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The Structure of the Tropical Rainforest

20 Feb

The structure of the tropical rainforest is very different from the temperate forests we’ve got up here in Rhode Island. There is a reason why people compare cities to jungles; the rainforest is every bit as architecturally diverse and lively as any city (but jungles are prettier and they smell better…) The structure of the rainforest is both vertical and horizontal, a three-dimensional panoply of microenvironments and niches. Here we are going to explore the nature of this structural diversity, and how niches make it possible. Animal species found in the same layer are said to feed in similar fashions as other animal species also found within the same layer. Plant species also follow a similar pattern of species convergence within the each layer. Many factors are responsible for the characteristics of the TRF. Those responsible include: the overall climate, sunlight, gaps, moisture, and nutrient. While not one factor  has been considered the limiting factor, many have commented on the availability of nutrients leading to the complexity of the TRF.

The structure of the rainforest is one of great complexity. The dense nature of the TRF illustrates evolution at its greatest challenge. With then number of plants and creatures encompassing these lands, much organization was done over time.

The TRF shows clear layers found in both its animal and plant grouping. According to P.W. Richards (1952), the vegetation of the TRF is divided into:

1)    Emergent Layers

2)    Main Story Canopy

3)    Understory

4)    Shrub Layer

5)    Herb Layer/ Forest Floor

These layers provide different microhabitats for animal and insect species which was defined by J.L. Harrison (1962) into six categories which are:

1)    Upper

2)    Canopy

3)    Middle- Flying animals

4)    Middle –Climbing animals

5)    Large Ground Animals

6)    Small Ground Animals

The strata of a TRF

Tropical rainforests are characterized by high temperatures, heavy rainfall and low seasonality. This creates a perfect greenhouse-like environment in which plants can grow, but also presents a large amount of competition between different plants struggling for limited light in the event of a tree-fall.

Less than a quarter of the sun rays reach the forest floor. All plants desire sunlight, it is especially apparent when a gap forms in the forest. The gap can be caused by fallen tree, storm damage, and the effects of humans clearing the forest. Once a gap is formed, competition for sunlight begins. The gap provides the opportunity for plants that have been dormant waiting for sunlight to become revitalized and create change. Beside the gaps formed by fallen trees, we must also include the gaps found within the canopy leading to the emergence of epiphytes.

A tree-fall lets light into the lower strata.

Each layer is home to millions of its own distinct species, in a vast cobweb of symbiotic relationships and other interactions.  Although there is a huge variety of trees in the forest, there is a “typical” rainforest tree structure; most trees have little branching, and wide roots to capitalize on as much of the poor soil as they can. They often have smooth bark to try to prevent epiphyte colonization. Buttress roots are common, too. Because some trees can have extremely widespread root systems (up to 3 acres in some paleotropical forests), most emergent trees are not that close to one another. The tallest and broadest trees are not found in the rainforest. The trees have their branches concentrated near the canopy, so as to not waste energy on leaves that are never going to see the sunlight. Most trees do not have large canopies or branches because of the lack of light in the undergrowth. Plants in the lower strata have “drip-tips” to prevent accumulation of water on their leaves, which would otherwise rot quickly.

Emergent trees exhibit the "typical" rainforest tree structure

       Drip-tips keep the leaf surface dry

Epiphytes are one of the elements of the rainforest’s architecture that is especially different from what we usually see in temperate forests. They are plants that grow on other plants. They usually do not parasitize the tree but simply use it as a “ledge to stand on” to get better access to sunlight. Epiphytes in the lower strata obtain their nutrients in other ways, by feeding on decaying matter, animal feces, and rainwater leftovers. Epiphytes have been found to live in greater abundance on trees with rougher bark. In the event of a tree-fall, the epiphyte population in that particular area will usually decrease.  They also receive nutrients from shallow roots and the vessel they have formed in their host plant.  Their location on the host plant occurs in gaps of leaves and branches found on the host. The emergence of epiphytes are one of the many adaptations plants have created to overcome competition further leading to the complexity of the TRF.

The soil of the TRF has often been called the “wet-desert” because of the lack of nutrients found within it. The nutrient cycle found within the tropical rain forest is very efficient but also in high demand. The majority of carbon is found from leaf litter, living vegetation, and dying wood trees. With the aide of fungi as recycling support any material that can be reused is converted to usable material and quickly taken in by neighboring plants. While nutrient necessary for plant growth, but so is the need for energy from sunlight.

Lianas help support tall rainforest trees

Liana plants are very characteristic of the structure of the rainforest; they are woody long-stemmed vines that grow in the soil and climb up trees to the top of the canopy for sunlight. They bridge many different trees together to help them withstand strong winds. The way the ubiquitous lianas function is a good metaphor for the rainforest as a whole, I think; if a liana is dragged down, it usually pulls several tall trees with it. In the same way,  the species of the rainforest are so intertwined that there’s no such thing as a solitary extinction, really; what would happen to Darwin’s magnificent orchid if its moth were gone?

NICHES

Perhaps the most critical factor enabling the spectacular structural and biological diversity of the rainforest is the plethora of biological niches in which the species establish themselves. Niches allow a vast number of organisms to thrive in a relatively small area by minimizing the competition between species. The niches are established by microclimates that are created throughout the forest’s vertical column. The upper layers, for example, are dry and extremely hot; water is scarcer up here. The lower regions are much cooler and wetter. Species segregate according to what microclimate they’ve adapted to thrive in.

The adaptations that many plants are created often revolve around the competition for nutrients or sunlight. Those that revolve around nutrients include the formation of shallow roots, stilts, buttresses, and pneumatophores, just to name a few. Each allows the plant to quickly gather nutrients upon their availability in the poor soils of the TRF. Other adaptions can also include the emergence of carnivorous plants as a way to gather nutrients outside the normal plant nutrient cycle. Sunlight adaptations include brood leaves and buttresses to provide the plant with support to reach maximum sunlight available.

Tropical rainforests have an intricate structure that makes them truly unique among biomes. Popular culture makes us think of rainforests as being some Indiana Jones-esque jungle that is impermeable to all who don’t come slinging machetes. The reality is that the rainforest’s structure is extremely varied and the underbrush is not all that thick due to lack of light. If you want to stray from the established trail, you’re more than capable of doing so.

just don't step on me.

WORKS CITED

ALL IMAGES FROM GOOGLE IMAGES

“The Distribution of Feeding Habits Among Animals in a Tropical Rain Forest”. J. L. Harrison Journal of Animal Ecology , Vol. 31, No. 1 (Feb., 1962), pp. 53-63

Richards, P. W. (1952). The Tropical Rain-forest, an Ecological Study. Cambridge.

http://www.wildmadagascar.org/overview/rainforests2.html

Kricher John, Tropical Ecology. 1st edition, Princton university Press 2011

http://www.sarracenia.com/faq.html
http://en.wikipedia.org/wiki/Tree

http://www.marietta.edu/~biol/biomes/troprain.htm

http://learnline.cdu.edu.au/units/sbi507/module3/fundamentals.html

Schnitzer S.A., Bongers F. (2002) The ecology of lianas and their role in forests. Trends in Ecology and Evolution, 17 (5):223-230.

Addo-Fordjour P, Anning A, Addo M, Osei M. Composition and distribution of vascular epiphytes in a tropical semideciduous forest, Ghana. African Journal Of Ecology [serial online]. December 2009;47(4):767-773.

Kricher, John. Tropical Ecology. First Edition. Princeton University Press 2011. Pg 103-105.

http://www.blueplanetbiomes.org/rainforest.htm

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