Life form evolution in epidendroid orchids: Ecological consequences of the shift from epiphytism to terrestrial habit in Hexalectris

Great diversity of life forms has favored the colonization and dominance of angiosperms in almost every terrestrial habitat. This is certainly the case in the Orchidaceae, in which the diversity of life forms includes epiphytes, terrestrial geophytes and mycoheterotrophs. In this study we investigated the evolution of life form in tribe Epidendreae focusing on the Glade comprised of Basiphyllaea, Bletia and Hexalectris. We estimated divergence dates, ancestral areas, patterns of niche evolution and divergence/convergence of climate preferences in order to understand whether shifts in climate preferences can be linked to the evolution of novel life forms, allowing species to extend their distribution ranges into novel habitats. Our analyses corroborated the shift from epiphytism to terrestrial life forms in Epidendreae, which occurred in a Glade with Basiphyllaea-Bletia-Hexalectris that originated in Mexico and Central America from the Mid-Eocene to the Oligocene, coincident with the origin of other aridity-adapted plant groups. Dispersal occurred to North America, South America and the Antilles. Some small aridity-adapted groups in Hexalectris diverged more recently in the Pliocene and Pleistocene when aridification in North America occurred. Of the climate variables analyzed, annual mean temperature, annual precipitation and precipitation of the wettest month were found to diverge for the terrestrial Glade relative to values associated with the other Epidendreae clades. For niche evolution, Ornstein-Uhlenbeck models indicated that the adaptive optima differed between epiphytes and terrestrial life forms, suggesting a shift of climatic niche between the terrestrial life form Glade of Basiphyllaea-Bletia-Hexalectris and the rest of the clades with epiphyte life forms. We suggest that the shift to terrestrial life forms, most noticeable in Hexalectris, favored the colonization of novel habitats with a large range of temperatures, extending the distribution of this genus northward.