Gametophytic Plasticity among Four Species of Ferns with Contrasting Ecological Distributions.

The goals of this study were to test the hypothesis that plasticity of life history traits is greater among species with broad ecological distributions than among species with narrow ecological distributions and to determine whether homosporous filicalean fern gametophytes exhibit adaptively meaningful responses to soil substrates. Sporophyte populations of Asplenium platyneuron and Polystichum acrostichoides have broader ecological distributions and were therefore expected to exhibit a greater amount of gametophytic plasticity than either Asplenium rhizophyllum or Diplazium pycnocarpon. Changes in reproductive effort (i.e., gametangia mm-2) were investigated across the four following soil treatments, which represent major edaphic environments on which sporophyte populations of these species occur: stream bank, Pinus strobus canopy, oak forest ridge top, and soil from a lime-bearing rock outcrop. Our results failed to support the "ecological breadth" hypothesis. Three of the four species-A. rhizophyllum, D. pycnocarpon, and P. acrostichoides-exhibited similar amounts and directions of plasticity, with reproductive effort greatest in treatments that were unfavorable to growth; A. platyneuron exhibited the least amount of plasticity. Nevertheless, species-specific patterns in gametophytic size and reproductive effort reflected the distributions of their sporophytic counterparts. Plasticity of size-related reproductive effort may be common among homosporous filicalean fern gametophytes because of their short life spans and limited capacity for vegetative competition.