The physiological ecology of vascular epiphytes: current knowledge, open questions

The current knowledge of the physiological ecology of vascular epiphytes is reviewed here with an emphasis on the most recent literature. It is argued that by far the most relevant abiotic constraint for growth and vegetative function of vascular epiphytes is water shortage, while other factors such as nutrient availability or irradiation, are generally of inferior importa nce. However, it is shown that the present understanding of epiphyte biolog y is still highly biased, both taxonomically and ecologically, and it is co ncluded that any generalizations are still preliminary. Future studies shou ld include a much wider range of taxa and growing sites within the canopy t o reach a better understanding how abiotic factors are limiting epiphyte gr owth and survival which, in turn, should affect epiphyte community composit ion. Finally, a more integrative approach to epiphyte biology is encouraged : physiological investigations should be balanced by studies of other possi ble constraints, for example, substrate instability, dispersal limitation, competition or herbivory.