| Summary: | Thesis (M.Sc.)--Memorial University of Newfoundland, 2009. Biology Includes bibliographical references (leaves 169-176) While Canada thistle (Cirsium arvense) is notorious as an aggressive, invasive non-native weed in agricultural fields, grasslands, and roadsides throughout North America, it has not typically posed a threat to boreal forests. However, in the balsam fir (Abies balsamea) -dominated lowland boreal forests in Gros Morne National Park (GMNP- Newfoundland, Canada), Canada thistle has recently invaded natural areas on a large landscape scale, occurring in 42% to 55% of anthropogenic and natural forest gaps, respectively, and frequently forming dense monocultures. It is important to determine if and how Canada thistle invasion will affect regeneration of native trees, particularly since regeneration of gaps in GMNP is already threatened by non-native, hyperabundant moose (Alces alces) populations, which exert extreme browsing pressure on forests. This study assessed the condition of forest gaps to support conifer regeneration by describing the current level of balsam fir regeneration, quality of seedbeds, and degree of Canada thistle invasion. Balsam fir seed and seedling addition experiments were performed in gaps to determine the effect of thistle presence on emergence, growth, and survival of balsam fir. Finally, the potential for allelopathic impacts on native conifers from Canada thistle was assessed in greenhouse experiments. Results revealed that gaps are not regenerating, contain poor seedbeds for conifer recruitment, and are heavily disturbed by moose browsing. Canada thistle invasion further threatens balsam fir emergence and early seedling survival. However, older, transplanted fir seedlings were not negatively affected by thistle, suggesting that seedling planting may be an effective management strategy to encourage fir regeneration in thistle-invaded gaps, and potentially even phase out shade-intolerant thistle plants over time.
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