of speciation in the eastern Mediterranean area. The native range of the genus extends from the western Mediterranean across North Africa and southern Europe, along east Africa to South Africa, and across Turkey, the southern former USSR and to India and Korea (Baum 1978). Tamarix is an ancient genus adapted to arid areas and saline soils. The order Tamaricales contains only two small families -- Tamaricaceae and Frankeniaceae, though some taxonomists place them as an isolated group in the large order Violales (Cronquist 1988). Tamaricaceae contains two small genera besides Tamarix, Myricaria and Reaumuria, both restricted to Eurasia; no species of Tamaricaceae are native in the Western Hemisphere (Table 4).
In the Frankeniaceae, the genus Frankenia occurs mainly in Australia, with several species native to Chile, a few species in Asia, and six species of mostly uncommon, low desert shrubs native to the western United States and northern Mexico, one of which, F. johnstonii, is endangered (Whalen 1987).
A total of 10 species of Tamarix have been introduced into North America (Baum 1967, Crins 1989). All except athel (Tamarix aphylla) are shrubs or small trees to 15 or 20 feet high (rarely to 35 feet high and 12 in. basal diameter), deciduous, with pink flowers in clusters of loose spikes along the terminal branches, and with bract-like foliage; all are facultative phreatophytes.
Most Tamarix species occur only as scattered ornamental plantings that still survive or that are barely naturalized. These occur throughout most of the United States, including the southeastern area to the Atlantic coast. Various species have been named in the U.S. literature as major pests in the West, i.e., T. ramosissima, T. chinensis, T. parviflora, T. gallica, and T. pentandra. However, specimens we have collected during the last five years from many of the major infestations of the West, from Texas to Wyoming to California, all were identified by B. W. Baum as T. ramosissima. Identification to the specific level in Tamarix is very uncertain for many species, and none of the U.S. identifiers claim to be Tamarix specialists. The major weedy species in the U.S. may be a hybrid, or hybrids, different from any Old World species. These taxonomic uncertainties are unlikely to be further resolved without resort to chemical, isoenzyme or DNA analyses, which have never been attempted.
Athel is quite distinct from the other naturalized species. It is evergreen, very large (to 50 feet high and 3-4 feet diameter), and with distinctive foliage and flowers. Athel is widely used as a shade tree and sometimes for windbreaks in the southwest (it is not tolerant of hard freezes) and is only minimally naturalized (southern California). It is not now regarded as a pest in the U.S. It has become a serious, invading pest in central Australia and it may have that potential in the U.S., but we do not now regard it as a target for biological control. We are at present avoiding selecting biocontrol agents for saltcedar that would cause obvious damage to athel trees. Its distribution across northern Africa and southern Asia does not overlap that of the more northern T. ramosissima and few of the insects that attack T. ramosissima also attack T. aphylla.
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For information on the outcome of this workshop or integrated weed management in the Pacific Region (Region 1), U.S. Fish and Wildlife Service, Portland, OR, contact: Scott_Stenquist@fws.gov