title: Understanding mechanisms of rarity in pteridophytes: competition and climate change threaten the rare fern Asplenium scolopendrium var. americanum (Aspleniaceae).
abstract: OBJECTIVE
  Understanding the ecology of rare species can inform aspects of conservation strategies; however, the mechanisms of rarity remain elusive for most pteridophytes, which possess independent and ecologically distinct gametophyte and sporophyte generations. To elucidate factors contributing to recent declines of the rare fern Asplenium scolopendrium var. americanum, we studied the ecology and ecophysiology of its gametophyte generation, focusing on responses to competition, temperature, and water stress.
  
  
  METHODS
  Gametophytes of A. scolopendrium var. americanum, its widespread European relative A. scolopendrium var. scolopendrium, and five co-occurring fern species were grown from spores. Gametophytes were grown at 20°C and 25°C, and germination rates, intra- and interspecific competition, desiccation tolerance, and sporophyte production were determined for all species.
  
  
  RESULTS
  Gametophytes of A. scolopendrium var. americanum had the lowest rates of germination and sporophyte production among all species studied and exhibited the greatest sensitivity to interspecific competition, temperature increases, and desiccation. Mature gametophytes of A. scolopendrium var. americanum grown at 25°C were 84.6% smaller than those grown at 20°C, and only 1.5% produced sporophytes after 200 d in culture. Similar responses were not observed in other species studied.
  
  
  CONCLUSIONS
  The recent declines and current status of populations of A. scolopendrium var. americanum are linked to its gametophyte's limited capacity to tolerate competition and physiological stress linked to climate change. This is the first study to develop a mechanistic understanding of rarity and decline in a fern and demonstrates the importance of considering the ecology of the gametophyte in plants with independent sporophyte and gametophyte generations.
authors.affiliation: Department of Plant Biology, University of Vermont, 111 Jeffords Hall, 63 Carrigan Drive, Burlington, Vermont USA 05405.
authors.forename: Weston L
authors.initials: WL
authors.lastname: Testo
authors.forename: James E
authors.initials: JE
authors.lastname: Watkins
mesh_terms: D057231:Climate Change; D004777:Environment; D029624:Ferns / Q000254:growth & development / Q000502:physiology*; D055993:Germ Cells, Plant / Q000254:growth & development / Q000502:physiology; D018525:Germination; D009518:New York; D011157:Population Dynamics; D013045:Species Specificity; D013696:Temperature
keywords: Aspleniaceae; Asplenium scolopendrium var. americanum; climate change; competition; conservation; fern; gametophyte; rare species