TROPICAL RAINFORESTS: The Canopy
ORCHIDS / EPIPHYTES
There are well over 15,000 epiphytes in the Neotropical realm alone, and over 30,000 world wide as well as numerous uncatalogued species. The term epiphytes 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 3300-6600 feet (1000-2000 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 2000 species), and orchids.
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Angraecum sesquipedale Orchid, Madagascar, 1997 |
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.
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'"(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 and 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 move through the tube, getting "tagged" with orchid pollen so the next flower he visits he will pollinate when he passes through its tunnel. Another interesting orchid reproduction strategy is that of the dancing lady orchid of South America. They 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, the bees 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.
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 which 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 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 cambers, 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 leafs. 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 3 kilograms of phosphorus, 2 kilograms of iron, and 10 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 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, crannies, 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.
Continued: Rainforest Overstory