Deepa is a freelance researcher and journalist. She writes and makes documentaries and videos.
The Tropical Rain Forest: A General Profile
Tropical rain forests are forests that never get dry and get rain almost every day of the whole year. They are a lush green, almost wet, home to innumerable living beings including plants, insects, animals etc., to the most luxurious kinds of flowering trees and plants, and in a nutshell, a heaven on earth for every nature enthusiast. The mystery of a rain forest is still unfolding before the eyes of the world as many species living in it still stay unidentified and unnamed by humanity. The tropical rain forests fall on both sides of the equator up to approximately 10 degrees of latitude (Lauer, 2012, p.7). Under the right kind of heat, water availability, earth’s radiation, atmospheric circulation, elevation and proximity to the equator, and thereby the position with respect to the sun, happens a tropical rain forest- the greatest treasure trove of nature (Lauer, 2012, p.7).
The Age of Rain Forest
The origin of the tropical rain forests is traced back to more than 200 million years ago when the entire 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 a new mode of propagation through them and the result was a new life form, the trees. Flowering plants came first and then the dinosaurs. The tall trees caused the rain forests to evolve into what they are today. Till 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 the temperate ecosystems and most of the species living today on earth originated in the tropics. The reason for the tropics being the cradle of the majority of species could be many- in the tropics, there is uniform climate throughout the year and there is no frost that inhibits life (Kurokawa et al., 2003). Some tree species in the rain forests 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 rain forest, there will be 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 as 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, during day time, 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 rain forest, 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.
When and How Often Does It Rain in a Rain Forest?
A rain forest is literally a raining forest. On an average, a tropical rain forest 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 rain forests- the “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 a rain created by the rain forest itself and by the “small water cycle” within the rain forest (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 travel, 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 with 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 rain forest after the noon time. 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 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, there is 95% humidity during the night in a tropical rain forest and 65-70 percent during the day (Silk et al., 2015).
Emergent Layer of a Rain Forest
The Strata of a Rain Forest
The tropical rain forests are like multi-storey buildings with different categories of people living in each storey. 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 storeys 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 rain forest may find such tree branches a place to rest and look for prey. However, at the best, they could see up to the next lower layer but the other deeper layers will 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 less light because of its dense leaf cover. The trees of this layer mostly are of the height, below 100 feet but not less than 80 feet (Tropical Rain Forest, 2012). The third layer of a rain forest is another shorter canopy with trees having about 50-60 feet 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 rain forest 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 are 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 rain forest experience is enhanced by this special lighting that nature has endowed a rain forest 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.
Air Quality, Wind and Sounds
There is very little wind in a rain forest (Lauer, 2012, p.21). As is expected, inside the thick vegetation of a rain forest, there cannot be free air flow and hence the lack of winds. It is basic science that the respiration of the plants supply 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. If one walks through a rain forest, a chill can run through one's spine because everything will seem to be on a standstill. However, to one's relief, one can listen to the music of water flowing and birds chirping. One can also hear near and far, the very many species of the forest acknowledging their presence through the sounds they make- crickets, frogs and mammals dominate this orchestra of rich and multifaceted acoustic life.
The soil of the rain forest 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 rain forest is a partly a self-feeding ecosystem (Sanchez, 2012, p.84). A farmer knows it is effort-some to cultivate a plant and get good yield. Without applying fertilizers and without irrigation, all the vegetation in the rain forests flower 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 a rain forest as much as possible while doing cultivation is the ultimate dream of any organic farmer.
The tropical rain forest ecosystem is 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 the 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 rain forest 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 rain forests increases if one starts looking far from the equator and move closer to it. In a tropical rain forest in Ecuador, the researchers have documented the presence of as much as 900 species of vascular plants just within the perimeter of one hectare of the 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 rain forests still remain unidentified and unnamed (Bermingham and Dick, 2005, p.14).
The Canopy Revisited
The canopy of a tropical rain forest 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 rain in leaves and branch cavities, the atmosphere, from 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 Rain Forests?
There are hunter-gatherer tribes living in the rain forests of today. Until recently the anthropologists had thought these people had no contact with the outside world as they have now and that they lived exclusively of the edible fruits, roots and the meat of the hunted animals available within the forest (Headland, 1987, p.463). However later evidences suggest that though rich in biodiversity, the tropical rain forests did not have had much food to offer to the human species (Headland, 1987, p.463). One could also see why humans started cultivating plants if one considers this 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 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 rain forest for long, say scientists (Headland, 1987, p.463). The romantic notions of exclusively forest dwelling early homo sapiens are only partially true.
The tropical rain forests continue to intrigue and marvel scientists and all nature enthusiasts. There is more to be discovered than already unraveled of this pristine world.
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
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.