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The Tropical Rainforest

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Deepa is a freelance researcher and journalist. She writes and makes documentaries and videos.

The Amazon

The Tropical Rainforest: A General Profile

Tropical rainforests are forests that never get dry and get rain almost every day of the whole year. Lush green and wet, they are home to innumerable living things, ranging from insects, and animals to the most luxurious kinds of flowering trees and plants; in a nutshell, heaven on earth for the nature enthusiast. The mystery of the rainforest is still unfolding before our eyes, as many species living in it still stay unidentified and unnamed by humanity.

Tropical rainforests fall on both sides of the equator up to approximately 10 degrees of latitude (Lauer, 2012, p.7). Factors such as temperature, water availability, radiation from Earth, atmospheric circulation, elevation and proximity to the equator, all come together to create a tropical rainforest—nature's greatest treasure trove (Lauer, 2012, p.7).

The Age of Rainforests

The origin of tropical rainforests is traced back to more than 200 million years ago when all land on planet earth was combined as a single continent and fraught with gigantic ferns, wild bananas, and wild yams. These plants with big leaves are the most primitive ones in the history of plant evolution. This ancestral forest is now beneath the soil, in the form of coal that we extract for energy (Age of tropical rain forests, 2018).

Then came the seeds, and with them, a new mode of propagation, resulting in a new life form: trees. Flowering plants came first, and then the dinosaurs. The tall trees caused the rainforests to evolve into what they are today. To date, the most successful species group in a tropical rain forest remains flowering plants and trees (Age of tropical rain forests, 2018).

Rain forests as an ecosystem are much older than temperate ecosystems and most of the species living today on earth originated in the tropics. The reasons for the tropics being the cradle of the majority of species are many. In the tropics, there is a uniform climate throughout the year and there is no frost that inhibits life (Kurokawa et al., 2003). Some tree species in the rainforests like Borneo Ironwood are known to live up to a thousand years (Kurokawa et al., 2003).

The Natural Air Conditioning in a Rain Forest

Being near the equator, the tropical rain forests get maximum solar radiation but only 10% of it escapes the thick canopy, the edifice of this majestic monument of nature (Lauer, 2012, p.12). If one examines the atmosphere that is just above the canopy of a rainforest, there will be a great amount of carbon dioxide and water vapor present- carbon dioxide released from the breathing of the trees and water vapor formed by the evaporation of water from the leaves (Lauer, 2012, p.12).

This water vapor and carbon dioxide trap the outgoing solar radiation is reflected back from the ground and this creates a greenhouse effect- exactly the same as we artificially create inside a greenhouse for enhancing the yield of the crops. The result is, that during the daytime, the canopy zone will be warm while the ground area will be cool, and during the night, the coolest part will be the upper zone of the rain forest and the ground will get warm (Lauer, 2012, p.12).

The cool and warm air interact to form a uniform climate (Lauer, 2012, p.15). This is why whenever you enter a rainforest, the ambiance is pleasingly cool. The temperature will never reach a point where the plants get dried up and it will also not fall to a point where there is frost. Hence the name, evergreen.

the-tropical-rain-forest

When and How Often Does It Rain in a Rain Forest?

A rainforest is literally a raining forest. On average, a tropical rainforest gets a rainfall of 4000 mm in a year (Silk et al., 2015). This rain is almost uniformly distributed throughout the year. There is also a type of rain specific to the rainforests: “zenithal” rain (Lauer, 2012, p.24). This is the rain occurring out of small cloud formations that gather their water vapor from the forest itself—that is, from the evaporation happening in the tree leaves (Lauer, 2012, p.24).

In other words, it is rain created by the rainforest itself and by the “small water cycle” within it (Lauer, 2012, p.24). Thus the water is returned to the ground as soon as it reaches the tree leaves, this being the smallest ever distance traveled by a water molecule when it gets involved with the phenomenon and the process called rain. There is no need to dare the atmospheric heights and fall upon unknown lands for the water molecules in question here. It is a small cyclical journey, just like on a swing, starting from the ground and returning to the ground in say, a day's time or less. This is the beauty of the zenithal rain.

