Leaf surfaces of tropical vascular plants provide homes for diverse groups of organisms, including epiphyllous (leaf-colonizing) bryophytes. Each leaf harbors a temporally and spatially discrete community of organisms, providing an excellent system for answering some of the most fundamental questions in ecology and evolution. In this dissertation, I investigated two main aspects of epiphyllous bryophyte biology: 1) adaptive traits of bryophytes to living on the leaf surface, and 2) community assembly of epiphyllous bryophytes in space (between-hosts) and time (succession).

For the first part, I used published trait data and phylogeny of liverworts in family Lejeuneaceae to demonstrate that only the production of asexual propagules appeared to evolve in response to living on the leaf surface, while other hypothesized traits did not have correlated evolution with epiphylly. The second portion dealt with the assembly of communities among different host types. In this part, I identified communities of epiphyllous bryophytes from the same set of three hosts across the gradient of water deficit fluctuation, as represented by the daily range of vapor pressure deficit (VPD) on the island Moorea, French Polynesia. Dissimilarity of epiphyllous communities among host types (beta diversity) increased with the increasing VPD range, suggesting that assembly of bryophyte community may depend on microclimate fluctuation. In the last portion of this dissertation, I investigated temporal changes of epiphyllous communities on Inocarpus fagifer, using chronosequence and long-term observation of tagged leaves in the field. Data from both approaches showed significant changes in abundance, species richness, and composition over time. These successional changes, however, did not follow any single trajectory, resulting in divergent communities among leaves of older age. Together, the results from this dissertation work improve our understanding of fundamental biology of epiphyllous bryophytes and allow future researchers to use this system to answer broader questions in ecology and evolution.