Evolution of Mesic and Xeric Habits in Tillandsia and Vriesea (Bromeliaceae)

Species of Tillandsioideae are usually epiphytic and portray a range of habits from st.rongly xeric with thick, narrow leaves that are densely covered with conspicuous, shield-shaped, moisture-absorbing trichomes to mesic with broad, thin leaves having few and inconspicuous trichomes. Phenetic groupings and evolutionary tree construction of the nine subgenera of Tillandsia and Vriesea and of the 36 species of T. subg. Phytarrhiza by means of cluster analysis, Wagner network, and character compatability, show subgenera with the xeric and mesic habit on the same major branches and with T. subg. Allardtia in an ancestral position on the subgeneric tree. Of the 36 species of T. subg. Phytarrhiza, the 17 of xeric habit are largely on separate branches and the five species of mesic habit, mostly Ecuadorian, are centrally located on the evolutionary tree between the xeric and the 14 semi-mesic species. A phenogram based on 43 characters and portraying overall similarity supports the separation of most of the xeric from the mesic species on the Phytarrhiza tree. The xeric and semi-mesic habits seem to have evolved several times; initially during evolutionary differentiation that we recognize at the generic and subgeneric levels. Xeric and semi-mesic species follow mesic ones on each of several different branches of the Phytarrhiza tree. Pleistocene climatic history in South America helps to explain the repeated evolution of species with the xeric habit. During the past decade, we have come to recognize from paleoclimatological studies in South America that the Quaternary climate, especially in upland areas, has undergone a series of humid and xeric phases. Such would have alternately favored evolution of xeric and mesic bromeliads and this seems to have occurred.