Falling in the trade wind routes that circulate the equatorial region, the rain forests also get heavy rains accompanied by thunder and lightning during the afternoons and nights (Lauer, 2012, p.20). Hence it is better to leave or find a shelter if you are inside a rainforest in the afternoon. The trade winds were named after the sea voyages for trade they facilitated during the time when Europe discovered the oceanic trade routes.

They are one of the most consistent phenomena of the earth, as they move through the very same pathways every year and also at the same time of the year. The heaviest rain comes to the rain forest 1-2 months after the sun reaches exactly overhead and it can be said, the “rain zone” of a rain forest migrates with respect to the position of the sun (Lauer, 2012, p.25). It is reported that there is 95% humidity during the night in a tropical rainforest and 65-70% during the day (Silk et al., 2015).

The emergent layer of a rainforest

The emergent layer of a rainforest

The Strata of a Rain Forest

The tropical rain forests are like multi-story buildings with different categories of people living in each story. Sometimes there are give and take relations between the populations of each stratum but sometimes a member species of a stratum will never meet a member species of another stratum.

Generally, the stories are counted as five. The topmost one is the emergent layer where the upper branches of the tallest trees stand aloof above the rest, doused by the warm sunlight (Tropical Rain Forest, 2012). These are the trees that are 100 feet tall or even taller. They grow upright and raise their heads above the general canopy, looking like nature's observatories. An eagle or falcon soaring high above a rainforest may find the branches of such trees a place to rest and look for prey. However, at the best, they could see up to the next lower layer; other deeper layers would be completely masked.

Most of the trees belonging to this stratum have smaller leaves than the other trees of a rain forest because they have to weather the winds that circulate at this height of the atmosphere with minimum stress and energy loss. There are plants like epiphytes growing on tree branches and ants and other insects living there too, exclusively endemic to this layer of a rain forest.

The second layer is the canopy of which the upper part is thoroughly exposed to sunlight (Tropical Rain Forest, 2012). However, the underbelly of the canopy has very little light because of its dense leaf cover. The trees in this layer mostly are of heights between 80 and 100 ft (Tropical Rain Forest, 2012).

The third layer of a rain forest is another shorter canopy with trees about 50-60 feet in height (Tropical Rain Forest, 2012). Monkeys, squirrels, and birds mostly live here. Naturally, with only the sunlight peeping through the upper canopy, this lower canopy is cooler and darker.

Fifth and the last is the floor of the forest. There will be saplings, ferns, insects, fungi, and a lot of decaying organic matter on this floor (Tropical Rain Forest, 2012). Small streams will be crisscrossing this entire forest floor, thanks to the incessant and intermittent rains that we discussed before. The lower strata plants and trees usually have large leaves in order to capture maximum sunlight for photosynthesis under the light-starved conditions of the inner forest (Tropical Rain Forest, 2012).

How Much Light Will be There Inside a Rain Forest During Day Time?

If the light density above the canopy of a rainforest is 100%, then the light density inside and near the ground is only 1% (Kira and Yoda, 2012, p.56). Very dark indeed! The light that reaches the ground is classified into many types by scientists based on quality and intensity (Kira and Yoda, 2012, p.56). When one looks up, there will be areas where no sky is visible, then there will be small openings in the canopy and also large openings in some places.

For humans like us, the mystery and charm of a rainforest experience are enhanced by this special lighting that nature has endowed a rainforest with. This is also why each and every leaf, flower petal, fruit, colorful insect, or animal will acquire a unique color and radiance when seen in their natural habitat inside a rain forest. It is indeed a photographer's paradise but a very risky one too.

the-tropical-rain-forest

Air Quality, Wind and Sounds

There is very little wind in a rainforest (Lauer, 2012, p.21). As is expected, inside the thick vegetation of a rain forest, there cannot be free airflow and hence the lack of winds. It is basic science that the respiration of the plants supplies carbon dioxide to the air and CO2 is also released by the soil by way of oxidation of the carbon content in the organic matter. The trees and plants naturally absorb this carbon dioxide for photosynthesis and release oxygen back and thus a balance of atmospheric quality is maintained.

