TROPICAL RAINFORESTS: HISTORY, COMPONENTS, STRUCTURE, SOILS, WEATHER

TROPICAL RAINFORESTS

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liana in Belize A tropical rainforest is defined as a forest in a tropical region, generally between the Tropics and Cancer and Capricorn, with a large amount of rainfall. By contrast, there are tropical dry forests and temperate rainforests. Tropical rainforests are known for being rich in both plant and animal life, a condition expressed by the term biodiversity.

There is about 1.2 billion hectares of tropical forest in the world. Tropical rainforests cover half the worlds woodlands, 14 percent of the world’s land surface and 6 percent of the Earth’s total surface area. Some rainforests have remained virtually unchanged for millions of years. In the same period lakes have appeared and dried up, volcanoes have produced new mountains worn down by erosion and grasslands have become deserts.

The first tropical rainforests began to form roughly 140 million years ago, during the age of the dinosaurs, when the climate of nearly the entire world was tropical. Flowering plants also evolved during this period, and with them their symbiotic relationship with wasps, beetles, bees, flies, butterflies, and moths who pollinated the flowers. [Source “Rainforest Canopy, the High Frontier” by Edward O. Wilson, National Geographic, December 1991]

Scientists define five different tropical habitats: 1) primary forests with trees 115 to 180 feet high; 2) secondary forests with trees 50 to 60 feet high; 3) Cecropia, mainly with palms up to 50 feet high; 4) scrub and shrubs up to 6.5 feet high; 5) pasture up to 1.5 feet.

There are a number of different kinds of rainforests: montane rainforests in the mountains, lowland rainforests in lowlands, riverine rainforests along rivers, esturine rainforests near estuaries and cloud forests where clouds collide with the slopes of tropical mountains. The word jungle is often times misused as a synonym for rainforest. A jungle refers to dense growth adjacent to an open area.

Based on discoveries made of a 300-million-year-old fossilized forest found in a coal seem in Illinois, club mosses grew over a meter thick and 40 meters high in primordial rainforests.

Websites and Resources: Rainforest Action Network ran.org ; Rainforest Foundation rainforestfoundation.org ; World Rainforest Movement wrm.org.uy ; Wikipedia article Wikipedia ; Forest Peoples Programme forestpeoples.org ; Rainforest Alliance rainforest-alliance.org ; Nature Conservancy nature.org/rainforests ; National Geographic environment.nationalgeographic.com/environment/habitats/rainforest-profile ; Rainforest Photos rain-tree.com ; Rainforest Animals: Rainforest Animals rainforestanimals.net ; Mongabay.com mongabay.com ; Plants plants.usda.gov

Books: “The Private Life of Plants: A Natural History of Plant Behavior” by David Attenborough (Princeton University Press, 1997); “Portraits of the Rainforest” by Adrian Forsythe.

Tropical Rainforest Components and Complexity

A group of professors wrote in The Conversation: Tropical forests are more than just trees — they are complex, dynamic networks of plants, animals and microbes. Active research areas typically emphasize how specific features of forests, such as the number of species they contain or tree biomass, change over time and space. [Source: Robin Chazdon, Professor Emerita of Ecology and Evolutionary Biology, University of Connecticut, Catarina Conte Jakovac, Associate professor of Plant Science, Universidade Federal de Santa Catarina, Lourens Poorter, Professor of Functional Ecology, Wageningen University, The Conversation, and Bruno Hérault, Tropical Forest Scientist, Forests & Societies Research Unit, Cirad, The Conversation December 10, 2021]

Important components a key areas of study within them include: 1) Soil: How much organic carbon and nitrogen does it contain, and how compacted is it? Soil that is too densely compacted — for example, by the hooves of grazing cattle — is hard for plant roots to penetrate and doesn’t absorb water well, which can lead to erosion. 2) Ecosystem functioning: How does the abundance and size of trees change as the forest regrows? What is the role in forest regrowth of trees that have root associations with nitrogen-fixing bacteria? How does regrowth affect the average density of wood and the durability of leaf tissues?

