The Rainforest Canopy

RAINFOREST CANOPY TREES

The canopy is the richest region of the diverse rainforest, and ranges in thickness from 10-40 feet (3-12 m). Countless species usually thought of as ground dwellers have adapted to life in the canopy—including worms, crabs, frogs, kangaroos, anteaters, and porcupines—where they feed on the abundance of fruits, seeds, and leaves or the numerous animals that are attracted these foods. The plant life of the canopy is nearly as rich due to the variety of epiphytes and lianas.

CANOPY TREES

As a result of the crowded growing conditions, canopy trees only branch near the top of their long, pole-like trunks (which also make them good for timber). Since there is no shortage of sunlight for the uppermost leaves of canopy trees, the exposed leaves are generally small and waxy to retain water. The leaves of the lower canopy branches, shaded from the harsh sun by the upper canopy ceiling, are often a darker blue-green color than the leaves of the upper canopy in order to absorb the red wavelengths necessary for photosynthesis. This red-wavelength light is generally missed by the upper leaves, which have chlorophyll pigment for capturing shorter wavelength light. Due to the lower light levels and differing types of light in the mid-canopy, the plants of this region and below have a greater variety of colors than the uniform sea of green as observed from above. New leaves in the canopy are generally not produced continuously, but instead, like fruits and seeds, often are produced in flushes of new growth. This feature helps protect young, vulnerable leaves by swamping leaf-predators. Sometimes new leaves are red or white in color warning leaf-eaters of the presence of distasteful compounds.

Unlike the overstory trees, the trees of the canopy cannot depend on wind for spreading their seeds, so they rely primarily on animals for dispersal and pollination. Insects are one of the largest groups responsible for flower pollination because many plant and insect species co-evolved together and today play intimate roles in the life cycles of each other. In fact it is estimated that 30 unique species of insect may be dependent on each species of tree. In turn, a tree species may be dependent on a number of species to complete its life cycle: a bat for pollination, and a bird to disperse and process its seeds. If the critical bat or bird is removed from the system, the tree may no longer propagate and the species may die out in the area.

Because there are no true seasons in equatorial rainforest, there is often no predictable flowering or fruiting season where all the trees of the forest may bloom, bear fruit, or shed all leaves. Instead, to outside observers the flowering cycles of lowland canopy trees may seem random and without obvious pattern. Although there must be some trigger for flowering, the mechanisms for most species are still largely unknown. For a few species the mystery been solved. The stimulus that triggers flowering can range from fire to species that flower after an especially rainy year. During a "dipterocarp year" in Southeast Asia, the canopy erupts in color as numerous emergent Dipterocarp trees flower almost simultaneously. These "mast flowerings" at irregular intervals (once or twice per decade) may be a strategy to swamp seed predators with so many of the large, energy-rich seeds (individual trees may have 120,000 fruits) that many seeds escape predation. Additionally the random nature of flowering means no seed predator has the opportunity to specialize on dipterocarp seeds. The principal pollinators of diperocarps—small insects known as thrips—have a short life cycle well adapted to the random flowering cycle of these trees. During the intervals between mast flowering events sparse thrip populations are sustained by feeding on understory flowers. When a mass flowering is triggered (studies show a strong correlation between flowering and droughts/El Nino events) the thirp population increases exponentially to take advantage of the massive number of flowers (about four million flowers per tree).

Since as many as 70-90 percent of canopy tree species depend on animals for pollination and seed dispersal, numerous species are equipped with special mechanisms to ensure the proper species will take and deposit pollen in the proper plant species and disperse seeds in a suitable place. Plants pollinated by certain animals often have certain characteristics. For example, flowers pollinated by birds have brightly colored, cup-shaped flowers, while flowers pollinated by bats are often white nocturnal blooms with copious amounts of nectar. Flowers pollinated by flies often have a rotting or mildew-like smell just as "bee-flowers" have a sweet odor. Butterfly flowers have a mild odor and are red or orange, since butterflies are one of few insects with good color vision. These flowers are most common in light-gap and forest-edge plants species, so butterflies tend to be most abundant in these areas.

