DISSERTATION RESEARCH: Evolution and Diversification of Epiphytic Ferns

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Vascular epiphytes-vascular plants that reside on an aboveground plant surface but do not extract water or nutrients from the host plant or the ground-make an important contribution to global plant diversity, comprising more than 10% of the known vascular plant species. Epiphytism has evolved numerous times and in all major vascular plant lineages, but not all transitions to the epiphytic habit have resulted in comparable levels of diversity and the contribution of epiphytes varies from lineage to lineage. Within ferns, 31% of the species are epiphytes, and the epiphytic habit has clearly been of considerable importance in the evolutionary history of this group. Nonetheless, remarkably little is known about the evolution and diversification of epiphytic ferns and several critical questions remain unanswered. How many times has epiphytism evolved in ferns? What morphological and ecological transformations are associated with the transition to the epiphytic habit? When did epiphytism evolve and what abiotic or biotic events may have spurred the diversification of epiphytic ferns? This award to Kathleen Pryer and Eric Schuettpelz will be used to address these questions through ancestral character state reconstructions and divergence time estimates within an explicit phylogenetic framework. There are three primary objectives to this study: (1) resolve a global phylogeny for ferns to serve as a tool for addressing evolutionary and ecological questions; (2) reconstruct the evolutionary history of epiphytism within ferns and identify the morphological and ecological innovations associated with the transition to epiphytism; and (3) estimate the relative timing of epiphytic fern diversifications and assess how these may have been impacted by the establishment of angiosperm-dominated everwet tropical forests-a possible diversification trigger. This study will provide training for Eric Schuettpelz in systematic biology, make significant contributions toward a complete understanding of plant evolution, and help in elucidating the origins of a part of global biodiversity that is much neglected. The resulting phylogenetic tree will further resolve the relationships among fern lineages, will allow for improved classification, and will serve as a framework for future evolutionary and systematic studies. Insight into the evolutionary history of epiphytism in ferns will likely extrapolate to other plant lineages, and information concerning the relative timing of epiphytic fern diversifications could have a profound impact on future thoughts concerning the basis of tropical diversity. If the establishment of angiosperm-dominated forest canopies is shown to have been a likely trigger for the diversification of epiphytic ferns, then it is highly probable that these canopies also spurred the diversification of epiphytic angiosperms and bryophytes, as well as many other canopy and terrestrial forest inhabitants. Much, if not all, of the extraordinary biodiversity present in the tropics may have been generated directly as a result of angiosperm-dominated everwet tropical forest establishment, but the magnitude and extent of this possible impact has yet to be evaluated.