Epiphytes are plants that grow on other plants, using them for support.
Introduction
High in the canopy of many of the moister forests is another world, made up not only of the upper branches of trees and vines rooted in the forest floor, but also a huge diversity of plant species exploiting the height advantage of the canopy location to reach sunlight without the need for terrestrial soil. Many of these plants are epiphytes.
Species diversity
According to the UCLA Botanic Gardens, there are more than 20,000 species of epiphytes worldwide, with many species remaining to be described. The largest diversities are to be found in the rainforests and cloud forests of Central and South America and South East Asia. Ferns in the Staghorn group (Platycerium species) and Birdsnest Ferns (Asplenium species), bromeliads (related to pineapples and found only in the Americas) and orchids are amongst the best known kinds, including as they do many species used as house and patio plants and in tropical gardens. Lower plants including mosses, liverworts and lichens are other widespread epiphytic species. Aboreal cacti are found mainly in the Americas, and are less likely to be spiny than ground dwelling cacti of drier regions.
Biology
Epiphytes do not take nutrients or water from their host plants, and must obtain these basic requirements from rainwater and the aerial compost that develops from trapped organic debris. The basket-like shape of some of the ferns and bromeliads helps to trap leaves and debris while other material collects in interwoven plant parts and in crooks and crannies in bark and tree trunks. In contrast, parasites, such as mistletoes, connect to the tissue of their hosts, draining them of sustenance.
Epiphyte load
The supporting trees can be badly affected by their epiphyte load and the heavy organic material that develops in and around them. Branches or whole trees can be physically weighed down, increasing the risk of breakage during storms and wind. Research by Nalini Nadarki showed that some host trees have a curious method of exploiting the aerial compost by developing adventitious roots (arising from the stem) that penetrate the nutrient mats developing under and around the epiphytes.
Drought resistance
The canopy environment can be harsh, drying rapidly at some times of the year. Drought resistance mechanisms include the occurrence of Crassulacean acid metabolism, a photosynthetic system that increases water-use efficiency. It is found commonly in plants with thick leaves or succulent green stems. Water can be collected and held in reservoirs within the plants, as seen in many bromeliads. Roots are used mainly for anchorage and sometimes for nutrient uptake, but in many orchid species become thick and fleshy and also function as water storage organs.
Interactions
Wind dispersal of dust-like seed (orchids and many bromeliads) or spores (ferns) is common in the canopy, allowing colonization of distant sites, but access to a range of vertebrate and invertebrate pollinators and seed dispersers is another advantage of life in the tree tops. Complex interactions with the many animal species (monkeys, possums, fruit bats, colourful bird species and countless invertebrates) sharing the space have developed. Some frogs and insects use the "tank" bromeliads for breeding.
Observing epiphytes
The canopy world of epiphytes and their neighbours is beyond the view of most human observers, though towers and canopy walks or specialized climbing equipment are available for the determined. Fortunately, epiphytes can also be seen growing on tree trunks and lower branches. Many epiphytic species also grow on rocks, in which case they are termed "lithophytes".
