Forest
A forest covers more than half a hectare, holds trees taller than 5 meters, and shades at least a tenth of the ground beneath them. That is one definition, set by the United Nations' Food and Agriculture Organization. There are more than 800 others in use around the world. The disagreement is not trivial. Under some rules, a stretch of land with no trees at all still counts as a forest, if it once grew them, will grow them, or was simply declared one by law.
In 2025, by the FAO's measure, forests covered roughly 31 percent of the world's land. They are the largest terrestrial ecosystems on Earth by area, found around the globe. They also do quiet, planetary work. How can a word so ordinary resist a single meaning? Why does the same Amazon tree that releases water from its leaves also sit two or three commodities away from being cut down? And how did an Old French word for royal hunting ground come to mean a place thick with trees at all?
The word forest came into English not to describe trees, but to fence off land for the chase. It derives from the Old French forest, meaning a vast expanse covered by trees. When English first borrowed it, the word denoted wild land set aside for hunting, with no requirement that trees grow there.
Carolingian scribes used the Medieval Latin foresta first, in the capitularies of Charlemagne, to name the royal hunting grounds of the king. The Romance languages had their own native words for woodland, all descended from the Latin silva, which survives in the English sylva and sylvan, and in the Italian, Spanish, and Portuguese selva. The cognates that look like forest, such as the Italian foresta and the Spanish and Portuguese floresta, all trace back instead to the French.
The Norman rulers of England carried the word in as a legal term. It appears in Latin texts such as Magna Carta, marking uncultivated land set aside for hunting by feudal nobility. These hunting forests did not necessarily contain a single tree. Because they often did enclose woodland, forest slowly came to mean woodland in general, whatever the tree density. By the early fourteenth century, English used the word in all three senses at once: common, legal, and archaic. Older English words for dense trees, including holt, weald, wold, and wood, were never borrowed. They grew from Old English itself.
The first known forests rose in the Middle Devonian, roughly 390 million years ago, with cladoxylopsid plants like Calamophyton. The real transformation came later, in the Late Devonian, with Archaeopteris. It was both tree-like and fern-like, and it grew to 20 meters tall or more.
Archaeopteris spread quickly across the world, from the equator to subpolar latitudes. It is the first species known to cast shade with its fronds and to form soil from its roots. It was deciduous, dropping its fronds onto the forest floor below. The shade, the soil, and the forest duff from those fallen fronds built the early forest. That shed organic matter changed freshwater itself, slowing the flow of streams and supplying food, which in turn promoted freshwater fish.
A forest splits into two kinds of parts: biotic and abiotic, the living and the non-living. The living side runs from trees, shrubs, vines, and grasses to mosses, fungi, insects, mammals, birds, reptiles, amphibians, and the microorganisms in the soil, all wired together by mycorrhizal networks.
The forest floor is carpeted in dead plant material, fallen leaves and decomposing logs that detritivores break down into new soil. That decaying layer lets insects overwinter and gives amphibians and birds shelter and food. It keeps the soil moist, halts erosion, and shields roots from heat and cold. The fungal mycelium of the mycorrhizal network ferries nutrients from rotting matter to living trees.
The understory sits above it, made of bushes, shrubs, and young trees built to live in shade. Higher still, the canopy is the tangled mass of branches and leaves from mature trees, where the dominant crowns catch most of the sunlight and produce the most food. In tropical rainforests one more level appears: the emergent layer, a few scattered trees that tower over the canopy. Trees rising as high as 35 meters add a vertical dimension, opening niches for arboreal animals, epiphytes, and species that thrive in the regulated microclimate beneath the leaves.
Forests account for 75 percent of the gross primary production of Earth's biosphere and hold 80 percent of its plant biomass. The world's forests carry about 606 gigatonnes of living biomass and 59 gigatonnes of dead wood. The forest carbon stock reaches an estimated 714 gigatonnes; 46 percent of it sits in soil, 44 percent in living biomass, and 10 percent in litter and deadwood.
A full-grown tree produces about 100 kg of net oxygen a year. Forests purify water, soften floods, serve as genetic reserves, and supply lumber and recreation. Millions of people lean on them almost entirely, depending on forests for fuelwood, food, and fodder. Research from 2017 shows that forests even induce rainfall; cut the forest and you risk drought, and in the tropics, heat stress for outdoor workers.
Primary boreal forests store 1,042 billion tonnes of carbon, more than the atmosphere holds, and they contain 60 percent of the world's surface freshwater. Primary tropical forests hold about two-thirds of all terrestrial animal and plant species. Yet the balance can flip. A forest can turn from a carbon sink into a carbon source if its diversity, density, or area falls, as has been seen in some tropical forests. The typical tropical forest may become a carbon source by the 2060s.
Most deforestation today happens in tropical forests, and the vast majority of it traces to just four commodities: wood, beef, soy, and palm oil. Over the past 2,000 years, the forested area of Europe has fallen from 80 percent to 34 percent. Large tracts have been cleared in China and the eastern United States, where only 0.1 percent of land was left undisturbed.
The Amazon shows what is at stake. Its trees release water from their leaves in anticipation of seasonal rains, triggering the wet season early. Because of this, seasonal rainfall there begins two to three months sooner than the climate alone would allow. Deforestation and human-caused climate change could break that process, pushing the forest across a threshold where it turns into savanna. Where rainfall is intermediate, forest flips to savanna fast once tree cover drops below 40 to 45 percent.
The damage is no longer accelerating everywhere. An estimated 420 million hectares of forest have been lost to deforestation since 1990, but the rate has fallen sharply. Between 2015 and 2020 the annual rate of deforestation was about 10 million hectares, down from 12 million in the prior five years. In 1997, the World Resources Institute found only 20 percent of the world's original forests remained in large intact tracts. More than 75 percent of those lay in three countries: the boreal forests of Russia and Canada, and the rainforest of Brazil.
China banned logging beginning in 1998, after the erosion and flooding it caused. Tree-planting programs there and in India, the United States, and Vietnam, joined by natural expansion, have added more than 7 million hectares of new forest a year. The shift from forest loss to net gain has a name: the forest transition. It comes through commercial plantations, agroforestry by small farmers, or land left to regrow after farms are abandoned.
Indigenous lands hold 36 percent or more of the world's intact forest, host more biodiversity, and suffer less deforestation. In the U.S. state of Wisconsin, forests managed by Indigenous people show more plant diversity, fewer invasive species, and higher tree regeneration. In India, roughly 22 percent of the population belongs to forest-dependent communities that practice agroforestry as a core part of their livelihood.
More than half of the world's forests, 54 percent, sit in only five countries: Russia, Brazil, Canada, the United States, and China. South America has the highest proportion of forest of any region, at 49 percent of its land area. In 2025, forest inside legally protected areas reached an estimated 813 million hectares, about 20 percent of the total. The IPCC has warned that to hold warming below 1.5 degrees, global forest cover must grow by an area equal to all of Canada by 2050.