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Factors that influence the distribution of ferns are poorly understood and likely reflect the ecology of both the sporophyte and the gametophyte generation. Little study has been done on the ecology of the gametophyte generation, especially in regard to tropical species. The goal of this study was to examine demography and the influence of light and disturbance on the distribution of the gametophytes of several tropical epiphytic, hemiepiphytic, and terrestrial fern species. Through a series of observational and experimental studies, we found that increased terrestrial gametophyte density and richness were related to both increased light and disturbance. By contrast, increased light had no influence, and increased disturbance negatively affected epiphytic density. Over a 25-mo demographic study, epiphytic and hemiepiphytic species had significantly greater longevities and lower recruitment rates than terrestrial species. Such unique strategies may have evolved in response to different disturbance regimens between the two habitats. Terrestrial species encounter and are adapted to more frequent disturbance and have invested in rapid gametophyte growth and recruitment. Epiphytic species may be more influenced by bryophyte competition, and in habitats of relatively low disturbance, they have invested in greater size and longevities. In such systems, gametophytes are able to survive for years waiting for favorable recruitment conditions. [ABSTRACT FROM AUTHOR]

To better understand changes in the distribution and diversity within plant functional types along an elevational gradient and the potential mechanisms driving such changes, we studied species richness of ferns at six elevations along a forested elevational gradient in Costa Rica, from La Selva Biological Station at 30 m a.s.l, up the slopes of Volcán Barva to 2960 m a.s.l. Among the samples from all the sites combined, we found 264 species from 60 genera. Sixty-nine species were terrestrial. 113 were canopy epiphytes, and 121 were low-trunk epiphytes. Only one species occupied both canopy and terrestrial habitats at any of the study sites. Overlap of canopy and low-trunk species composition was relatively low (18%), and lower still was the overlap of terrestrial and low-trunk species (12%). Total species richness peaked at the 1000-m site for canopy and low-trunk epiphytic species. In contrast, the richness of terrestrial species rose to a mid-elevation maximum and remained relatively constant at higher elevations. In an effort to explain elevational patterns of species richness, we examined mean annual rainfall and temperature, light intensities in the canopy and at ground level, and the mid-domain effect. Of the explanatory factors examined, the mid-domain effect accounted for most of the elevational pattern. We found little evidence that environmental gradients drive patterns of fern species richness on this spatial scale. [ABSTRACT FROM AUTHOR]