Tropical Broadleaf Evergreen Forest: The Rainforest
Introduction. The tropical rainforest is earth's most complex
biome in terms of both structure and species diversity. It occurs under
optimal growing conditions: abundant precipitation and year round
warmth. There is no annual rhythm to the forest; rather each species
has evolved its own flowering and fruiting seasons. Sunlight is a major
limiting factor. A variety of strategies have been successful in the
struggle to reach light or to adapt to the low intensity of light
beneath the canopy.
Climate: Mean monthly
temperatures are above 64 ° F; precipitation is often in excess of 100
inches a year. There is usually a brief season of reduced
precipitation. In monsoonal areas, there is a real dry season, but that is more than compensated for with abundant
precipitation the rest of the year.
Vegetation layer of trees is apparent. These layers have been identified as A, B, and C layers:
- A layer: the emergents. Widely spaced trees 100 to 120 feet
tall and with umbrella-shaped canopies extend above the general canopy
of the forest. Since they must contend with drying winds, they tend to
have small leaves and some species are deci
duous during the brief dry season.
- B layer: a closed canopy of 80 foot trees. Light is readily available at the top of this layer, but greatly reduced below it.
- C layer: a closed canopy of 60 foot trees. There is little
air movement in this zone and consequently humidity is constantly high.
- Shrub/sapling layer: Less than 3 percent of the light
intercepted at the top of the forest canopy passes to this layer.
Arrested growth is characteristic of young trees capable of a rapid
surge of growth when a gap in canopy above them opens.
- Ground layer: sparse plant growth. Less than 1 percent of the
light that strikes the top of the forest penetrates to the forest
floor. In such darkness few green plants grow. Moisture is also reduced
by the canopy above: one third of the precipitation is intercepted
before it reaches the ground.
Growthforms: Various growthforms represent strategies to reach sunlight:
- Epiphytes: the so-called air plants grow on branches
high in the trees, using the limbs merely for support and extracting
moisture from the air and trapping the constant leaf-fall and
wind-blown dust. Bromeliads (pineapple family) are especially abundant
in the neotropics; the orchid family is widely distributed in all three
formations of the tropical rainforest. As demonstration of the relative
aridity of exposed branches in the high canopy, epiphytic cacti also
occur in the Americas.
- Lianas: woody vines grow rapidly up the tree trunks
when there is a temporary gap in the canopy and flower and fruit in the
tree tops of the A and B layers. Many are deciduous.
- Climbers: green-stemmed plants such as philodendron
that remain in the understory. Many climbers, including the ancestors
of the domesticated yams (Africa) and sweet potatoes (South America),
store nutrients in roots and tubers.
- Stranglers: these plants begin life in
the canopy and send their roots downward to the forest floor. The fig
family is well represented among stranglers.
- Heterotrophs: non-photosynthetic plants can live on the forest floor.
- Parasites derive their nutrients by tapping into the roots or stems of photosynthetic species. Rafflesia arnoldi,
a root parasite of a liana, has the world's largest flower, more than
three feet in diameter. It produces an odor similar to rotting flesh to
attract pollinating insects.
- Saprophytes derive their nutrients from decaying organic matter. Some orchids employ this strategy common to fungi and bacteria.
Common characteristics of tropical trees. Tropical species frequently possess one or more of the following attributes not seen in trees of higher latitudes.
- Buttresses: many species have broad, woody flanges
at the base of the trunk. Originally believed to help support the tree,
now it is believed that the buttresses channel stem flow and its
dissolved nutrients to the roots.
- Large leaves are common among trees of the C layer.
Young individuals of trees destined for the B and A layers may also
have large trees. When the reach the canopy new leaves will be smaller.
The large leaf surface helps intercept light in the sun-dappled lower
strata of the forest.
- Drip tips facilitate drainage of precipitation off the
leaf to promote transpiration. They occur in the lower layers and among
the saplings of species of the emergent layer (A layer).
Other characteristics that distinguish tropical species of trees from those of temperate forests include
- Exceptionally thin bark, often only 1-2 mm thick. Usually very smooth, although sometimes armed with spines or thorns.
- Cauliflory, the development of flowers (and hence fruits) directly from the trunk, rather than at the tips of branches.
- Large fleshy fruits attract birds, mammals, and even fish as dispersal agents.
Infertile, deeply weathered and severely
leached soils have developed. Rapid
bacterial decay prevents the accumulation of humus. The concentration
of iron and aluminum oxides gives the soil a bright red color and sometimes produces
minable deposits (e.g., bauxite).
Fauna: Animal life is highly diverse. Common characteristics
found among mammals and birds (and reptiles and amphibians, too)
include adaptations to an arboreal life (for example, the prehensile
tails of New World monkeys), bright colors and sharp patterns, loud
vocalizations, and diets heavy on fruits.
Distribution of biome: The tropical rainforest is found between
10 ° N and 10 ° S latitude at elevations below 3,000 feet.