Rainforest

• 1. Introduction

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  Rainforests have been called the "jewels of the Earth" and the "world's largest pharmacy" - and the numbers behind those names are staggering. Somewhere between 40 and 75 percent of all living species on the planet make their home in rainforests. More than one quarter of all natural medicines have been discovered there. And yet millions of species of plants, insects, and microorganisms in tropical rainforests remain completely unknown to science. What exactly defines a rainforest? How do these ecosystems actually work, layer by layer? Who lives inside them, and what happens to the planet if they disappear?

• 2. Defining the Forest

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  A closed and continuous tree canopy is the single feature that unites every rainforest on Earth. Add to that a moist layer of leaf litter, moisture-dependent vegetation, the presence of epiphytes and lianas, and the complete absence of wildfire, and you have the core definition. Tropical rainforests sit within 10 degrees north and south of the equator, where mean monthly temperatures stay above 18 degrees Celsius in every month of the year. Annual rainfall runs no less than 168 centimetres and can exceed 1,000 centimetres, though most tropical rainforests receive between 175 and 200 centimetres per year.

  Many tropical forests cluster around the monsoon trough, the band meteorologists call the Intertropical Convergence Zone. The reach of these forests is genuinely global: they stretch across Southeast Asia from Myanmar to the Philippines, Malaysia, Indonesia, Papua New Guinea, and Sri Lanka; across Sub-Saharan Africa from Cameroon to the Congo; across South America in the Amazon basin; across Central America through Bosawas and the southern Yucatan Peninsula; and onto Pacific islands including Hawaii.

  Temperate rainforests occupy a far smaller footprint. They appear in the Pacific Northwest of North America, from Alaska down through British Columbia, Washington, Oregon, and California, and also in the Appalachian Mountains. Parts of coastal Ireland, Scotland, southern Norway, the western Balkans, Galicia, and the eastern Black Sea coastlines including Georgia and Turkey qualify as well. East Asia contributes southern China, Taiwan's highlands, much of Japan and Korea, and Sakhalin Island. Southern Chile and parts of Australia and New Zealand round out the picture.

  Dry rainforests complicate the category further. They carry a more open canopy and grow in areas of lower rainfall, often sheltered in the rain-shadow of mountain ranges. In Australia, they develop a structurally complex species composition, with trees adapting through deciduousness, reduced leaf size, thick leathery leaves, and sharp thorns. Their understory can be split into sub-types including Araucarian vine forest and semi-evergreen vine thicket.

• 3. A World in Layers

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  The emergent layer crowns the forest at heights of 45 to 55 metres, with occasional giants reaching 70 to 80 metres. Trees growing here face hot temperatures and strong winds that other trees below never experience. Eagles, butterflies, bats, and certain monkeys have adapted to life at these exposed heights.

  Below that, the canopy layer runs from roughly 30 to 45 metres and holds the densest concentrations of biodiversity found anywhere on the planet. The canopy, by some scientific estimates, shelters 50 percent of all plant species. Epiphytic plants anchor themselves to trunks and branches, drawing water and minerals directly from rain and the debris that collects on their host trees rather than from the ground. A quarter of all insect species on Earth are believed to live in the rainforest canopy alone.

  Scientists long suspected the richness of the canopy but lacked practical ways to study it. In 1917, naturalist William Beebe declared that "another continent of life remains to be discovered, not upon the Earth, but one to two hundred feet above it, extending over thousands of square miles." Real exploration only began in the 1980s, when researchers developed techniques such as firing ropes into the treetops using crossbows. Balloons, airships, cranes, and walkways anchored to the forest floor followed. The science of reaching the canopy by airship or similar aerial platform even acquired its own name: dendronautics.

  The understory lies between the canopy and the forest floor, receiving only about 5 percent of the sunlight that strikes the top of the forest. Jaguars, boa constrictors, and leopards hunt here alongside birds, snakes, and lizards. Leaves at this level grow noticeably larger than those above, a direct response to the reduced light. Many of the seedlings that will one day push into the canopy wait out their early years in this dim middle world.

  The forest floor receives just 2 percent of incoming sunlight. Away from riverbanks, swamps, and clearings, the ground stays relatively open because so little light reaches it. What does accumulate there - dead plant material, fallen animals - disappears rapidly. Warm, humid conditions accelerate decay, and fungi are central to that process, feeding on organic waste and cycling nutrients back into the system.

• 4. Soils and the Nutrient Paradox

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  Despite the overwhelming abundance of life above ground, the soil beneath a tropical rainforest is often quite poor. Rapid bacterial decay strips organic matter before it can build into humus. The laterization process concentrates iron and aluminium oxides in the soil, turning it a bright red and sometimes producing mineral deposits such as bauxite. Most trees keep their roots close to the surface because nutrients below are insufficient; the minerals those trees need come primarily from the thin top layer of decomposing leaves and animal matter.

  The acidity of rainforest soils makes the problem worse. Plant roots absorb nutrients by exploiting the difference in acidity between themselves and the surrounding soil. When the soil is already highly acidic, that gradient collapses, and nutrient uptake slows dramatically. The clay particles common in tropical rainforest soils also fail to trap nutrients effectively, allowing rain to wash them away before plants can use them. Even artificial fertilisation largely fails for the same reason: added nutrients leach out in the heavy rainfall before they can be absorbed.

