How Forests Work: Layers, Networks, and What We Lose When They Go

Stand in an old-growth forest and look at what appears to be empty air between the trunks. It is not empty. Every cubic meter holds insects, spores, microbes, and the water vapor the trees have just pulled out of the ground. Beneath your boots, fungal threads wrap around tree roots and trade minerals for sugar, in deals that have been running for centuries. Cut the trees and you do not just lose timber. You dissolve a working system that stores carbon, makes rain, and builds soil, and that takes far longer to rebuild than to remove.

Key takeaways

• Forests have distinct vertical layers. Canopy, understory, shrub, herb, and forest floor each have their own light, humidity, and community of species
• Fungi do much of the work. Decomposer fungi release the nutrients locked in dead wood, and mycorrhizal fungi extend the reach of living roots
• Nutrient cycling is mostly a closed loop. Dead material is broken down and the nutrients are taken up again, and in many tropical forests most of the nutrients sit in living biomass rather than in the soil
• Forests move water and carbon at planetary scale. They cover about 31% of Earth's land, and the FAO's latest assessment puts the carbon they hold at roughly 714 gigatonnes
• Deforestation cascades. Clearing a forest changes rainfall, erosion, species, and stored carbon together, and agriculture is the main driver

What counts as a forest, and how much is left

The FAO says forests cover 31 percent of the world's land, and that they hold more than half of the carbon stored in the world's soils and vegetation and more than half of its land animal and plant species (FAO). The most recent FAO Global Forest Resources Assessment, as summarized on Wikipedia, puts the total at about 4.14 billion hectares. Tropical forests account for 45% of that area, and five countries, Russia, Brazil, Canada, the United States, and China, hold 54% of it (Wikipedia, "Forest"). For a country-by-country breakdown, see Countries With the Most Forest Cover.

"Forest" is a loose label for very different things. The main types differ in climate, tree species, and how they keep their nutrients.

Type	Where	Typical trees	Defining feature
Boreal (taiga)	Subarctic Canada, Russia, Scandinavia	Spruce, fir, pine, larch	Cold, slow decomposition, thick organic soil
Temperate needleleaf	Cool mid-latitudes, mountains	Pines, firs, redwoods	Evergreen, often fire-adapted
Temperate broadleaf and mixed	Eastern North America, Europe, East Asia	Oak, beech, maple	Seasonal leaf drop, fertile soils
Tropical moist	Amazon, Congo Basin, Southeast Asia	Hundreds of species per hectare	Dense layers, fast nutrient turnover
Tropical dry	Seasonal tropics	Drought-deciduous trees	Leafless for months
Plantations	Worldwide	Usually one or two species	Managed for timber, pulp, or fruit

Source: Categories follow the UNEP-WCMC forest classification as summarized in Wikipedia, "Forest"; characteristics are standard ecology.

One more distinction matters for the rest of this article. Primary or old-growth forests are naturally regenerated forests of native species with no clearly visible signs of human activity. Wikipedia's synthesis of FAO data puts the world's primary forest at about 11.3 million square kilometers as of 2025, with Brazil, Canada, and Russia holding 62% of it (Wikipedia, "Old-growth forest"). Plantations and regrowth forests count as forest in the statistics, but they do not behave like primary forest, as later sections explain.

The layers of a forest

Think of a forest as a building with several stories, each with its own tenants. The tenants are sorted by how much light they can get.

The emergent layer consists of the few tallest trees that rise above the main canopy, found only in tropical rainforests. Their crowns sit in full sun, strong wind, and wide temperature swings. Many are pollinated by bats or large insects that can cross the gaps between isolated crowns, and some tropical emergents exceed 60 meters.

The canopy is the forest's solar roof. Interlocking crowns of mature trees catch most of the incoming light, which is why the canopy is where most photosynthesis happens. It is also a habitat of its own: birds, insects, arboreal mammals, and epiphytes (plants such as orchids and bromeliads that grow on branches rather than in soil) live here and may rarely touch the ground.

