Plant life occurs in layers in a tropical rainforest. The
uppermost layer consists of very tall trees—some over 150 feet tall—rising out
of the dense canopy below. The canopy consists of a dense mass of treetops, vines, and
other plants occuring in a layer, usually about 100 feet above ground. Below that is the
understory, which consists of various seedlings, saplings, trees stunted from lack of
light, bushes and shrubs, some growing up to 75 feet tall. Finally, there is the forest
floor.Because of the very dense canopy, little
sunlight reaches the ground. In a dense forest, the canopy trees can block out over 95% of
the sunlight. Though Tarzan movies would have us think otherwise, there is actually
relatively little vegetation on the rainforest floor. Aside from some seedlings, young
creeping vines, and scattered fallen leaves, the ground is relatively bare.
Let’s look at some characteristics of rainforest
plants, and explore how these characteristics might help plants compete for light.
Since light levels increase as you rise into the upper
layers, rapid vertical growth would seem to be a good strategy, and indeed it is. One very
common type of rainforest plant is the liana, which is a general term for a woody vine. In
the rainforest, lianas grow to a much larger size than most of us are used to seeing in
our temperate climates. Vines may reach a length of 200' or more, with stems approaching
the girth of a medium-sized tree (6" or more in diameter). With a growth rate of up
to 2' to 3' per day, lianas certainly use the strategy of rapid vertical growth. And once
they reach the upper canopy, the vines can command their share of the sunlight.
Many rainforest trees also use the strategy of vertical
growth. In contrast with a sprawling oak, most rainforest trees grow straight and
vertical, with few or no branches on the lower trunk. Many species’ saplings are able
to remain in a relatively dormant state until a gap in the canopy opens up, perhaps due to
a fallen tree. Once sunlight is available, the sapling begins more rapid growth.
Interestingly, the leaves of many rainforest trees are
strikingly similar. Most of their leaves are oblong or oval, with a pronounced "drip
tip"—think of the common houseplant philodendron, with its heart-shaped leaves
ending in a long point. This shape allows excess water to run off for more efficient
photosynthesis.
Another interesting group of rainforest plants are the
epiphytes. These plants grow up in the branches of the canopy—rather than getting
their water and nutrients from the soil, they get them from the air and from organic
matter trapped in the trees’ bark and branches. Bromeliads and orchids are both
common epiphytes found in the rainforest.
Of course once you get into the upper canopy, plants face
the opposite challenge — intense, relentless sunlight. Leaves in the upper canopy are
leathery with a thick cuticle for protection. And some plants are able to actively alter
the orientation of their leaves. By varying the turgor pressure in the leaf joints, plants
can raise the leaves perpendicular to the sun for maximum exposure, or lower them to
minimize sun stress.
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