A walk through a rainforest can send a chill up your spine; everything appears to be at a standstill. Even so, you can still hear the music of water flowing and birds chirping. And if you listen carefully, you might also hear the many species of the forest making their presence known through their sounds; crickets, frogs, and mammals dominate this orchestra of rich and multifaceted acoustic life.

The Soil

The soil of the rainforest provides nitrogen and phosphorus to the trees and plants, while the decaying parts of the flora and fauna supply potassium, magnesium, and calcium for plant growth (Sanchez, 2012, p.84). It can be said a rainforest is partly a self-feeding ecosystem (Sanchez, 2012, p.84).

A farmer knows it takes effort to cultivate a crop and get a good yield. Without applying fertilizers and without irrigation, all the vegetation in the rain forests has flowers and fruit in plenty. This is owing to the nutrient recycling that happens through falling leaves and branches and their decomposition by fungi, termites, ants, bacteria, and the enzymes that the roots of the live plants and trees release. The natural diversity of the forest ensures all nutrients are available in plenty. To emulate this wholesomeness of the rainforest as much as possible while doing cultivation is the ultimate dream of any organic farmer.

Biodiversity

The tropical rainforest ecosystem is as rich as one could ever imagine. It is estimated there are between 40,000 and 53,000 tree species in them (Silk et al., 2015). This is in stark contrast to the European territories having a temperate climate, with only around 124 tree species to claim as their own (Silk et al., 2015). There are epiphytes (plants that grow on other tree branches. E.g. orchids), lianas (tree-like hard-stemmed climbers that grow up to the canopy), climbers (that climb up only to the lower strata), stranglers (plants that start living on tree branches and then grow their roots down to get nutrients from the ground and anchor there. E.g. figs), and heterotrophs (plants that grow on the ground and do not carry out photosynthesis E.g. fungi) (Tropical Rain Forest, 2012).

The biodiversity of a tropical rainforest varies immensely as geographical location changes, indicating that evolution also played an important role (Bermingham and Dick, 2005, p.15). For example, the tree diversity one can see in the Western Ghats in India is entirely different from the neo-tropical tree collections in the Americas. There are also many regional factors that decide the biodiversity of a tropical forest. The species diversity of rainforests increases if one starts looking far from the equator and moves closer to it.

In tropical rainforests in Ecuador, researchers have documented the presence of as many as 900 species of vascular plants just within the perimeter of one hectare of forest land (Bermingham and Dick, 2005, p.14). It is also a fact that about 20 to 30 percent of the tree species of tropical rainforests still remain unidentified and unnamed (Bermingham and Dick, 2005, p.14).

The Canopy Revisited

The canopy of a tropical rainforest is an amazing place. It consists not only of the branches and leaves of tall trees but plants like epiphytes that have made the very canopy their habitat. There are canopy beetles, ants, epiphyte-eating canopy birds, and other anthropods that have made the canopy their habitat. Holo-epiphytes are one category of epiphytes that anchor themselves on tree canopies but are not parasitic and they spend their entire life on the canopy without ever touching the ground (Benzing, 2012, p.133). They use the canopy of other trees only for anchorage.

All these species get their nutrients and water from the trees, the decaying litter, the water stored from the rain in leaves and branch cavities, the atmosphere, mist and fog, and so on. It is a micro-habitat that has its own unique ways of life- most of the members most of the time completely unaware of the world that is there 80-100 feet below.

Did Some of Our Ancestors Live Exclusively in the Rainforests?

There are still hunter-gatherer tribes living in the rainforests today. Until recently, anthropologists had thought these people had no contact with the outside world and that they lived exclusively on fruits, meats, and animals available within the forest (Headland, 1987, p.463). However, recent evidence suggests that though rich in biodiversity, the tropical rainforests did not have much food to offer to the human species (Headland, 1987, p.463).