The understory holds shrubs, small trees, and saplings living in filtered shade. Plants here tend to have larger, thinner leaves to capture scarce light. Many are waiting. When a canopy tree falls and opens a gap, the saplings below suddenly get light and race to fill it. Ecologists call this gap dynamics, and it is how forests renew themselves one patch at a time.

The herb and shrub layers hold ferns, mosses, wildflowers, and seedlings. In dense tropical forest, the floor can be so dim that few plants grow there at all, while in a temperate deciduous forest this layer bursts into flower each spring before the canopy leafs out. That timing is no accident. The spring wildflowers are racing the leaves above them.

The forest floor is a carpet of leaf litter, fallen branches, and decaying wood over a darker layer of decomposed organic matter. It looks like debris, but it is the engine room, where most of the recycling happens. Our companion explainer on why leaves change color covers what trees pull back out of their leaves before the leaves drop.

The vertical structure matters because it multiplies habitat. A single hectare of rainforest offers many different light, humidity, and temperature conditions stacked within about fifty meters, and different species specialize in each.

Nutrient cycling: a loop, not a pile

If you ask where a rainforest keeps its nutrients, the surprising answer is: in the trees. Many tropical forest soils are old, heavily weathered, and low in the minerals plants need. Heavy rain flushes nutrients downward and out of reach. Yet the forest above them is spectacularly productive.

The resolution is speed. When a leaf falls in a tropical forest, decomposers can break it down within weeks in warm, wet conditions, and fine roots and their fungal partners recapture the released nutrients before rain washes them away. Think of a rainforest as someone who keeps cash in a wallet and spends it constantly, rather than in a savings account. Boreal forests are the opposite. Cold slows decomposition, so nutrients accumulate for centuries in thick layers of organic soil, like money in a bank.

The cycle has several players, each with a job.

Group	Examples	Role in the cycle
Fungi (saprotrophs)	White-rot and brown-rot fungi	Produce enzymes that break down lignin and cellulose, the tough compounds in wood
Bacteria	Nitrogen fixers, general decomposers	Convert atmospheric nitrogen into usable forms and process simpler organic compounds
Detritivores	Earthworms, millipedes, pill bugs, termites	Shred litter, increasing its surface area for microbial attack
Mycorrhizal fungi	Ectomycorrhizal and arbuscular types	Extend roots' reach for water and phosphorus in exchange for plant sugars
Predators and grazers	Springtails, mites, nematodes	Control microbial populations and release nutrients as waste

Fungi deserve special credit. Lignin, the compound that stiffens wood, resists most organisms. Certain fungi evolved enzymes that can dismantle it, which is why dead trees do not pile up indefinitely. Without them, forests would be buried in their own debris, and the carbon in that wood would stay locked away. Dead wood is not waste in this system. Standing dead trees (snags) and rotting logs shelter woodpeckers, beetles, salamanders, and seedlings. Old-growth forests are defined partly by having plenty of them, along with large living trees, multilayered canopies, and gaps where individual trees have died.

The wood wide web: what we know about mycorrhizal networks

Most land plants form partnerships with mycorrhizal fungi. The fungus colonizes the root and sends out thread-like hyphae far into the soil, acting as an extended root system. The plant pays in sugars from photosynthesis, and the fungus pays in water, phosphorus, nitrogen, and other minerals it mines from soil particles. This part of the story is well supported and fundamental to how land ecosystems function.

The more famous claim is that these fungi connect many trees into a shared underground network that moves resources between them. A 1997 paper by Suzanne Simard and colleagues in Nature, "Net transfer of carbon between ectomycorrhizal tree species in the field," showed that carbon labeled with isotopes could move between trees in field conditions. Later labeling work showed carbon moving in both directions between paper birch and Douglas fir, with the net direction varying over time (Wikipedia, "Mycorrhizal network").

