General Impact
Fagan and Peart (2004) state that, "The invasion of F. alnus into forests and riparian areas in the Northeast has alarmed foresters and conservationists (Reinartz, 1997; Post et al. 1989 and Converse, 1984). Buckthorn attains high cover in the understory of closed white pine (Pinus strobus L.) forests, especially in gaps. Tree seedling density and diversity are known to be inversely related to buckthorn basal area, implying that buckthorn lowers the survival or germination rates of tree seedlings ( Frappier et al. 2002). Fagan and Peart (2004) state that, "Forests dominated by white pine are common throughout New England and are economically important sources of timber. Because F. alnus has frequent seed crops in sunny areas and can form seed banks in nearby forests (Frappier et al. 2002), it is likely to continue increasing in pine forests in the northeastern US for the foreseeable future. White pine does not regenerate under a closed canopy." In the study conducted by Fagan and Peart (2004), they state that, "F. alnus in the forest understory clearly reduced the extension and radial growth of saplings of all species. F. alnus has an extensive shallow root system (M.E. Fagan, pers. obs.) and may be a strong below-ground competitor. F. alnus altered the relative abundance of seedlings towards a shade-tolerant species (sugar maple). F. alnus reduced the performance (growth and/or survival) of all tree seedlings, except for sugar maple. The survival of the most shade-intolerant species, white pine, was so reduced by F. alnus cover that pine survival to sapling size beneath buckthorn is highly unlikely. Removal experiments and surveys by Frappier et al. (2002) also indicate that glossy buckthorn reduces the diversity and density of tree seedlings." Converse (1984) states that, "F. alnus affects the survival of co-occurring species. Other woody plants such as Viburnum opulus L. (in Europe) and Betula pumula L. may be replaced by F. alnus, or are unable to invade F. alnus thickets (Godwin 1936, Lovely 1982)." Catling and Porebski (1994) state that, "Currently in some areas of Ontario, F. alnus comprises more than 90% of the green biomass over areas of several acres and it has become a major component of regionally and provincially significant plant communities." Converse (1984) states that, "In addition to naturalized habitats, these species are problems in parts of some natural areas. F. alnus sometimes invades similar woodland habitats (Brue 1980), but more often invades wetlands that are comparable to its European wetland habitats." The author states 3 other possible reasons why F. alnus may have an easier time invading wetland habitats. These include, "Acidification of surface peat of calcareous fens (Godwin 1974); Exposed mineral soil providing a seed bed (Andreas 1983); and Fire supression and cessation of routine mowing (Godwin 1936, Curtis 1946, Vogl 1969, Godwin et al. 1974, White 1965, Zimmerman 1978, Moran 1981, Gawler 1983)."
Location Specific Impacts:Manitoba (Canada) 
Competition: Haber (2001) states that, "It forms extensive populations, commonly to the exclusion of other species spreading from introduced centres in urban areas to natural sites in adjoining woodlands and wetland sites. It is one of the most aggressive alien plants in wetland habitats of southern Ontario." Nova Scotia (Canada)
Competition: Haber (2001) states that, "It forms extensive populations, commonly to the exclusion of other species spreading from introduced centres in urban areas to natural sites in adjoining woodlands and wetland sites. It is one of the most aggressive alien plants in wetland habitats of southern Ontario." Ontario (Canada)
Competition: Catling and Porebski (1994) state that, "Currently in some areas of Ontario, F. alnus comprises more than 90% of the green biomass over areas of several acres and it has become a major component of regionally and provincially significant plant communities." Quebec (Canada)
Competition: Haber (2001) states that, "It forms extensive populations, commonly to the exclusion of other species spreading from introduced centres in urban areas to natural sites in adjoining woodlands and wetland sites. It is one of the most aggressive alien plants in wetland habitats of southern Ontario." Connecticut (United States (USA))
Competition: Fagan and Peart (2004) state that, "Forests dominated by white pine are common throughout New England and are economically important sources of timber. Because F. alnus has frequent seed crops in sunny areas and can form seed banks in nearby forests (Frappier et al 2002), it is likely to continue increasing in pine forests in the northeastern US for the foreseeable future. White pine does not regenerate under a closed canopy."
