Epiphytes - adaptations to an aerial habitat

What are epiphytes?

Epiphytes are plants which grow above the ground surface, using other plants or objects for support. They are not rooted in the soil nor are they parasitic (ie they do not directly harm the other plant). By growing on other plants, the epiphytes can reach positions where the light is better or where they can avoid competition for light. Many mosses and lichens are epiphytes, as are approximately 10 per cent of all seed plants and ferns. Epiphytes are particularly common in some groups of plants, such as ferns, bromeliads (members of the pineapple family, Bromeliaceae) and orchids: over half of the 20,000 species of orchids are epiphytic.

Where do most epiphytes grow?

Most epiphytic seed plants and ferns are found in tropical and subtropical rainforests because they require conditions of high humidity. The areas richest in epiphytes are the montane rainforests (at altitudes around 1500 m). The trees provide many habitats with different conditions of temperature, exposure and illumination. In temperate regions, epiphytes are most common in moist forests, for example in Cornwall, south-western Ireland and along the Pacific north-west coast of the USA.

Obtaining water and nutrients

Since epiphytes have no contact with the soil, the major difficulties they encounter are shortages of water and nutrients. Many have similar adaptations to desert plants, enabling them to collect, absorb and store water and restrict its loss. Some epiphytic mosses and lichens, in contrast, shrivel and stop growing when water is short, but when it becomes available they are able to absorb it rapidly and resume growth.

The major sources of water are dew, moisture in the air and rainwater running down the support tree. In the so-called tank or urn bromeliads of the South American rainforest, such as Aechmea, the leaves are arranged as a tightly rolled funnel, forming a water-tight container. Rain and plant debris are trapped in these funnels or 'tanks', and absorbing tissues at the base of the leaves take up both water and nutrients. The tanks provide habitats for a large number of insects and sometimes even small frogs.

Nutrients in the dust washed off the support tree's leaves by the trickling rainwater are absorbed by epiphytes. Other nutrients are obtained from plant debris trapped by the branches of the support tree or amongst the roots of the epiphytes. The bird's nest fern (Asplenium nidus) and stag's horn ferns (Platycerium), both epiphytic ferns from the rainforests of South-east Asia, trap falling leaves and other debris amongst their fronds. Roots grow out from their stems into the decaying plant matter to absorb nutrients. Various members of the plant family, Araceae, such as Anthurium, and orchids also collect plant debris which is retained in a lattice made up of intertwined aerial roots. Almost all orchids have fungi (mycorrhiza) associated with their roots which provide the plants with micronutrients.

Absorbing water

Many epiphytic orchids have aerial roots covered with a layer of dead cells known as the velamen which absorbs water from the humid atmosphere. When filled with water, the velamen becomes transparent allowing light to reach the green tissue in the roots, where photosynthesis occurs. (Photosynthesis is the process of food manufacture in the plant, when carbon dioxide from the air is combined with water to make sugars using sunlight as energy). When water is in short supply, the dry velamen restricts water loss from the roots.

Spanish moss (Tillandsia) is an epiphytic bromeliad from tropical and subtropical America which appears as hanging grey strands draped over habitats as barren as telegraph wires and cacti. This plant lacks roots and has a greatly reduced shoot covered with absorbing scales (trichomes) which take up water from the atmosphere.

Storing water

Cacti, which are best known as desert plants, also occur as epiphytes, for example the Christmas cactus (Schlumbergera) from Brazil. Their fleshy stems or leaves store water. Epiphytic orchids have swollen bulb-like stems called pseudobulbs and thick succulent leaves for water storage.

Restricting water loss

Plants lose water from all above-ground surfaces. Many epiphytes have a waxy layer on their surfaces which reduces water loss. The surface area from which water can be lost is often restricted by having small leaves. In extreme cases, some epiphytic orchids have leaves that are reduced to tiny scales. The green aerial roots of these plants carry out photosynthesis.

The major route of water loss is through the stomata, pores in the surface which open to take up carbon dioxide from the air for photosynthesis. Many plants open their stomata during the day when temperatures are high, resulting in increased loss of water. Epiphytes and many desert plants have a modified form of photosynthesis which enables them to store carbon dioxide taken up at night and release it during the day for sugar production. Because their stomata open at night when temperatures are lower, water loss is reduced.

Seed production and dispersal

Epiphytes often have brightly coloured flowers or bracts to attract their pollinators. For example, the bird-pollinated Bilbergia nutans has a drooping flower head with pink or red bracts and pink sepals. Among the epiphytic orchids are many with highly specific relationships with their pollinators. They have very specialised flower structures and produce scents which are similar to the specific sex-attractants of particular groups of bees or flies.

Orchids produce thousands of dust-like seeds which are distributed by the wind, often over considerable distances. Some bromeliads have seeds surrounded by a sticky pulp which helps to glue the seedling in position until it is established.

Ant plants

The ant plants are a particularly specialised group of epiphytes found in South-east Asia. In Myrmecodia and Hydnophytum, the bases of the stems are swollen into giant structures containing chambers where ants build their nests. Some chambers store water which is available to the plant. Dischidia rafflesiana has pouched leaves which house colonies of ants. The ants drag in earth and leave behind corpses and faeces, all providing nutrients for the plant.

'Stranglers'

The stranglers are a group of plants that begin life as epiphytes; the best known examples are the strangling figs (Ficus) including the Asian peepul tree (F. religiosa). They usually establish on old canopy trees and outlive their supports. The plants are known as stranglers because their roots interlace around the supporting trunk, giving the appearance of strangling it.

Where can epiphytes be seen at Kew?

Epiphytic ferns, bromeliads and orchids can be seen in the wet tropical zones of the Princess of Wales Conservatory with examples of Tillandsia in the cloudforest zone. In the Palm House, other epiphytic plants are grown, as they would be in their natural surroundings, supported on trees.

Additional sources of information

Ayensu, E. S. (ed.), 1980. Jungles, Jonathan Cape
Luttge, U. (ed.), 1989. Vascular plants as epiphytes: evolution and ecophysiology, Springer-Verlag