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

Abstract


Purple loosestrife (Lythrum salicaria L.) is an emergent aquatic plant of Eurasian origin; with the rise of marine commerce, it became established on all mid-latitude continents, except South America. L. salicaria arrived along the northeastern maritime coast early in the 19th century and has subsequently spread across mid-latitude North American wetlands. Several modes of colonization or escape were probable, including ship's ballast, livestock bedding and forage, wool, and purposeful import as seeds or rootstocks for early gardens and herb beds. The gene pool shared by these immigrants accumulated over 50 years or more from a wide climatic and geographic range across Europe and Asia Minor. Rapid gene flow is inferred from the more or less equal distribution of three forms of flowers (short-, mid-, and long-style) occurring over Europe and North America. Purple loosestrife's plant associates in North America are cattails (Typha spp.), reed canarygrass (Phalaris arundinacea), sedges (Carex spp.), bulrushes (Scirpus spp.), willows (Salix spp.), and horsetail (Equisetum fluviatile), in the order mentioned. All of these plants have conspecifics (or nearly identical taxa) that are native to Eurasia, suggesting that L. salicaria was strongly preadapted to be a successful immigrant.

The bisexual flowers of purple loosestrife are insect-pollinated. Self-pollination is possible, but cross-pollination prevails. The period of bloom in most areas is from late June to early September. An old plant can produce more than 2 million seeds per growing season; sexual reproduction is of overwhelming importance. Seed dispersal is largely by drift in moving water; long distance spread is possible by seeds imbedded in mud on water birds, trucks or off-road vehicles, or in the cooling systems of outboard motors. Erratic spread can also occur by purposeful introduction as a honey bee forage plant or by accidental escape from horticultural plantings. Seed samples from commercial suppliers of wildlife cover and prairie restoration plants have contained purple loosestrife seeds as an impurity. Seed longevity is at least 3 years with viability of 80 + %. Floating seeds or propagules must lodge on open, moist soil or saturated organic debris to take root. Once established, purple loosestrife can survive with 50% of full sun, but declines in vigor at lower light levels. The plants grow on a wide range of substrates, but are more successful on slightly acid or neutral soils. L. salicaria has many ways of adapting to a wide range of habitats. It responds to soil nutrient (P and N) deficiencies by increasing the root to shoot ratio, to rising water level by the growth of aerenchyma in submerged stem tissue, and to trampling, cutting, or crushing of stems with shoot and root buds at the site of damage.

The spread and dominance of purple loosestrife in North American wetlands has shown a pattern of exponential increase that corresponds to the rate of exploitation of these habitats. With the construction of the eastern canal systems, and the extension of marine commerce into the Great Lakes, L. salicaria colonized the glacial marshes of the Midwest by 1900. By 1940, it was established in the Pacific Northwest and had begun to spread onto the Great Plains; by 1985, Alaska and Montana were the only States north of the 35th parallel that had not reported purple loosestrife.

The impact of purple loosestrife on native vegetation has been disastrous, with more than 50% of the biomass of some wetland communities displaced. Monospecific blocks of this weed have maintained themselves for at least 20 years. Impacts on wildlife have not been well studied, but indicate serious reductions in waterfowl and aquatic furbearer productivity. Several declining species of vertebrates are threatened with further degradation of their breeding habitats with the continued expansion of purple loosestrife. The plant's low palatability to livestock makes it a problem in wetland pastures in the Northeast; it also threatens riparian hay meadows and off-water swales in irrigated areas in the West. Although L. salicaria can invade relatively undisturbed habitat, the spread and dominance of this weed have been greatly accelerated in disturbed habitats. Despite early control efforts in Quebec, and subsequent work in the United States, little research on purple loosestrife ecology and control has been done. Glyphosate has been used successfully, but no effort has been made to measure the impact of this broad-spectrum herbicide on native plant communities. Enough preliminary work has been done with biological control of purple loosestrife to suggest that this promises to be a valuable approach.

On both regional and local scales, the rate of spread of L. salicaria has followed the logistic curve. At present, coping with purple loosestrife hinges on early recognition of the arrival of the weed and a conservative pattern of marsh management that avoids stressing native plant communities. Early detection allows local eradication to be carried out with minimum damage to the native plant community. Some form of integrated control is needed. California, Illinois, Ohio, Minnesota, and Wisconsin have taken prompt action to deal with this weed.


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