3) Forest structure: How do maximum tree size, variation in tree size, and total biomass — the quantity of plant matter above ground in tree trunks, branches and leaves — change as forests regrow? 5) Diversity and composition of tree species: How do the numbers of tree species present and the diversity and abundance patterns of species change and become more similar to nearby old-growth forests?

Rainforest Canopy

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above canopy The upper branches of the rainforest trees and the vegetation found in them are referred to as the canopy. It extends for about five to seven meters (16 to 23 feet) deep from the tops of the trees. The vastness and complexity of system is one reason why the rainforest contains over half of all the world's species.

The trees that make up the canopy vary in height between 30 and 45 meters (100 and 150 feet), with a few towering 60 meters (200 feet) or more. The trees have trunks free of branches for the first sixty feet or so and nearly horizontal upper branches. Scientists have estimated that the total surface of the leaves and branches on the world's forest canopies is equal to the total surface of the earth.

Virgin rainforests are made up of a nearly continuous canopy of trees with flattened crowns. Describing what it is like be up there, David Attenborough wrote: "Suddenly the humid twilight is replaced by fresh air and sunshine. Around you stretches a limitless meadow of leaves...dimpled like the surface of enormously enlarged cauliflower. Here and there, standing 10 meters or more over the rest, rises a single giant tree...Up here the wind blows freely through their crowns...the air is warm and humid."

Life and Soil in the Rainforest Canopy

The canopy captures 90 percent of sunlight, conducts the bulk of the photosynthesis activity and is home to nearly all the animals, plants, insects and birds. Many creatures live their entire lives and die in the canopy without every setting foot on the ground. While many species in the canopy are very similar to each other, they often have very little in common with species found near the ground.

Once a tree has grown tall enough to become part of the canopy it serves as a host for palm trees, parasitic vines like figs and flowering plants. Many forms of life rely on “soil” found on tree branches rather than on the ground soil.

On the 100-foot-high branches of some large trees there is foot-thick soil with earthworms. This soil made up of wind-borne dust, fallen leaves, decomposed fungi and moss, other detritus (decayed organic material), animal droppings and plant and animal remains. The mixture is like a bog, and especially thick where the branches come together.

Living on top of this soil are leafy plants such as orchids, philodendrons, ferns, bromeliads, and flora related to azaleas, violets and pepper plants. Growing on the leaves and the undersides of the branches are lichens and mosses. And living among all this are birds, insects, small mammals, worms, caterpillars, spiders, butterflies, hummingbirds, lizards and salamanders. [Source “Rainforest Canopy, the High Frontier” by Edward O. Wilson, National Geographic, December 1991]

Mid-Level and Low-Level Rainforest Life

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Rainforest strata The area below the canopy is sometimes called the understory. Very little light penetrates the canopy and the species of trees below it include adolescent canopy trees working their way up and mature tree species that thrive in the twilight region.

Lower down are at the height of human beings is a lower canopy of saplings and herbaceous plants. The layer of leaves, not as dense as the canopy but still significant. These are produced by the small trees and palms and other plants adept at living in dim light. The lower canopy is kind of like a way station for animals on their way between the canopy and the ground.

There isn't much visible activity going on in the rainforest at ground level, which sometimes receive only one percent of the sunlight that reaches the canopy. The canopy is so thick that people who work in the rainforest almost never see the sky and rain from a cloudburst take ten minutes before it reaches the ground.

But that doesn’t mean there isn’t stuff going on. David Attenborough wrote: "It is very hot and stagnant and loaded with moisture. Such conditions suit the process of decay. bacteria and mold work unceasingly. Fungi proliferate, spreading their filaments through the leaf litter and erecting their bodies. The speed of decomposition is extraordinarily rapid."

The canopy trees block all but one percent of the sunlight. Despite rainfall of up to 16 feet a year saplings on the floor of primary forests only grow around an third of an inch a year. At that rate it would take 3,000 years to reach the canopy.

Rainforest Soils

Tropical rainforest soil is very thin and low in nutrients. With no winters or frosts to kill insects or microorganisms, and with lots of heat and humidity to help them grow and multiply, organic matter such as fallen leaves and twigs decomposes so quickly that only a thin layer of organic material covers the soils.

