Evolution of woody life form on tropical mountains in the tribe Spermacoceae (Rubiaceae)

Spermacoceae are mainly an herbaceous group in the Rubiaceae. However, a few lineages are woody, and are found in a diverse range of habitat types. Three of the largest woody lineages (Arcytophyllum, Hedyotis, and Kadua) are characterized by their distribution in the moist tropical mountains, and have disjunct distribution patterns with respect to their closest relatives. In this study, we explore the cases of derived woodiness in these three lineages and their diversification dynamics in the tropical mountains of Asia, the Pacific, and the Americas. By combining phylogenetic results with wood anatomical studies, we estimated timing of origin of the three woody groups, inferred their ancestral traits and ancestral distribution ranges, analyzed their associations with the tropical upland habitat, and elucidated their diversification across tropical mountains. The three woody clades originated and diversified from herbaceous ancestors in close association with the tropical upland habitat...