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More info for the terms: cohort, genet, marsh, natural, succession
The ways and extent to which purple loosestrife affects succession in wetland plant communities are not altogether clear. It is evident that purple loosestrife requires open, moist, bare substrate for establishment (see Site Characteristics and Regeneration Processes). It is generally agreed that purple loosestrife is a pioneer or gap-colonizing species that quickly responds to site disturbance by recruiting often-substantial numbers of new genets from a pre-existing seed bank [1,30,110].
Purple loosestrife displays many characteristics typical of pioneer species, such as rapid maturity, high seed production, tolerance of nutrient-poor environments, and high germination success. Yet North American populations, once established, also are potentially long-lived (22+ years), capable of growing to a relatively large size, and have shown the propensity for near-continuous, low-level recruitment in the absence of large-scale disturbance [1,129]. While evidence is somewhat limited, it is speculated natural mortality rates in adult plants are quite low .
Purple loosestrife, once established, can persist within a site for relatively long periods, even in the absence of frequent disturbance. After examining purple loosestrife population structure within several different communities in eastern Massachusetts, Anderson  concluded low levels of nearly-continuous recruitment are likely to occur in areas where mature plants (and the inevitable prodigious purple loosestrife seed bank) are present. Additionally, this trend is punctuated by occasional disturbances that provide conditions suitable for short-lived recruitment episodes in which relatively large cohorts of new plants are established.
But there is some question regarding the view that purple loosestrife inevitably dominates invaded sites in virtual monotypic stands. Anderson  points out that in a widely cited review by Thompson and others , estimates of the proportion of stand biomass attributed to purple loosestrife, which ostensibly increased over time following establishment, may instead have been attributable to increases in the number of stems per genet rather than greater numbers of individual plants. The number of annually produced stems per single genetically distinct plant has been shown to be a good predictor of the age of that individual . Anderson  also notes observations described in Thompson and others  were strictly visual assessments, and since no hard data was collected, there is no way to definitively ascertain what, if any, changes in biomass distribution among species may have occurred.
In its native range, European populations of purple loosestrife may also form large monospecific stands following pregrowing season disturbance, but are prone to invasion by other species soon after stand establishment [110,111]. Whitehead  described the gradual yielding of monospecific stands of purple loosestrife to mixed species communities in England as being due to slow growth of purple loosestrife during periods of cool spring temperatures compared with competitors possessing low-temperature growth capabilities such as cattails or reeds (Phragmites spp.) It is likely that an aggregate of factors act to limit purple loosestrife site dominance in its native habitats .
Thompson and others  have reviewed several historical accounts of purple loosestrife stands, both in its native Europe and elsewhere. They determined that while purple loosestrife seldom maintains strong community dominance in native (European) habitats, it commonly forms dense, long-lasting, virtually monospecific stands in areas where it is not native, especially temperate North America. They considered 3 factors that could possibly account for this phenomenon: 1) the absence of many key insect predators that effectively reduce competitiveness of European purple loosestrife plants, 2) predominance of the muskrat in its native North American habitat and the impact of its selective Foraging behavior on cattails (see Importance to Livestock and Wildlife or Impacts and Control), and 3) the possibility that North American purple loosestrife may have evolved adaptive traits which make it more vigorous and competitive than its European relatives.
Many factors are likely to affect the ability of purple loosestrife to form and maintain extensive monodominant stands in North American wetlands. Characteristics particular to certain classes of habitat may lead to monodominance. Auclair and others  have noted some trends in 2 distinct plant communities of Huntington Marsh, located along the St. Lawrence River near the junction of the Quebec, Ontario and New York borders. In the emergent aquatic community, the dominant emergent taxa tended to exclude each other, resulting in a mosaic of nearly monospecific communities. In particular, river bulrush (Schoenoplectus fluviatilis), common reed (Phragmites australis) and narrow-leaved cattail (Typha angustifolia) displayed this phenomenon. In contrast, sedge meadow communities were much more diverse and lacked the dominance and segregation of species. Instead they demonstrated subtle gradients in composition that were generally difficult to discern.
The nature of particular disturbance events may also impact initial floristics and subsequent successional trajectories. For instance, the relative competitiveness of purple loosestrife seedlings following disturbance may depend upon when initiation of the new seedling community occurs within the growing season. Because purple loosestrife growth rates are closely linked to day length , early summer establishment of a seedling cohort or community, compared with late summer establishment, is more likely to result in a monospecific stand of purple loosestrife because purple loosestrife seedlings will be more competitive .
More research is needed to help elucidate the means and extent to which purple loosestrife alters successional trajectories and community dynamics. Long-term studies that examine preinvasion vs. postinvasion data would be particularly helpful.
- 1. Anderson, Mark G. 1991. Population structure of Lythrum salicaria in relation to wetland community structure. Durham, NH: University of New Hampshire. 93 p. Thesis. 
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- 102. Rawinski, Thomas James. 1982. The ecology and management of purple loosestrife (Lythrum salicaria L.) in central New York. Ithaca, NY: Cornell University. 88 p. Thesis. 
- 110. Shamsi, S. R. A. 1976. Some effects of density and fertilizer on the growth and competition of Epilobium hirsutum and Lythrum salicaria. Pakistan Journal of Botany. 8(2): 213-220. 
- 111. Shamsi, S. R. A.; Whitehead, F. H. 1974. Comparative eco-physiology of Epilobium hirsutum L. and Lythrum salicaria L. I. General biology, distribution and germination. Journal of Ecology. 62(79): 272-290. 
- 112. Shamsi, S. R. A.; Whitehead, F. 1974. Comparative eco-physiology of Epilobium hirsutum L. and Lythrum salicaria L. II. Growth and development in relation to light. Journal of Eocology. 62: 632-645. 
- 118. Skinner, Luke C.; Rendall, William J.; Fuge, Ellen L. 1994. Minnesota's purple loosestrife program: history, findings, and management recommendations. Special Publication 145. St. Paul, MN: Minnesota Department of Natural Resources, Division of Fish and Wildlife, Ecological Services Section. 27 p. 
- 129. Thompson, Daniel Q.; Stuckey, Ronald L.; Thompson, Edith B. 1987. Spread, impact, and control of purple loosestrife (Lythrum salicaria) in North American wetlands. Fish and Wildlife Research 2. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service. 55 p. 
- 141. Whitehead, F. H. 1971. Comparative autecology as a guide to plant distribution. In: Duffey, E. O.; Watt, A. S., eds. The scientific management of animal and plant communities for conservation: Proceedings of the 11th symposium of the British Ecological Society; [Date unknown]; [Location unknown]. Oxford, England: Blackwell Scientific: 167-176. 
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