THE OVERSTORY LAYER OF THE RAINFOREST CANOPY
The overstory consists of giant emergent trees that tower above the surrounding canopy. These trees are huge, at least by tropical standards, some exceeding a height of 213 feet (65 meters) with horizontal limbs that stretch over 100 feet (30 m). These trees live in a different climate from the trees of the canopy. The air is much drier and moderately strong winds blow through their branches. These overstory species have adapted to take advantage of the wind for seed dispersal and typically the seeds of these species are light and equipped with some sort of mechanism to allow the winds to carry the seeds great distances away from the parent tree. The Kapok (Ceiba), or Silk-cotton tree
, of South America releases its seeds attached to cotton-like material, which drifts in the wind currents for miles before reaching earth. Before fruiting, the tree sheds all its leaves so breezes pass unimpeded through its branches. In Asia, the seeds of emergent tree species Dipterocarps
are equipped with "wings" that cause the spinning seed to slow as it falls and enable it to be carried long distances by the breeze.
Emergent tree in Venezuela
(Photo by R. Butler).
These emergent tree species are often covered with epiphytes (non-parasitic plants which take no nutrients from the host plant but use it for support). Over 2,000 epiphytes may be found on a single tree, making up one-third of the tree's total weight and 40 percent of the leaf biomass in some forests. Lianas, too, cling in mass numbers to emergent trees with over 1,500 regularly found on a single tree, making up about 20 percent of the total leaf biomass of the forest.
The most successful and most plentiful predators of vertebrates in the canopy are the birds of prey. Each continental forest region has its own species of giant eagle characterized by short wings, a long tail, and razor-sharp talons. These birds are most abundant in the overstory where they build nests near the top of giant emergent trees and raise single hatchlings. Because these eagles generally nest in the tallest trees, which are often the most valuable timber trees, they are especially threatened by selective logging, which not only destroys their habitat and nesting grounds, but also frightens away their prey. These giant eagles fly through the canopy with great speed and agility as they hunt their prey of primates, parrots, and other large mammals. When their prey is spotted, the eagle dives beneath the canopy and attacks its prey from underneath. The harpy eagle of South and Central America is the largest of these eagles attaining a height of 3 feet (1 m) and a wingspan of 6 feet (1.8 m). Because of its size, one of the harpy eagle's favorite foods is the sloth. From Southeast Asia, although now limited to four islands in the Philippines, comes the highly threatened monkey-eating eagle (100-300 left), while the crowned eagle hails from West Africa.
Giant Kapok tree in Brazil. Click image for more photos. (Photo by R. Butler)
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