The soils are heavily leached of nutrients by rain water. But in many cases that doesn't matter much because the nutrients are absorbed by other life forms before the rain has a chance to carry them away. Trees absorb the nutrients with thick mats of rootlets that grow close to the surface of the soil.

Rainforest soils are generally terrible for agriculture, producing crops for one to three years before being depleted of nutrients (See Slash and Burn Agriculture). How is it then that a tropical rainforest is so rich in life. The secret is a quick turnover of nutrients.

When leaves, trees, plants of animals die they fall to the forest floor where they are quickly recycled into mulch by insects and fungi. Bacteria then plays a crucial part converting the mulch and litter into nutrients consumed by plants. The air at ground level in a tropical rainforest, according to Harvard biologist and ant specialist Edmond Wilson is "humid and saturated with odors of healthy decay." [Source “Rainforest Canopy, the High Frontier” by Edward O. Wilson, National Geographic, December 1991]

Only two organisms can break down dead plants into usable nutrients: bacteria and fungi. In wet, humid, dark and hot conditions in the rainforest fungi and bacteria can break down cellulose very quickly. Since there is little wind in the forests. leaves tend to fall straight down. Thus, many plants fed on nutrient they themselves deposited. Termites also play a major roll converting cellulose to nutrients that can be consumed by other animals. The termites themselves a favorite food of many animals.

Peat Rainforests

A peat rainforest, also known as a peat swamp forest, is a tropical forest where waterlogged soil prevents dead plants, leaves and wood from fully decomposing. Over time, this creates a thick layer of acidic peat. Peat swamp forests are typically surrounded by lowland rain forests on better-drained soils, and by brackish or salt-water mangrove forests near the coast. Some of the largest peat rainforests are in Indonesia and on the island of Borneo. Large areas of these forests are being logged at high rates. [Source Wikipedia]

A a peat bog is a wetland bog with sandy peat soil and woody shrubs. These areas typically occur in broad, low-lying shallow basins that do not drain naturally. They are formed by the accumulation of organic matter, resembling black muck, which builds up over thousands of years. This accumulation of material causes the soil to be highly acidic and nutrient-deficient. [Source: NOAA]

At one time, peat bogs were prominent ecosystems in many coastal areas but have disappeared rapidly due to their perceived low value for the environment and high value for land development. In reality, restoring healthy pocosin wetlands provides important benefits to terrestrial and aquatic ecosystems, as well as human communities: They provide wildlife habitat, lessen the frequency and severity of wildfires, sequester carbon, nitrogen, and mercury (known as a carbon sink), protect the water quality of estuaries, and control flooding in low-elevation coastal areas. Peat bog restoration also plays a key role in the adaption of ecosystems to sea level rise by preventing soil loss and promoting soil formation.

Borneo

Rainforest Weather

Compared to the changing weather conditions in temperate forest, the condition in tropical rainforests remains pretty much the same: being close the equator the amount of sunlight and warmth are almost the same everyday. The main variations are the rainfall amounts which often time change between the wet and dry season.

Rainforests receive anywhere from 80 inches to extremes of 400 inches of rain a year. Generally, a third of the rain evaporates, a third is absorbed by roots and greenery and third runs off. Even during strong thunderstorms sometimes little rain penetrates the canopy and reaches the ground. Instead it is absorbed by plants and collects in pools in the leaves and branches in the upper reaches of the rainforest.

The heat in the tropics is strong enough to melt the paint on the walls of steel hulled ships. The shade provided by all the trees in the rainforest, however, keeps the temperatures lower than they otherwise would be. The humidity though makes being in the forest very uncomfortable for many people.

