The epiphytic habitat on a living host: reflections on the orchid-tree relationship

In a living tree crown, the surface expands from year to year and the limbs of the tree thicken, as a consequence of primary and secondary growth. An epiphytic plant fixed on a bark surface is thus subject to constant and, at times, rapid successional change, to which the epiphyte must acclimate or die. These changes concern physical and chemical development in the bark substrate and changes in microclimatic conditions. Thus, the width of the tolerance range of an epiphytic species in principle defines its typical longevity. Epiphytes are distributed in a non-random fashion with respect to crown part occupied, the zone of habitation being delimited by the youngest crown position offering conditions for establishment and the position preventing further survival of the epiphytic species in question. The orchid life cycle is crucially dependent on other organisms, such as pollinating animals and symbiotic fungi, but the relationship between epiphytic orchid and phorophyte tree is currently receiving the least attention, despite the likely relevance for conservation of rare and threatened orchid species. Although many correlative studies have recorded occurrence of mature epiphytic orchids, the dynamics created by the growth of the phorophyte are often disregarded. We call for more experimental approaches, such as transplantation studies of seeds and seedlings, to reveal the still largely unknown mechanisms by which orchid distribution is biased towards certain phorophyte species, age stages and crown positions. We provide a compilation of noteworthy cases that could serve as starting points for studies into bias mechanisms.