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Spread, Impact, and Control of Purple Loosestrife (Lythrum salicaria) in North American Wetlands

Field Identification, Classification, and Distribution


The name "loosestrife" has been associated with several members of the loosestrife family (Lythraceae) as well as with the genus Lysimachia in the primrose family (Primulaceae). Although Lythrum salicaria has more than 10 common names in America and Great Britain, the most widespread and best established usage is "purple loosestrife. " L. salicaria (Fig. 1) is most easily identified during its long season of bloom (late June to early September in most areas). At this time, the characteristic reddish-purple floral masses can be easily identified at 100 m. In North America, purple loosestrife may be confused with one of the following: fireweed (Epilobium angustifolium), blue vervain (Verbena hastata), blazing stars (Liatris spp.), and in the Pacific Northwest, spirea (Spiraea douglasii). Nevertheless, once observers sharpen their search images for the subtle differences in color and floral structure between these plants, positive field identification is easy. Differences in site preferences also help to separate purple loosestrife from superficially similar species; L. salicaria usually occurs on moist or saturated soils. Moreover, it is the only reddish-purple flowered plant to develop massive monospecific blocks of showy floral displays over large wetland tracts. In early autumn, a distinctive but transitory color change usually occurs at the end of the growing season when L. salicaria leaves dehydrate and turn bright red; this color may persist for 10 days. Dead stalks remain standing through winter and have a characteristic brown tone that, in combination with spire-shaped capsule clusters, readily identifies the species.

GIF-Purple loosestrife
Fig. 1. Structure, growth forms, and field identification characters of Lythrum salicaria (purple loosestrife).

Classification

Lythrum is the type genus of the loosestrife family (Lythraceae). About 22 genera and 500 species occur worldwide; representatives include herbs, shrubs, and trees that reach their greatest abundance in the American tropics where two of the largest genera (Cuphea and Diplusodon) are endemic (Gleason 1963). Several of the genera include species that are cultivated as ornamentals, but none are food or fiber producers. About 35 species of Lythrum occur throughout the world; Shinners (1953) recognized 12 species in his synopsis of Lythrum in the continental United States. He concurred with most American botanists that 3 of these 12 species (L. hyssopifolia, L. virgatum, and L. salicaria) were nonindigenous. L. hyssopifolia, an annual, very likely came from Europe where it is listed as a weed in seed of clover and lucerne (Salisbury 1961). We found no mention of L. hyssopifolia as an agricultural weed in North America, despite its widespread occurrence in waste places and disturbed wetlands.

L. virgatum is a purple-flowered loosestrife, somewhat similar to L. salicaria, but distinctive in that it is glabrous throughout, with leaves narrowed at the base. The leaves of L. salicaria are wide (cordate) at the base and more or less covered with fine hairs (downy). Fernald (1950) considered L. virgatum to have come from Europe, and to have escaped from cultivation in local areas in Massachusetts and New Hampshire; it does not seem to be spreading beyond these areas. Hultén (1971) suggested a close relationship between L. virgatum and the nearly glabrous races of L. salicaria recognized by Hara (1956) from the USSR and Japan. Shinners (1953) was not able to obtain North American specimens of L. virgatum, but, from examination of three Eurasian specimens he concluded that they were "not very similar to L. salicaria, many of the flowers being solitary or merely paired."

Hultén (1971) considered the several races of L. salicaria occurring over the world to be difficult to distinguish, especially on the basis of pubescence. He referred to the L. salicaria complex and noted that its variability was also reflected in reports of chromosome numbers of 30, 50, and 60. Although we did not attempt to evaluate the genetic status of the L. salicaria complex in North America, we examined specimens of L. salicaria from nearly all sectors of its continental range in North America and found that, morphologically, they were fairly uniform with all of the specimens assignable to L. salicaria. Moreover, the work of Shamsi and Whitehead (1974b) showed that L. salicaria responds to a decrease in light intensity with increased leaf area and decreased amount of tomentum. This suggests that some of the variability reported in herbarium specimens may have been influenced by light levels at the site of collection. We recognize that the variability of L. salicaria needs investigation, but believe that we are dealing with a well-defined species. From this perspective, we are thankful not to be struggling with the high degree of variability characteristic of the leafy spurge (Euphorbia esula) complex (Croizat 1945) wherein 11 closely related taxa have been identified in collections of this widespread and pernicious agricultural weed.

Origin and World Distribution

Although the geographical origins of L. salicaria are uncertain, Hultén (1971) showed European and Asian centers of distribution (Fig. 2). The European segment is much the larger and extends from Great Britain across Europe to the Central USSR. L. salicaria is abundant in England, Wales, and Ireland but becomes rare in Scotland (Clapham et al. 1962; Perring and Walters 1976). It reaches its northwestern limit of occurrence near Sogne Fjord on Norway's North Sea coast and extends through southern Sweden and Finland into Russia with its northern limit near the 65th parallel. It is common throughout central and southern Europe (Palhinha 1939; Polunin 1969), but is not known from the Azores, Balearic Islands, Crete, the Faeroes, Iceland, or Svalbard (Tutin et al. 1968). In the Mediterranean basin it occurs in northern Morocco and along the coastal fringe of Algeria. It is common, although not abundant, in Italy (D. Q. Thompson, 1984 unpublished field notes). It continues eastward through the Balkans to the western shore of the Caspian Sea. Zohary (1972) reported purple loosestrife to be "locally fairly common" in riverbank and swamp habitats in Palestine.

GIF-Distribution maps of the Northern Hemisphere
Fig. 2. Distribution of purple loosestrife (Lythrum salicaria), broad-leaved cattail (Typha latifolia), and reed canarygrass (Phalaris arundinacea), in the Northern Hemisphere (after Hultén 1964, 1971). Shaded areas indicate zones of more or less continuous distribution; dots represent collection sites; solid lines indicate probable outer distribution boundary; dashed lines are tentative outer limits.

North and east of the Caspian Sea, a considerable gap in the distribution of L. salicaria occurs in arid western Mongolia, western and central China, and Tibet. Hultén (1971) indicated the main islands of Japan as the core of the Asian segment of the species' native range, with outlying populations extending from the Amur River south across the lowlands of Manchuria and China to Southeast Asia and northern India. In historic times, L. salicaria has expanded to a circumpolar distribution in the northern hemisphere, a pattern shown by Hultén (1958, 1971) to be typical of more than a hundred other European or Asian dicots. L. salicaria has also invaded other temperate and subtropical areas and is now known in eastern Africa (Ethiopia), Australia, and Tasmania. In an atlas of world weeds, Holm et al. (1979) showed L. salicaria as present in New Zealand. We are not aware of confirmed L. salicaria records in South America or South Africa. As with many weeds, the distribution and timing of the spread of L. salicaria suggest a close association with exploration and human emigration from northern Europe and the development of international maritime commerce.


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