Orchidaceae: Ancestral Habitats and Current Status in Forest Canopies

This report considers two related subjects: the habitat preferences of ancestral orchids and the unparalleled expansion of Orchidaceae, particularly in tropical forest canopies. We chal- lenge a proposal that extant terrestrial family members evolved from epiphytic antecedents (Rob- inson and Burns-Balogh 1982) on the grounds that much evidence and several precedents in Or- chidaceae and other taxa were ignored or misinterpreted. These authors' hypothesis rests on certain key orchid characteristics that they claim are indicative of an epiphytic bottleneck in orchid phy- logeny. But the same features could have emerged in soil-rooted ancestors. Some, such as micro- spermy and carbon mycotrophy, are almost certainly terrestrial in origin. We conclude that the more conventional view of epiphytism in Orchidaceae as secondary is probably correct. We support this notion with a conceptual model of orchid ecological-physiological phylogeny that is consistent with functional correlates of microspermy, mycotrophy, and orchid root anatomy. Also offered is a hypothetical scheme depicting forces that may have promoted this family's great diversity through selection of certain adaptive features. Questions that must be pursued to develop greater insight on the adaptive history of Orchidaceae are identified.