One could also see why humans started cultivating plants if one considers the scarcity of food in the forests. It is suggested by new studies that the hunter-gatherers living in the tropical rain forests would have established a barter connection with the farming communities living in the outskirts of the forests very long back and would have depended on them for cultivated food (Headland, 1987, p.463). In return, they should have exchanged the forest goods they collected from within the tropical woods (Headland, 1987, p.463-491).

This is the norm today for many tribal people and it seems this is a very ancient practice too. The tribal people living inside the rain forests might also have carried out some farming activity on their own inside the forest, though in a limited sense (Headland, 1987, p.463). Otherwise, they would not have been able to survive inside a tropical rainforest for long, say scientists (Headland, 1987, p.463). The romantic notions of exclusively forest-dwelling early homo sapiens are only partially true.

The tropical rainforests continue to intrigue and marvel scientists and all nature enthusiasts. There is more to be discovered in this pristine world.

References

Age of tropical rain forests, (2018), Rainforest Conservation Fund, Retrieved from http://www.rainforestconservation.org/rainforest-primer/rainforest-primer-table-of-contents/c-age-of-tropical-rainforests/

Bermingham, D and Dick, C.W. (2005), Overview: The history and ecology of tropical rainforest communities, In Tropical Rainforests: Past, Present and Future (pp.7-15), Bermingham, E., Dick, C.W. and Mortiz, C. [Eds.], Chicago: University of Chicago Press.

Headland, T.N. (1987) The wild yam question: How well could independent hunter-gatherers live in a tropical rain forest ecosystem, Human Ecology, 15 (4), pp.463-491. Retrieved from https://link.springer.com/article/10.1007/BF00888000

Kira, T. and Yoda, K. (2012), Vertical stratification in micro climate, In Tropical rain forest ecosystems: Biographical and ecological studies, Lieth, H. and Werger, M.J.A. [Eds.], New York: Elsevier.

Kurokawa et al., (2003) The age of tropical rain forest canopy species, Borneo ironwood (Eusideroxylon zwageri), determined by 14C dating, Journal of Tropical Ecology, 19 (1), pp.1-7. Retrieved from https://www.cambridge.org/core/journals/journal-of-tropical-ecology/article/age-of-tropical-rainforest-canopy-species-borneo-ironwood-eusideroxylon-zwageri-determined-by-14c-dating/5439228B44EA889527FDF21970E34DFA

Lauer, W. (2012), Climate and weather, In Tropical rain forest ecosystems: Biographical and ecological studies, Lieth, H. and Werger, M.J.A. [Eds.], New York: Elsevier.

Benzing, D.H. (2012), Vascular epiphytism in America, In Tropical rain forest ecosystems: Biographical and ecological studies, Lieth, H. and Werger, M.J.A. [Eds.], New York: Elsevier.

Silk et al., (2015), An estimate of the number of tropical tree species, PNAS, 112 (24), pp.7472-7477. Retrieved from http://www.pnas.org/content/112/24/7472.short

Tropical rain forest, (n.d.), Biomes of the World: Department of Geospatial Science, Radford University, Retrieved from https://php.radford.edu/~swoodwar/biomes/?page_id=100

Questions & Answers

Question: Why is every day the same in the tropical rainforest?

Answer: It is because of the thick vegetation mainly. However, there are different categories of tropical rain forests and some of them have seasonal climate variations. For example, a monsoon tropical rain forest will experience wet and dry spells during the rainy season and summer respectively. The days are the same also because the rain forest climate is a self-sustained system. The evaporated water is less in the atmosphere above a rain forest because of the thick vegetation but water transpires in great quantity from the tree leaves to the atmosphere. This water is sufficient to create rain clouds and the same water rains back to the forest. This cycle is eternally repeated. Hence the stability and uniformity of the climate.

© 2018 Deepa

Comments

Deepa (author) from India on November 06, 2019:

Thank you. I have always been fascinated by the science behind the way life behaves.

Halemane Muralikrishna from South India on October 15, 2019:

Nice to read your article on rainforests and the biodiversity, which took me back to my college days of Botany study.