Epiphyte

Epiphyte is one of the subdivisions of the Raunkiær system. The term epiphytic derives from Greek epi- 'upon' and phyton 'plant'. Epiphytic plants are sometimes called "air plants" because they do not root in soil. However, that term is inaccurate, as there are many aquatic species of algae that are epiphytes on other aquatic plants (seaweeds or aquatic angiosperms).

Epiphytes in terrestrial ecosystems

Epiphytic plants include mosses, orchids, and bromeliads such as Spanish moss (of the genus Tillandsia), but epiphytes may be found in every major group of the plant kingdom. About 89% of (or about 24,000) terrestrial epiphyte species are flowering plants. The second largest group are the leptosporangiate ferns, with about 2,800 species (10% of epiphytes). About one-third of all fern species are epiphytes. The third largest group is clubmosses, with 190 species, followed by a handful of species in each of the spikemosses, other ferns, Gnetales, and cycads. Within the Neotropics, there are estimated to be total of 15,500 species of epiphytic vascular plants.

A monograph on epiphytic plant ecology was written by A. F. W. Schimper (Die epiphytische Vegetation Amerikas, 1888). Assemblages of large epiphytes occur most abundantly in moist tropical forests, but mosses and lichens occur as epiphytes in almost all biomes. In Europe there are no dedicated epiphytic plants using roots, but rich assemblages of mosses and lichens grow on trees in damp areas (mainly the western coastal fringe), and the common polypody fern grows epiphytically along branches. Rarely, grass, small bushes or small trees may grow in suspended soils up trees (typically in a rot-hole).

Terrestrial epiphytes

Holo-epiphyte or hemi-epiphyte

Epiphytes can generally be categorized into holo-epiphytes or hemi-epiphytes. A holo-epiphyte is a plant that spends its whole life cycle without contact with the ground and a hemi-epiphyte is a plant that spends only half of its life without the ground before the roots can reach or make contact with the ground. Orchids are a common example of holo-epiphytes and strangler figs are an example of hemi-epiphytes.

Plant nutrient relations

Epiphytes are not connected to the soil, and consequently must get nutrients from other sources, such as fog, dew, rain and mist, or from nutrients being released from the ground rooted plants by decomposition or leaching, and dinitrogen fixation. Epiphytic plants attached to their hosts high in the canopy have an advantage over herbs restricted to the ground where there is less light and herbivores may be more active. Epiphytic plants are also important to certain animals that may live in their water reservoirs, such as some types of frogs and arthropods.

Epiphytes can have a significant effect on the microenvironment of their host, and of ecosystems where they are abundant, as they hold water in the canopy and decrease water input to the soil. Some non-vascular epiphytes such as lichens and mosses are well known for their ability to take up water rapidly. Epiphytes create a significantly cooler and more moist environment in the host plant canopy, potentially greatly reducing water loss by the host through transpiration.

Plant metabolism

CAM metabolism, a water-preserving metabolism present among various plant taxa, is particularly relevant to epiphytic communities. For example, it is estimated that among epiphytic orchids, as many as 50% are likely to use it. Other relevant epiphytic families which display such metabolism are Bromeliacee (e.g. in genera Aechmea and Tillandsia), Cactaceae (e.g. in Rhipsalis and Epiphyllum) and Apocynaceae (e.g. in Hoya and Dischidia).