So far, solid. The argument begins with how much it matters. Popular accounts often describe "mother trees" nurturing their seedlings through the fungal web, and the forest as a cooperative superorganism. Critics say the evidence does not support the strongest versions of those claims. A 2023 review by Karst, Jones, and Hoeksema in Nature Ecology & Evolution argued that claims about common mycorrhizal networks suffer from positive citation bias and overinterpreted results, and a 2023 paper by Henriksson and colleagues in New Phytologist re-examined the evidence for resource sharing among trees through ectomycorrhizal networks (Wikipedia, "Mycorrhizal network"). Open questions include:

• whether nutrients move through direct fungal connections or are released into the soil and taken up by neighbors
• whether any transfer measurably improves the survival or growth of the receiving plant
• how widespread such networks are in natural forests, as opposed to experimental plots
• whether trees preferentially help relatives, a claim described as emerging rather than established

A fair summary is that carbon and other compounds can move through fungal networks, that the fungi themselves are essential, and that the ecological significance of tree-to-tree sharing is still being tested. The analogy of a marketplace fits better than the analogy of a family: the fungi are traders who profit from every exchange, and whether they also move goods between the stalls on purpose is exactly what researchers are measuring.

One practical consequence is less disputed. Fungi need living hosts, and clear-cutting can disrupt the networks they form with tree roots. That is one reason replanting a cleared site with nursery seedlings is not the same as letting a forest regrow.

How forests move water

Trees are pumps. Roots pull water from the soil, it travels up through the trunk's vessels, and it leaves through tiny pores in the leaves as vapor, a process called transpiration. A large tree can move substantial amounts of water on a sunny day, and across a whole forest this puts a great deal of moisture into the air.

That water does several jobs at once.

It cools the surface. Evaporation uses energy, so a transpiring forest is cooler than a bare field under the same sun. This is the same physics that makes sweating cool your skin.

It feeds rainfall. In large continental forests, a portion of rain that falls is moisture the forest itself recycled from earlier rain. Droughts in the Amazon in 2005, 2010, and 2015 to 2016 have been linked to deforestation and warming (Wikipedia, "Deforestation of the Amazon rainforest"). Read the broader mechanism in How the Water Cycle Works.

It steadies rivers. Forest soil full of roots and leaf litter acts like a sponge. It soaks up rain, releases it slowly, and holds soil in place. Cleared land sheds the same rain quickly, so streams flood harder and run lower between storms.

It filters water. Forested watersheds typically deliver cleaner water than farmland or developed land, which is why many cities protect forests upstream of their reservoirs.

Forests and the carbon cycle

Photosynthesis is how forests turn air into wood. Each tree pulls carbon dioxide through its leaves and uses sunlight to build sugars and, eventually, trunks, branches, and roots. Our explainer on how photosynthesis works covers the chemistry.

The FAO's Global Forest Resources Assessment 2025, as reported on Wikipedia, estimates the total carbon stock of the world's forests at about 714 gigatonnes. Roughly 46% of it is in soil, 44% in living biomass, and the remainder in litter and dead wood (Wikipedia, "Forest"). That split is worth remembering, because it means that nearly half the carbon is underground, where it is easy to disturb and hard to measure.

Where a forest keeps its carbon varies by type. Tropical forests hold much of theirs in living trees. Boreal forests, with cold, slow decomposition, hold a large share in soil. Neither is simply "better" at storage, and the claim sometimes made that tropical forests hold more carbon per hectare than any other land ecosystem depends on what you count.

A related debate concerns old-growth forests. For decades, many assumed that mature forests stop absorbing carbon because growth balances decay. A 2008 study by Luyssaert and colleagues in Nature, summarized by Wikipedia, found that net ecosystem productivity was usually positive in forests aged roughly 15 to 800 years, meaning old forests often keep accumulating carbon. That finding is part of why conservationists argue that protecting old forest is not a "no-gain" strategy (Wikipedia, "Old-growth forest").

Forests are not a magic fix for climate change. A forest that burns, dies of drought, or is logged returns its stored carbon to the air. For the physics of that problem, see Climate Change Science: How It Actually Works.

Why forests hold so many species

The FAO says forests are home to more than half of the world's land animal and plant species. Several mechanisms help explain why.

Habitat in three dimensions. The layered structure and the variety of microhabitats, from rotting logs to canopy branches, give many species a place to specialize.

Disturbance creates patchwork. Treefall gaps, floods, and fires create a mosaic of young and old patches. Some pine species have cones that open fully only after fire. A forest that has never burned is not necessarily healthier than one that has.