The temperature and humidity are relatively constant at ground level in the tropical rainforest, but varies in the upper branches of the trees. Sunlight bakes the vegetation there, occasionally raising the temperature of the surrounding air to a full 10̊ higher than the ground level temperatures The relative humidity in the upper levels of the rainforest can range from as high as 100 percent at night to less than 30 percent at midday. [Source “Rainforest Canopy, the High Frontier” by Edward O. Wilson, National Geographic, December 1991]

Fluctuating temperatures and humidity, intense sunlight, torrential rains and winds can create "unpredictable microclimates" and eddies of wind that move trees and pump air and nutrients into canopy in an "accordion-like" fashion. [Source: Carol Kaesuk Yoon, New York Times, November 22, 1994; "Forest Canopies" Dr. Nadkarni, Academic Press]

First Forests Appeared About 390 Million Years Ago

The first land plants appeared about 470 million years ago but trees and forests didn't emerge until around 390 million years ago. Michael Dhar wrote in Live Science: During that interval, plant life slowly evolved genetic precursors needed to produce trees, which then outcompeted other plants, Chris Berry, a paleobotanist at Cardiff University in the United Kingdom, told Live Science. In 2019, Berry and colleagues reported on the oldest forest on record, in the journal Current Biology. This forest, uncovered in Cairo, New York, revealed that features characteristic of trees and forests — namely wood, roots and leaves amid a population of dozens of plants — appeared "far earlier than previously suspected": in the early Devonian period, 385 million years ago, the researchers said in the study. [Source Michael Dhar, Live Science, July 2, 2022]

The Cairo site preserved fossilized root systems of ancient trees, pinpointing where they would have appeared in life, Berry said. "We don't see fossils of the trees, but we see a map of exactly where those trees were standing," he said. "So what we learn is the ecology of the forest." That fossilized "map" features Archaeopteris, an ancient plant that boasted "large woody roots and woody branches with leaves," like modern trees, according to a report from Binghamton University in New York. Previously, the earliest known Archaeopteris find had placed the plants' arrival 20 million years later, Science reported.

The development of these early forests depended on the evolution of precursors to defining tree traits, Berry said. "I think the trigger is evolutionary, the development of anatomies which allow more complex branching," he said. Such anatomies arrived once plants had evolved "the genetic toolkit to be able to build" tree-like structures, Berry said. Early branching systems, for example, developed just before the Devonian, in the Silurian period (443.8 million to 419.2 million years ago), while the first roots arrived in the early Devonian, according to two reports from the Brooklyn Botanic Garden. Tree traits thereafter conferred major advantages, particularly the ability to rise above competition to soak up sunlight.

Some environmental changes may have made at least one important tree feature possible, however. Megaphylls, leaves that are common today and are characterized by branching veins, can grow much larger than their predecessors, thus absorbing more sunlight. They first appeared about 390 million years ago but started out small and became widespread only 30 million years later, at the end of the Devonian, according to a 2001 study in the journal Nature and a 2021 PBS "Eons" episode featuring the study.

That delay occurred because high carbon dioxide (CO2) levels made Earth too hot for large, megaphyll leaves, the study found. They'd simply absorb too much sunlight and overheat. Plummeting CO2 levels in the Devonian, however, doubly benefited megaphylls: Reductions in this greenhouse gas cooled the planet, while large megaphylls could fit in more pores called stomata to take in higher levels of dwindling CO2. Such leaves could then help push forward the forest revolution.

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Meteor That Killed Dinosaurs Also Created Modern Tropical Forests

The cataclysmic asteroid strike 66 million years ago off the Yucatán Peninsula that killed off the dinosaurs — the Chicxulub impact event — also changed tropical forests in distinct ways as evidenced by fossils. "This single historical accident changed the ecological and evolutionary trajectory of the rainforest forever," said Carlos Jaramillo, a paleontologist at the Smithsonian Tropical Research Institute in Panama and co-author of the study published in Science. "Without the meteorite, living in the tropical forest today would be very different." [Source: Alison Snyder, Axios, April 2, 2021]

Axios reported: “Over 12 years, Jaramillo and his colleagues collected and analyzed more than 6,000 specimens of fossilized leaves and 50,000 fossilized pollen samples in Colombia spanning the Late Cretaceous and early Paleogene period. The researchers measured changes in the density of leaf veins, the diversity of insect damage and other attributes to quantify how the forest plants' diversity and structure changed from the impact. Before the Chicxulub impact event, the forests were a mix of flowering plants, ferns and conifers with an open canopy that allowed light to pour into the forest. After the impact, 45% of the plant species went extinct (pollen fossils for the species vanished in the record) and flowering plants came to dominate a more closed canopy, forming the forests seen today.

