Geoscience Reference
In-Depth Information
plants and dead organisms. This decomposing layer can be 6-8 in (15-20 cm) in
depth. It is the lifeblood of the Tropical Rainforest Biome. The rapid decomposi-
tion of dead plants and animals is undertaken by many organisms, from insects to
smaller bacteria and fungi. Scientists have just begun to research the bacteria and
fungi that are so essential in the maintenance of the rainforest.
Rainforest vegetation has adapted to soils with little fertility by rapidly recy-
cling nutrients from decomposing material in the forest litter. Beneficial aerobic
and anaerobic bacteria convert unusable compounds into usable minerals and
nutrients that are necessary for plant growth. The uptake of decomposed, mineral-
ized matter is facilitated by roots and their accompanying beneficial fungi (mychor-
rhiza) that help the plants utilize nutrients. The roots of tropical plants tend to be
shallow and often above the ground to allow for the greatest use of these nutrients.
Small complex roots form networks with mychorrizal fungi that rapidly absorb the
nutrients and makes them available to the plant. In exchange for these services, the
plants provide the microorganisms with food and shelter among the roots. These
microbes also have been found to help a tree resist drought and disease.
Vegetation
This ancient biome occurs under optimal growing conditions: abundant precipita-
tion and year-round warmth. With no annual rhythm to the forest, each species
has evolved its own flowering and fruiting season.
Forests have evolved to accommodate many species of various shapes, sizes,
and heights that fill in the forest. Tropical broadleaf evergreen trees are the most
common growthform (see Plate III). Sunlight is a limiting factor in the lower can-
opy and a variety of strategies and growthforms have been successful in the struggle
to adapt to varied light, above and beneath the canopy. A vertical stratification of
three canopy layers of trees is usual. Along with these tree layers is an accompany-
ing array of woody vines, orchids, bromeliads, and epiphytes. The tree layers have
been identified as A, B, and C layers with a shrub layer and ground layer below
(see Figure 2.6).
The emergents are in the A layer. These are widely spaced trees 100-120 ft (30-
36 m) tall with umbrella-shaped crowns that extend above the general canopy of
the forest. These trees contend with strong drying winds and tend to have small
leaves. Some may be deciduous during the brief dry season. The next layer, the B
layer, is composed of a closed canopy of 60-80 ft (18-24 m) trees. Light is readily
available at the top of this layer but greatly reduced below it. The C layer is often
the third and last layer of the canopy. The C layer is composed of smaller trees
around 30-60 ft (9-18 m) tall. This layer along with the B layer creates a closed
canopy with little air movement and high humidity.
Below these three layers is the shrub and sapling layer where little light pene-
trates through the canopy layers. Less than 3 percent of the light reaching the top