  On younger substrates, particularly those of volcanic origin, tropical soils can be genuinely fertile. But clearing the forest triggers a cascade of problems. Rain falls directly on exposed soil surfaces, creating run-off and beginning erosion. Streams and rivers form where none existed. Flooding becomes possible. The intricate system that sustained the forest had depended on the canopy intercepting water and the roots holding soil in place. Remove the trees, and the whole mechanism unravels.

• 5. Flora, Fauna, and the Refuge Effect

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  Rainforests support mammals, reptiles, amphibians, birds, and invertebrates across dozens of families. Primates and felids are among the mammal groups present. Reptiles range from snakes and turtles to chameleons. Bird families include the Vangidae and Cuculidae. Fungi are common throughout, decomposing the remains of plants and animals and returning their elements to the cycle.

  The extraordinary diversity of rainforest species traces in large part to what biologists call physical refuges: places where plants are inaccessible to herbivores, or where animals can hide from predators. The sheer number and variety of these refuges allows far greater total biomass than a simpler environment would support.

  Not every population is stable. Some reptile species that feed on amphibians show declining numbers, a trend scientists say requires close monitoring. The seasonality of rainfall shapes the reproductive cycles of amphibians. Because those cycles feed directly into the food supply of specialist reptile predators, any disruption can ripple through to species with no obvious connection to the original change. Specialist feeders are especially vulnerable because they are less likely to switch to alternative prey.

• 6. People of the Forest

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  On the 18th of January 2007, Brazil's indigenous affairs agency FUNAI reported that it had confirmed the presence of 67 different uncontacted tribes within Brazil's borders, up from 40 in 2005. That single announcement pushed Brazil past the island of New Guinea to become the country with the largest number of uncontacted tribes in the world. The province of Irian Jaya, also known as West Papua, is estimated to be home to 44 uncontacted groups of its own.

  Central African rainforest is home to the Mbuti pygmies, a hunter-gatherer people whose average height falls below one and a half metres, or 59 inches. Anthropologist Colin Turnbull published a study of them, The Forest People, in 1962. In Southeast Asia, related short-statured peoples are known collectively as Negrito.

  In Sarawak, the Malaysian state that forms part of Borneo - the third largest island in the world - many distinct peoples live in the rainforest interior. The Kayan, Kenyah, Kejaman, Kelabit, Punan Bah, Tanjong, Sekapan, and Lahanan are among them. Together these groups are called Dayaks or Orangulu, a name that translates as "people of the interior." About half of Sarawak's 1.5 million people are Dayaks. Anthropologists generally believe their ancestors migrated from the Southeast Asian mainland, a conclusion that the groups' own mythologies appear to support.

• 7. Deforestation and Its Costs

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  Almost 90 percent of West Africa's rainforest has already been destroyed. Since humans arrived on Madagascar, the island has lost two thirds of its original rainforest. At rates of loss documented before projections were made, tropical rainforests in Indonesia faced being logged out within 10 years, and Papua New Guinea within 13 to 16 years. Rainforest Rescue identified the expansion of oil palm plantations as a significant driver in Indonesia, where palm oil was already cultivated on nine million hectares. Together, Indonesia and Malaysia produce about 85 percent of the world's palm oil.

  Biologist E. O. Wilson of Harvard University estimated that if species loss continued at the pace associated with rainforest destruction, a quarter or more of all species on Earth could be gone within 50 years. The possible extinction rate attached to that projection ran to more than 50,000 species a year. Brazil declared its deforestation a national emergency, and official government data showed that Amazon deforestation jumped 69 percent in 2008 compared to the previous year.

  A natural, undisturbed rainforest absorbs and emits carbon dioxide in rough long-term balance. Human-induced deforestation breaks that balance, turning forests from neutral actors into net carbon sources. Burning and drought compound the effect. Some climate models projected a large loss of Amazonian rainforest around 2050 driven by drought and the dieback that would follow. The popular idea that tropical forests act as the "Earth's lungs" by adding oxygen to the atmosphere turned out to be misleading; an undisturbed rainforest contributes little net oxygen addition through photosynthesis. What rainforests do contribute is the recycling of water vapour, with measurable effects on cloud formation and regional climate.

  A 2009 report offered one counterpoint: for every acre of rainforest cut down each year, an estimated 50 or more acres of new forest were growing back in the tropics, including secondary forest on former farmland. That regrowth, however, is not equivalent to primary forest in biodiversity or in the depth of its ecological function.

• 8. What the Forest Gives

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  Tropical rainforests supply timber and animal products including meat and hides. They draw tourists and provide ecosystem services that extend far beyond their borders. Many foods that now appear on tables worldwide originated in tropical forests and are still grown on plantations that replaced primary forest. Plant-derived medicines from rainforest species treat fever, fungal infections, burns, gastrointestinal problems, pain, respiratory conditions, and wounds.

  The uncontacted tribes confirmed by FUNAI in 2007 represent communities whose relationship to that pharmacopeia, and to the forest itself, developed over generations entirely outside the industrial economy. Their continued existence depends on forest cover that non-native peoples have rarely managed sustainably; exploitation and agricultural clearance remain the dominant pattern. The distinction the source draws between native and non-native relationships to the forest is not incidental. It points toward the question of what kind of knowledge disappears when both the forest and its longest-standing inhabitants are lost together.