Pests may protect diversity. One long-standing hypothesis, associated with the ecologists Daniel Janzen and Joseph Connell, proposes that seedlings die at higher rates close to adults of their own species, because seed-eating insects and soil pathogens concentrate there. If so, a single species cannot take over, and room opens for others. The idea is influential and has support in tropical studies, though how much it explains varies from forest to forest.

Fragmentation shrinks what is left. When a forest is chopped into patches, species disappear from small, isolated fragments faster than they can recolonize. Island biogeography theory predicts this: smaller and more isolated patches support fewer species. Tropical forests are particularly vulnerable, with many species restricted to small ranges. See also Countries With the Most Biodiversity for where the richest places lie.

What happens when forests are cleared

Deforestation is not one event. It is a chain.

The clearing itself. Our World in Data, drawing on research by Singh and Persson, puts global deforestation at roughly 10 million hectares per year across 2001 to 2022, while the FAO's assessment gives a lower average of about 8.5 million hectares per year. The numbers differ because they use different methods and time windows. Both agree that the rate peaked in the 1980s and early 1990s and has declined since, though it remains high (Our World in Data).

The drivers. The same analysis finds that agriculture is the leading cause overall. Beef accounts for about 41% of global deforestation (with Brazilian beef alone about a quarter), oilseeds such as soy and oil palm for about 16%, and forestry and cereals for about 12% each. About 70% of deforestation is driven by domestic demand and about 30% by traded products (Our World in Data).

Soil damage. Without roots and litter, soil loses its structure and washes away. In many tropical regions, cleared land produces good yields for only a few years before nutrients run out, which pushes farmers to clear new land. This tight nutrient cycle that made the forest productive makes the land poor once the trees are gone.

Changed water. Less transpiration means less cloud formation and rainfall locally, and runoff increases. Rivers run flashier, with bigger floods and lower dry-season flow.

Carbon release. Burning or decaying biomass sends stored carbon into the air, and soil disturbance releases more. This is why deforestation is a significant source of emissions. The IPCC estimated that agriculture, forestry, and other land use together were responsible for about 22% of net anthropogenic greenhouse gas emissions around 2019 (Wikipedia, "REDD+").

Species loss. Habitat loss and fragmentation are leading causes of biodiversity decline.

The Amazon: a test case

The Amazon shows all of these effects at once. Amazon Watch, cited on Wikipedia, reported that by 2022 about 20% of the rainforest had been transformed and another 6% was highly degraded. Brazil's annual deforestation in the Brazilian Amazon, measured by the national space agency INPE, has swung widely with policy:

Year	Brazilian Amazon deforestation (km²)
2004	about 27,000 (sources give 27,423 and 27,772)
2012	4,571
2022	11,600
2023	7,952
2024	6,288

Source: INPE satellite monitoring, as compiled in Wikipedia, "Deforestation of the Amazon rainforest". Figures are rounded and the page's own 2004 values conflict.

The table shows that deforestation responds to enforcement and economics. It fell by more than 80% from 2004 to 2012, rose again, and fell again in 2023 and 2024.

Climate scientists worry about a tipping point, a threshold beyond which the forest can no longer generate enough of its own rainfall and shifts toward savanna. Modeling studies suggest the forest may be approaching such a point, but the exact threshold is uncertain and debated. The practical message does not depend on the exact number. A forest that makes its own weather is vulnerable to being cleared past the point where it can recover.

Can forests be rebuilt?

Partly. Forest area has increased in some regions, including parts of Europe and East Asia, as farmland has been abandoned or planted. Regrowth forests recover quickly in some respects: canopy cover and simple structure can return within decades. Other qualities come back much more slowly, including the full diversity of species, large old trees, deadwood, and soil carbon built up over centuries.

Plantations differ most. A rows-of-one-species plantation can store carbon and produce timber, but it lacks the layered structure and species richness of natural forest. That is why a hectare lost from old-growth is not offset by a hectare planted elsewhere.

