Ecology and Evolution of the Ferns of Moorea and Tahiti, French Polynesia

Ferns are the only major lineage of land plants with haploid (gametophyte) and diploid (sporophyte) stages that can grow separately from each other for extended periods. Gametophytes, as the sexual stage, are critical to fern evolution. However, the ecology of fern gametophytes is poorly known due to their small size and cryptic morphology. In this dissertation, I use the ferns of Moorea and Tahiti, French Polynesia to investigate the relative roles of sporophytes and gametophytes in community assembly and evolution. In Chapter 1, I use DNA sequences to identify field-collected gametophytes to species and compare community diversity between fern sporophytes and gametophytes on Moorea and Tahiti. I find that phylogenetic community diversity decreases with elevation in sporophytes, but not gametophytes. I observe several species with gametophytes that are distributed beyond the range of conspecific sporophytes, and at least one species that may lack sporophytes on the islands completely. My results suggest that the transition from gametophyte to sporophyte functions as a filter restricting phylogenetic diversity of fern communities. In Chapter 2, I use the filmy ferns (Hymenophyllaceae) of Moorea to investigate how changes in physiology between fern gametophytes and sporophytes are correlated with ecological niche. I find that the gametophytes of filmy ferns are not more stress-tolerant than sporophytes. Rather, they are adapted for microhabitats that are buffered relative to those of sporophytes. I suggest that the gametophytes of filmy ferns rely on asexual reproduction via gemmae to achieve dense population sizes over a wide range. In Chapter 3, I conduct a phylogenetic investigation of epiphytic traits in the ferns of Moorea. I find that epiphytes tend to have shorter stipes, smaller rhizomes, and gametophytes with non-cordate morphotypes relative to terrestrial species. I demonstrate that epiphytic communities are clustered phylogenetically and functionally relative to terrestrial communities, supporting a scenario of extreme environmental conditions in the forest canopy acting as a filter on epiphytic ferns. In Chapter 4, I describe a new hybrid fern species, Microsorum × tohieaense, from Moorea based on morphological and molecular phylogenetic analysis, and discuss the significance of my findings to the taxonomy of microsoroid ferns.