The Rainforest Canopy


July 30, 2012

There are well over 15,000 epiphytes in the neotropical realm alone, and over 30,000 worldwide as well as numerous uncatalogued species. The term epiphyte describes a plant which, like a parasite, grows on a host, but unlike a parasite, takes no nutrients from the tree itself and relies on nutrients from the air, falling rain, and the compost that lies on tree branches. Their epiphytic way of life gives these plants advantages in the rainforest, allowing them access to more direct sunlight, a greater number of canopy animal pollinators, and the possibility of dispersing their seeds via wind. Epiphytes may be familiar to people in temperate climates because many house plants and "air-plants" are actually epiphytes from the rainforest.

Epiphytes are found throughout rainforests, but exist in the greatest abundance in the so-called cloud forests which exist at the fog-laden elevation of 3,300-6,600 feet (1,000-2,000 m). Epiphytes belong to 83 families, of which the majority are ferns and flowering plants. Some of the better-known epiphytes include ferns, lichens, mosses, cacti, bromeliads (over 2,000 species), and orchids.

Orchids are the most diverse group of flowering plants with over 18,000 species, representing about 8 percent of all flowering plants found worldwide. It is estimated that there are another 10,000 to 12,000 species that have yet to be described. Many of these species are endemic to microhabitats like a single Andean valley or a canyon of a tepui in the Guyana Shield and are very rare. Every year, it is likely that hundreds of orchid species go extinct as valley systems, especially those along the Andes, are destroyed. Nonetheless, there is a huge variety of orchids ranging from species that grow on the ground to the 70 percent of orchids which grow as epiphytes.

Orchids are very well-adapted to life in the canopy. They have roots with a large surface area for rapid absorption of nutrients and water. Their secondary stems can hold stores of water so the plant can withstand periods of drying.

One major reason orchids are so successful in the forest is because they produce tiny seeds (measured in microns) that number in the hundreds of thousands. The balloon-like seed coat coupled with the small seed size enables orchid seeds to be dispersed over great areas by wind currents.

Epiphyte in the Venezuelan rainforest. Click here for more photos of epiphytes. Photo by Rhett A. Butler

Orchids also utilize insects to spread their pollen. Several species from Madagascar release a strong odor to attract sphinx moths, which drink the nectar of the white flower and then carry away pollen with which they fertilize, inadvertently, other orchid plants. One species, the hawkmoth—which superficially resembles a hummingbird—has a tongue that exceeds 14 inches (35 cm.) so only it can penetrate the long trailing spurs of the flower of one orchid species, Angraecum sesquipedale. Numerous orchid species have tiny, almost microscopic, blooms which release a mildew-like odor that attracts small flies for fertilization. Another orchid, the bucket orchid of Central America, is equipped with a small bucket structure behind the flower. The flower produces an oil which drips into the "bucket" and attracts bees with its unique odor. Each bucket orchid species has its own scent, thus each attracts its own species of bee. When the male bee smells the perfume, it goes to the orchid to collect an oily substance which he will use to attract females (he is only attracted to one orchid species scent since he wants to attract only females of the same species). However, often as he is collecting his oil, the bee falls into the bucket. The only way out is through a tube. The bee moves through the tube, getting "tagged" with orchid pollen, so when he visits the next flower he will pollinate it as he passes through its tunnel. Another interesting orchid reproduction strategy is that of the dancing lady orchid of South America. These produce many tiny flowers that are positioned so they "dance" even with the slightest breeze. These flowers are lively enough so that small aggressive bees—thinking they are intruders—attack them and in the process are dusted with pollen.

The cacti of the rainforest are quite different from the cacti of more temperate and desert zones of the world. The cacti found in deserts grow in the soil or sand to get moisture and are outfitted with round, waxy leaves to reduce water loss. These cacti are often protected by sharp spines. However, the majority of cacti from tropical regions grow in the canopy as epiphytes, lack sharp quills, and have elongated leaves for light absorption, not water retention.

Epiphyte photo gallery

Epiphytes add a new dimension to the forest, creating new niches to be exploited by a wide range of species. One of the best examples of a tiny ecosystem based in an epiphyte is the tank bromeliad of South America whose stiff, upturned leaves can hold more than two gallons (8 L) of water. These reservoirs of water not only provide a drinking supply for many canopy animals but also create an entire habitat which species use for shelter and breeding. A multitude of insect larvae exist in these pools and are fed upon by other animals. The water catchments of the tank bromeliad serve as a nursery for poison-arrow frog tadpoles. The female frog lays its eggs on a leaf or in burrows on the forest floor. When the tadpoles hatch, she allows them to climb upon her back and she makes her way up to a bromeliad where she deposits the tadpoles into one of the plant's pools that is free of potential predators. The tadpole feeds on the developing insect larvae of the catchment. Some species of frog employ another strategy; they actually return to the bromeliad every few days to deposit an infertile egg into the water. The tadpole can then feed on the egg yolk. Bromeliads, especially those with interconnecting chambers, are often colonized by stinging ants, which provide the plant with nutrients produced by ant waste and their collection of decaying debris.

Epiphytes are superbly adapted to the often harsh conditions of the canopy: the serious lack of water and the shortage of minerals and nutrients. Many species, like the orchids, have developed structures to conserve water. Some have thick stems that store water; others have leaf hairs that effectively close the plant stomata when it is dry; and the tank bromeliads hold water in their stiff, upturned leaves. To counter the lack of nutrients, plant species have either developed symbiotic relationships with animals or have mechanisms, like a basket shape, for catching fallen debris which decomposes and provides sustenance. A surprisingly high amount of nutrients is provided by falling rain. For example, at a site near Manaus, Brazil, rain brought three kilograms of phosphorus, two kilograms of iron, and ten kilograms of nitrogen per hectare annually. As already mentioned, both tank bromeliads and other epiphytes rely on symbiotic relationships: tank bromeliads use the excrement produced by inhabitants of its water catchments, while other bromeliads, including nest epiphytes and mymecophytes, rely on waste created by resident ant colonies.

Epiphytes produce far more seeds than their grounded counterparts because so many of their seeds fail to reach suitable places to grow. Many epiphytes have wind-dispersed, microscopic seeds equipped with wings, gliding apparatuses, or parachutes. Even epiphytes that offer fleshy fruits may have several thousand seeds in a single berry. Mistletoe, actually an arboreal parasite, is a typical example of an aerial epiphyte-like plant with seeds that are spread in such a way as to ensure continuance of the species. Its berries have a laxative coating so they pass rapidly through the digestive system of the birds that consume them. In addition, the seeds have a sticky coat so when they pass out of the bird, they stick to the tail feathers. Hence, when the bird rubs the seeds on canopy branches, complete with natural fertilizer, the seeds end up in just the right place for growth.

Epiphytes grow most readily in cracks, grooves, and pockets where organic debris has collected and provides sustenance for initial growth. Surprisingly there is an abundance of canopy compost created by the decay of fallen leaves, wood, and animal waste. The layer of mulch provides moisture and trace minerals for epiphyte growth.

Birdnest fern. Click image for more photos of epiphytes. Photo by Rhett A. Butler

Review questions:

  • What are epiphytes?
  • True or false—Orchids can be a type of epiphyte.

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