Why did the forests change rather than return to their pre-impact state, especially since the climate was similar in both periods? Jaramillo offers three possibilities: 1) The disappearance of dinosaurs, which flattened the forest floor and fed on the canopy, reduced competition for light between plants and allowed flowering flora to take off. 2) Ash from the impact brought phosphorus to the ground, creating a rich niche for fast-growing, nitrogen-fixing flowering plants to take off. Jaramillo says the team plans to collect samples of soil preserved in rocks to measure their nutrient content. 3) The conifers that evolved to be canopy trees in the tropics went extinct, whereas flowering plants were diversifying and "doing all sorts of things," he says.

It took 6 million to 7 million years for the diversity of plants in the forest to return to levels before the collision. “"The lesson that comes out of these studies looking at rebuilding is that diversity doesn't come back right away," says Sean Gulick, who studies the Chicxulub impact and its effect on marine ecology at the University of Texas Institute for Geophysics and wasn't involved in the new research.

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Rainforests in the Arctic and Antarctic

There is evidence that rainforests flourished in the Arctic about 50 million years ago. Studies conducted by the Chinese Academy of Sciences in Beijing and the University of Colorado in the early 2020s indicated that winter temperatures tended to remain above freezing, while summer temperatures were around 21̊ C (70̊ F). As is true today there were months of darkness in the winter, and daylight all day at the height of the summer, plus lots plant life. Archaeologists have found the toe bone of a Gastornis, a now-extinct 6-foot-tall flightless bird that is though to have lived in dense forests, as well as indirect weighed hundreds of pounds. Around 58 million years ago Greenland was covered by temperate rain forests. [Source Lizzy Rosenberg, Green Matters, January 21 2021]

The discovery of amber in Antarctica sediments dated to 83 million and 92 million years ago by scientists from Germany and Britain shows that rainforest existed near South Pole. Amer is produced from the harden sap of coniferous trees. According to Science Alert: Along with fossils of roots, pollen, and spores, the amber provides some of the best evidence yet that a mid-Cretaceous, swampy rainforest existed near the South Pole, and that this prehistoric environment was "dominated by conifers", similar to forests in New Zealand and parts of Patagonia today. The study was published in Antarctic Research.[Source Carly Cassella, ScienceAlert, November 14, 2024]

Scientists have unearthed fossilized wood and leaves in Antarctica since the early 19th century, but many of these discoveries date back hundreds of millions of years to when the southern supercontinent Gondwana existed. As Antarctica drifted away from Australia and South America toward the south pole, it's not entirely clear what happened to its forests. In 2017, researchers drilled into the seafloor near West Antarctica and pulled up exceptionally well-preserved evidence of these long-lost habitats. After several years of analysis, Klages and a team of researchers announced in 2020 that they had found a web of fossilized roots that dated back to the mid-Cretaceous. Under the microscope, they also identified evidence of pollen and spores.

That same drilling has now offered up concrete proof that resin-producing trees once existed in Antarctica. In a 3-meter (10-foot) long layer of mudstone, Klagen and a new team have described several tiny slices of translucent amber, just 0.5 to 1.0 millimeters in size. Each hosts a variation of yellow to orange colors with typical scalloped fractures on the surface. This is a sign of resin flow, which occurs when sap leaks out of a tree to seal the bark against injuries from fires or insects. The Cretaceous was one of the warmest periods in Earth's history, and volcanic deposits found on Antarctica and nearby islands show evidence of frequent forest fires during this time.

Image Source: Mongabay mongabay.com ; Wikimedia Commons

Text Sources: “The Private Life of Plants: A Natural History of Plant Behavior” by David Attenborough (Princeton University Press, 1997); National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated November 2024