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Citation. Hager, H. 2002. Invasion dynamics of purple loosestrife in wetlands. Ph.D. Thesis, University of Regina.

Abstract. Invasion of species into novel ecosystems is a problem because they can cause changes in ecosystem structure and processes. Plant invasion is context-specific and depends on propagule pressure and the interaction of environmental characteristics and traits of invading species. Understanding these factors facilitates control and prevention of plant invaders. Therefore, I investigate factors affecting invasion of non-native Lythrum salicaria L. (purple loosestrife, Lythraceae) in North American, Typha -dominated marshes.

I use field surveys to assess the widely accepted hypothesis that invasion of L. salicaria is associated with a reduction in the abundance of native plant species. I find that plant diversity is higher in invaded than in univaded wetland patches. However, seed bank species richness does not differ between invaded and uninvaded patches. Therefore, invasion of L. salicaria can be associated with changes in vegetation composition and higher native plant species richness.

I use greenhouse, mesocosm and field experiments to examine several hypotheses regarding factors promoting L. salicaria germination, establishment, survival and growth in wetlands. Specifically, I test the effects of dispersal, nutrient addition, ambient seedling herbivory, vegetation type and disturbance of neighbouring plants and plant litter on L. salicaria recruitment, and the effects of vegetation type and disturbance of neighbouring plants and plant litter on L. salicaria seedling survival and growth. I find that L. salicaria distribution is limited by both the ability of its seeds to disperse among wetlands, and by environmental conditions for its germination and establishment within wetlands. Recruitment is most affected by disturbance; disturbance that removes aboveground neighbouring vegetation and particularly, that removes plant litter, results in increased recruitment. There are lesser effects of vegetation type on L. salicaria recruitment, and no effect of fertilization or herbivore exclosure. L. salicaria growth is also disturbance-dependent, and removal of litter particularly increases the growth of L. salicaria seedlings. L. salicaria seedlings are not better competitors than native seedlings in established vegetation; however, established L. salicaria vegetation is more suppressive for growth of seedlings than are native vegetation types. Regardless, some seedlings can recruit and grow in established vegetation and once L. salicaria vegetation is established, it can suppress establishment of new seedlings.

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