| Summary: | 碩士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 107 === Epiphytism has become an important evolutionary trend among different land plant lineages to adapt to the angiosperm-dominated forests in the Cenozoic. These epiphytes have been rapidly diversified through exploring novel niches provided by angiosperm trees. Nonetheless, some epiphytes in ferns are found never on angiosperm hosts but grow only on the root-mantle-covered trunks of tree ferns. From fossil records, such a “fern-on-tree-fern” relationship was suggested as the earliest plant epiphytism, and can be traced back to the time period much earlier than the known episodes of rapid diversification of epiphytes in the Cenozoic. Based on these findings, I hypothesized that tree fern epiphytism have an ancient origin earlier than the Cenozoic angiosperm epiphytism. To test this hypothesis, I reconstructed the habitat evolution history of four fern genera with tree fern specialists, including Tmesipteris, Abrodictyum, Polyphlebium and Vaginularia. In addition, I studied the ecology of tree fern specialist ferns in Taiwan to better understand what environmental and developmental factors likely shape their host specificity. The results indicate that Tmesipteris and Polyphlebium each has the origin time of tree fern epiphytism earlier than the Cenozoic. Tree fern specialists in Tmesipteris inherited the ancient habitat state from their Mesozoic common ancestor, while tree fern specialists in other three genera evolved through recent niche contractions in the Cenozoic. The niche contraction act on both sporophytic and gametophytic stages. In the case of tree fern specialist Vaginularia junghuhnii, gametophytes of the tree fern specialist perform the worst drought resistance among the vittarioid relatives, which is opposite to the general idea of evolution of epiphytism involving the improvement of drought resistance in the epiphytic environments, and may be the reason for the restricted distribution only on moist tree fern trunks. That is, the evolution of tree fern specialists may be a different phenomenon of evolution of epiphytism.
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