troops of leaf- cutter ants, and some individuals are
evident, carrying their burdens of clipped leaves to
their massive underground colony.
North American broad- leaved forests are often
layered: there is a nearly uniform canopy, the height
to which the tallest trees such as the oaks and maples
grow; a subcanopy of understory trees, such as Sassafras
and Flowering Dogwood; a shrub layer of viburnums
or Mountain Laurel; and a herbaceous layer of ferns
and wildflowers.
The tropical rain forest, in contrast, is not neatly
layered. Trees are far more variable in height, and
identifying horizontal strata within the forest is
difficult in most cases. Forest structure is complex.
Some trees, called emergents, tower above most other
trees, making the forest canopy irregular in height.
Trees of varying heights, including numerous palms,
occur both in the understory and the canopy. Most
trees are monotonously green, but a few may be
bursting with colorful blossoms, while some may be
essentially leafless, revealing the many epiphytes, or
air plants, that have attached themselves to their main
branches. A thin covering of herbaceous plants shares
the heavily shaded forest floor with numerous seedling
and sapling trees, ferns, and palms. It is difficult to
perceive a simple pattern in the overall structure of a
rain forest. Complex indeed.
Rain Forest Characteristics: A
Checklist
Now that you have some sense of what to expect as
you wander through a tropical rain forest, here is a
bullet list of characteristics to note as you make your
observations. These will be explained in additional
detail below.
- Tall trees of many species, some reaching higher
than 30 m (98 ft). - Species identification is in many cases quite difficult
without expert help because so many trees look
generally the same and leaf shape is very similar
among many species. - Deep shade with scattered sun flecks at ground level.
This is because the tall canopy of leaves is generally
quite good at absorbing sunlight. - Scattered emergent (unusually tall) trees reaching
well above most of the tree canopy. This characteristic
gives the forest an uneven canopy. Though emergent
trees also may be seen in many temperate forests, they
are usually much more apparent in tropical forests.- No clearly delineated understory, shrub layer, or herb
layer. Many tree species of various sizes typically
compose the understory and mid- elevation levels.
Palms often dominate in the understory, and palms
may reach canopy height and persist for long time
periods. - Forest gaps, openings due to such events as tree
fall, ranging from single tree falls to moderate- size
blowdowns, are common. Gaps are flooded with
sunlight, and rapidly growing sun- dependent plant
species are common; large gaps become jungle-
like. Gaps add a major component of horizontal
patchiness to forests, accounting for the shifting
mosaic pattern typical of rain forests. (The term
shifting mosaic refers to patches of forest that change
with time, the result of accumulated disturbances in
various areas of forest.) - Many tree species tend to rise nearly to canopy height
before widely branching; thus the pattern is often
umbrella- like, the branches appearing to radiate as
spokes from the trunk. - Trees most commonly have animal- pollinated
flowers, many of which are colorful, though some tree
species are wind- pollinated and have nondescript
flowers. - Some flowering and fruiting is evident in any month.
You should see some trees and shrubs bearing fruit
at pretty much any time of year, though patterns
of flowering and fruiting are affected by rainfall
amount. - Leaves tend to be thick and waxy. They are usually
oval, with little or no lobing but with pointed drip
tips, sharp points at the apex that permit water to
drain easily from the leaf. Compound leaves are
common in many species, such as those within the
legume family (Fabaceae). Large leaves are common. - Most trees are broad- leaved evergreens, but some
deciduous tree species occur. Within rain forest,
deciduous leaf drop (confined to isolated trees of
certain species) is often correlated with flowering
and fruiting. Bursts of colorful flowers will be
evident on bare branches. Within moist forest, where
dry season is pronounced, deciduous leaf drop is
associated with water stress. - Trees often display buttressed roots; some have prop
(or stilt) roots, and many exhibit surface roots that
radiate across the forest floor. A buttress is a flaring
40 chapter 3 rain forest: the realm of the plants