On the policy side, the main international tool is REDD+, a voluntary framework under the UN climate convention that pays developing countries for results in reducing deforestation and degradation. It grew from a 2005 proposal by Papua New Guinea and Costa Rica, was built out through decisions at Bali in 2007 and Cancún in 2010, and was completed in the Warsaw and Paris rounds in 2013 and 2015. Critics point to additionality (would the forest have been cleared anyway?), leakage (clearing moves elsewhere), and permanence (gains can be reversed by fire or logging). A 2024 multi-country evaluation found modest average forest and welfare gains that were not always sustained (Wikipedia, "REDD+").

Common mistakes about forests

"Forests are the lungs of the Earth." The phrase is catchy and misleading. A mature forest uses most of the oxygen its photosynthesis produces, through the respiration of trees, soil organisms, and decomposers. What forests do store is carbon, and what they provide is water regulation, habitat, and soil protection.

"Trees equal forest." A plantation is not equivalent to a natural forest, even if both appear green in satellite images.

"Forest fires are always bad." Many forests evolved with periodic fire. Problems arise when fire patterns change because of climate, fuel buildup, or human ignition, not when fire exists at all.

"Fungal networks prove trees cooperate." The networks are real and the fungi are essential. The strongest claims about tree-to-tree altruism are not settled.

"Cutting old trees and planting new ones is carbon neutral." The carbon in a century-old tree cannot be replaced by a seedling on a human timescale, and soil carbon may also be disturbed.

FAQ

Q: How much of the world is covered by forest? A: The FAO says forests cover about 31 percent of the world's land. Its most recent assessment, as summarized on Wikipedia, puts the area at roughly 4.14 billion hectares, with tropical forests accounting for 45% of it.

Q: Is all deforestation bad? A: Not categorically, but the type of forest matters. Converting primary forest is generally the most damaging for biodiversity, carbon, and water. Sustainable forestry and agroforestry can keep a good deal of forest function. Primary forest is disproportionately valuable because it cannot be rebuilt quickly.

Q: Can planted forests replace natural ones? A: Only partly. Plantations can produce timber, store carbon, and protect watersheds. They typically lack the structural complexity, species richness, and deadwood of natural forest, which develop over centuries.

Q: Why are old-growth forests so valuable? A: They have large old trees, multilayered canopies, standing dead trees, and fallen logs that specialist species depend on. Research such as Luyssaert and colleagues' 2008 study suggests they often keep accumulating carbon rather than stopping, though the details are debated.

Q: What is REDD+? A: REDD+ is a voluntary UN climate framework in which developing countries receive results-based payments for reducing deforestation and forest degradation. It is contested because of questions about additionality, leakage, and permanence, but it remains the main international financing mechanism for forest protection.

Q: Do trees really talk to each other through fungi? A: Fungal networks exist and carbon can move through them. Whether this amounts to deliberate communication or meaningful help for neighboring trees is contested, and critics argue that popular accounts have overstated the evidence.

The takeaway

A forest works because its parts depend on each other: canopy layers that sort light, fungi that crack open dead wood, roots that anchor soil, leaves that pump water into the sky. Remove one piece and the others change, which is why deforestation spreads outward from the clearing and why replanting does not simply reverse it. The most useful way to think about a forest is as infrastructure that happens to be alive. It is cheaper and far easier to keep it running than to rebuild it once it stops.

Sources

• FAO, forestry overview: https://www.fao.org/forestry/home/en/
• Our World in Data, "Forests and Deforestation": https://ourworldindata.org/forests-and-deforestation
• Wikipedia, "Forest" (summarizing FAO Global Forest Resources Assessment 2025): https://en.wikipedia.org/wiki/Forest
• Wikipedia, "Mycorrhizal network" (summarizing Simard et al. 1997; Karst et al. 2023; Henriksson et al. 2023): https://en.wikipedia.org/wiki/Mycorrhizal_network
• Wikipedia, "Old-growth forest" (summarizing Luyssaert et al. 2008): https://en.wikipedia.org/wiki/Old-growth_forest
• Wikipedia, "Deforestation of the Amazon rainforest" (INPE data): https://en.wikipedia.org/wiki/Deforestation_of_the_Amazon_rainforest
• Wikipedia, "REDD+": https://en.wikipedia.org/wiki/REDD%2B

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