What are the ecological consequences of allopolyploid speciation in ferns? In northeastern North America, this question is relevant to the Adiantum pedatum complex. Adiantum viridimontanum (Green Mountain maidenhair fern) is an allotetraploid hybrid derived from a sterile cross between the serpentine-restricted diploid species A. aleuticum (Aleutian maidenhair fern) and the rich-woods diploid species A. pedatum (northern maidenhair fern). In this study, we characterized the ecological niche of the allotetraploid hybrid maidenhair fern relative to the niches of its two progenitors by establishing 48 survey plots at 13 sites where the three species occur in Vermont and by measuring a set of ecological attributes in each plot, including the habitat type, light availability, topography, substrate characteristics, and soil chemistry. These data were analyzed using principal component analysis (PCA) to characterize the multivariate ecological niche of each species as circumscribed by the position of survey plots in environmental space. Along the first principal component, the ecological niche of the allotetraploid was situated between those of its progenitors in environmental space, closely overlapping the niche of A. aleuticum and slightly overlapping that of A. pedatum. Along the second principal component, a portion of the ecological niche of the allotetraploid was transgressive relative to the niches of its progenitors in environmental space; in geographic space, these survey plots correspond to northern hardwood serpentine-influenced forests, a habitat type that is novel for the species complex. Thus, the ecological outcome of allopolyploid speciation in the A. pedatum complex appears to include elements of both niche intermediacy and niche expansion into novel habitats.

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