Due to the great diversity of flowering plant species in the rainforest, no one species dominates. Therefore it is sometimes difficult for a particular species, say bee species, to sustain itself feeding on the nectar of one species of tree. It may feed on the nectar of several species of tree within the same family. However, by doing this, there is a great risk of hybridization and much wasted pollen. Coevolution—the process where a trait in species A has evolved to a trait in species B, which had previously been affected by a trait in species A—has reduced this problem. The flower of one plant species (call it species A) has structures to deposit pollen on one particular part of the bee, maybe its left back leg. When the bee visits another species (call it species B), the pollen from species A will remain on the bee"s left back leg without being deposited in flower B. In the meantime, species B may plant its pollen on the upper part of the bee"s right wing. As the bee flies away from flower B, it will have pollen from both flower B and the previous flower it visited. When the bee lands in another flower A, the flower picks up the pollen off the bee's left hind leg because it is equipped with an apparatus to do so, and the flower is pollinated.

Bees are one of the major insect groups responsible for pollination of rainforest trees. Plant species are often highly specialized to be pollinated by one unique species of bee. For example, the castanharanas tree, a relative of the Brazil nut tree, have flowers with spring-hood covers which must lifted to get nectar. In the process, the bees are brushed with pollen, so when they visit the next castanharanas flower, the bee pollinates it. Many bees use "buzzing" to get the flower to release pollen. The flower only releases pollen when the bee beats its wings at the right frequency; thus only certain bee species can harvest the pollen of specific plant species. Bees recognize color, odor, and shape, and flowers pollinated by bees are often yellow, white, or blue with a showy shape.

The insects of the order Lepidoptera, better known as butterflies and moths (pictures), contribute to the continuance of canopy trees, among many other plant species. The Lepidoptera order is the second largest order after Coleoptera (the beetles) with over 150,000 member species encompassing about 80 families. Most people think of butterflies (17,500 species over 14 families) as primary pollinators, but moths (130,000 species over 65 families), too, have an important role in pollination. For example, the piranha tree is pollinated by moths, just after the peak of the Amazonian flood season, when the tree loses all of its leaves and new ones appear immediately. The new crop of young leaves is soon completely covered with moth caterpillars. Once a tree is stripped, caterpillars form cocoons which are suspended from the tree branches. The tree produces a second crop of leaves, untouched by the caterpillars, now in the pupal stage of development. The adult moths emerge from the cocoons and pollinate the flowers. Moths, being nocturnal, are attracted to dull white to yellow flowers that open and release odor after sunset.

Flies are important pollinators of forest flowers and are attracted by the rot-like odor produced by some flowers. Small flies (drosophilids) are common pollinators of tiny orchid blooms that have an odor like decaying matter.

Beetles are pollinators with a good sense of smell and are attracted to odors of fermentation, spice, and fruit. Much is still unknown about the significance of beetle pollination in the forest, although with the incredible diversity of beetles—over 400,000 described species making up about 25 percent of all described species on Earth—they probably play an important role. One example is a beetle species that pollinates a species of canopy tree, annona. Annona flowers produce an odor after sundown that attracts beetles and flies which must push through the flower petals. Once the flower is penetrated, pollen is released on the insects. The petals drop off at dawn and the pollen-covered insects fly away to another flower the next evening.

Birds are important pollinators, and the hummingbirds of the New World and their Old World counterparts, the sunbirds, are attracted to flowers with large, cup-like flowers with bright colors and large amounts of sweet nectar. Hummingbirds are able to avoid the problem that afflicts most nectar-feeding birds: finding a suitable perch, by having the ability to beat their wings fast enough to hang in the air. Birds are showered, marked, or even stabbed with pollen when they arrive to feed.

Although most people do not think of mammals as pollinators, they play a crucial role in both pollination and seed dispersal of rainforest trees. Bats are the most important pollinators among mammals in the rainforest. Bats, active at night, are attracted primarily to nocturnal blooms with a strong, musty odor and generous amounts of nectar. Fruit bats of the New World and flying foxes of the Old World are responsible for the pollination and seed dispersal of many canopy trees. Other mammals known to pollinate plants are Australian/Papuan marsupials, rodents, and primates.

Review questions:

• Why is most rainforest life found in the canopy?
• How do rainforest canopy trees disperse their seeds?
• What are common pollinators in the rainforest?

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Continued / Next: Epiphytes