PARENT SESSION
Organized Oral Session 22: Comparative ecology of tropical trees: Linking physiology to dynamics and distribution
Organizer(s): SC Thomas and KE Harms
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 511 B, Level 5, Palais des congrès de Montréal
Comparative functional ecology of terrestrial and epiphytic gametophytes of tropical ferns.
Watkins, James*,1, Mack, Michelle1, Mulkey, Stephen1, 1 University of Florida, Gainesville, FL, U.S.A.
ABSTRACT- In tropical forests, canopy habitats experience greater extremes in light and moisture than terrestrial habitats. Not surprisingly, fern species appear to exist exclusively as either an epiphyte or terrestrial entity and do not exhibit reciprocal establishment. Since ferns rely on a relatively sensitive free-living gametophyte generation, we hypothesized that the gametophytes of epiphytic species would be more tolerant to excessive light exposure and drought relative to terrestrial species. The goal of this study was to develop a mechanistic understanding of the functional traits important for the survival of fern gametophytes and their recruitment into the sporophyte generation in tropical wet forests. A series of experiments were used to understand how germination rates, drought tolerance and photobiology differ between species. We reciprocally transplanted the spores of 30 tropical fern species on soil collected from canopy and terrestrial (mineral) sites, and exposed them to 150 
mol m-2 sec-1 of light. After 3 months, we simulated the effects of light stress on chlorophyll fluorescence by exposing 10 gametophytes per species to 1200 mol m-2 sec-1 for one hour. We then examined drought stress by exposing 10 individuals per species to 60% humidity for 70 min. Immediate and long-term recovery were monitored for both experiments. Germination rate was higher on canopy soils (95%) than terrestrial soils (34%). All terrestrial species were able to grow on canopy soil: a condition never observed in nature. Recovery from both light and drought stress was largely driven by species natural light environment and not necessarily the epiphyte/terrestrial dichotomy proposed in our original hypothesis. These data suggest that gametophytes exhibit differential abilities to cope with environmental stress, which translate to differences in distribution and abundance of species.
Key words: gametophyte, fern, epiphyte, drought