Habitat alteration: Fagan and Peart (2004) state that, "F. alnus altered the relative abundance of seedlings towards a shade-tolerant species (sugar maple)."
Reduction in native biodiversity: In the study conducted by Fagan and Peart (2004), they state that, "F. alnus in the forest understory clearly reduced the extension and radial growth of saplings of all species. F. alnus has an extensive shallow root system (M.E. Fagan, pers. obs.) and may be a strong below-ground competitor." Maine (United States (USA))
Competition: Fagan and Peart (2004) state that, "Forests dominated by white pine are common throughout New England and are economically important sources of timber. Because F. alnus has frequent seed crops in sunny areas and can form seed banks in nearby forests (Frappier et al 2002), it is likely to continue increasing in pine forests in the northeastern US for the foreseeable future. White pine does not regenerate under a closed canopy."
Habitat alteration: Fagan and Peart (2004) state that, "F. alnus altered the relative abundance of seedlings towards a shade-tolerant species (sugar maple)."
Reduction in native biodiversity: In the study conducted by Fagan and Peart (2004), they state that, "F. alnus in the forest understory clearly reduced the extension and radial growth of saplings of all species. F. alnus has an extensive shallow root system (M.E. Fagan, pers. obs.) and may be a strong below-ground competitor." Massachusetts (United States (USA))
Competition: Fagan and Peart (2004) state that, "Forests dominated by white pine are common throughout New England and are economically important sources of timber. Because F. alnus has frequent seed crops in sunny areas and can form seed banks in nearby forests (Frappier et al 2002), it is likely to continue increasing in pine forests in the northeastern US for the foreseeable future. White pine does not regenerate under a closed canopy."
Habitat alteration: Fagan and Peart (2004) state that, "F. alnus altered the relative abundance of seedlings towards a shade-tolerant species (sugar maple)."
Reduction in native biodiversity: In the study conducted by Fagan and Peart (2004), they state that, "F. alnus in the forest understory clearly reduced the extension and radial growth of saplings of all species. F. alnus has an extensive shallow root system (M.E. Fagan, pers. obs.) and may be a strong below-ground competitor." Rhode Island (United States (USA))
Competition: Fagan and Peart (2004) state that, "Forests dominated by white pine are common throughout New England and are economically important sources of timber. Because F. alnus has frequent seed crops in sunny areas and can form seed banks in nearby forests (Frappier et al 2002), it is likely to continue increasing in pine forests in the northeastern US for the foreseeable future. White pine does not regenerate under a closed canopy."
Habitat alteration: Fagan and Peart (2004) state that, "F. alnus altered the relative abundance of seedlings towards a shade-tolerant species (sugar maple)."
Reduction in native biodiversity: In the study conducted by Fagan and Peart (2004), they state that, "F. alnus in the forest understory clearly reduced the extension and radial growth of saplings of all species. F. alnus has an extensive shallow root system (M.E. Fagan, pers. obs.) and may be a strong below-ground competitor." Vermont (United States (USA))
Competition: Fagan and Peart (2004) state that, "Forests dominated by white pine are common throughout New England and are economically important sources of timber. Because F. alnus has frequent seed crops in sunny areas and can form seed banks in nearby forests (Frappier et al 2002), it is likely to continue increasing in pine forests in the northeastern US for the foreseeable future. White pine does not regenerate under a closed canopy."
Habitat alteration: Fagan and Peart (2004) state that, "F. alnus altered the relative abundance of seedlings towards a shade-tolerant species (sugar maple)."
Reduction in native biodiversity: In the study conducted by Fagan and Peart (2004), they state that, "F. alnus in the forest understory clearly reduced the extension and radial growth of saplings of all species. F. alnus has an extensive shallow root system (M.E. Fagan, pers. obs.) and may be a strong below-ground competitor."
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