Deforestation has important local, regional, and global consequences. Forests provide important ecosystem services including regulation of water cycles, stabilization of soils, moderation of local and regional climates, and habitat support for diverse plant and animal species. Forests also sequester carbon in the form of wood and other biomass as the trees grow, taking up carbon dioxide from the atmosphere (see carbon cycle). When forests are burned, their carbon is returned to the atmosphere as carbon dioxide, a greenhouse gas that has the potential to alter global climate (see greenhouse effect; global warming), and the trees are no longer present to sequester more carbon or provide other services.

In addition, most of the planet’s valuable biodiversity is within forests, particularly tropical ones. Moist tropical forests such as the Amazon have the greatest concentrations of animal and plant species of any terrestrial ecosystem; perhaps two-thirds of Earth’s species live only in these forests. As deforestation proceeds, it has the potential to cause the extinction of increasing numbers of these species.

On a more local scale, the effects of forest clearing, selective logging, and fires interact. Selective logging increases the flammability of the forest because it converts a closed, wetter forest into a more open, drier one. This leaves the forest vulnerable to the accidental movement of fires from cleared adjacent agricultural lands and to the killing effects of natural droughts. As wildfires, logging, and droughts continue, the forest can become progressively more open until all the trees are lost. Additionally, the burning of tropical forests is generally a seasonal phenomenon and can severely impact air quality. Record-breaking levels of air pollution have occurred in Southeast Asia as the result of burning for oil palm plantations.

In the tropics, much of the deforested land exists in the form of steep mountain hillsides. The combination of steep slopes, high rainfall, and the lack of tree roots to bind the soil can lead to disastrous landslides that destroy fields, homes, and human lives. With the significant exception of the forests destroyed for the oil palm industry, many of the humid forests that have been cleared are soon abandoned as croplands or only used for low-density grazing because the soils are extremely poor in nutrients. (To clear forests, the vegetation that contains most of the nutrients is often burned, and the nutrients literally “go up in smoke” or are washed away in the next rain.)

Although forests may regrow after being cleared and then abandoned, this is not always the case, especially if the remaining forests are highly fragmented. Habitat fragmentation divides large forest blocks into smaller disconnected pieces or patches, isolating populations of plant and animal species from each other. The effects of fragmentation on different forms of life vary according to species, and the effects depend on the distance between remaining pieces of habitat and the threats posed by the region that separates one patch of habitat and another. For example, habitat fragmentation may have little effect on birds and other animals that can move quickly between one habitat and the next, if the deforested area is small, such as a narrow forest cut or a power-line corridor. With increased distance between habitat patches, an animal is at a higher risk of encountering predators, being struck by a vehicle, staying too long in less-than-hospitable conditions while moving between patches, or becoming isolated from others. For immobile species, such as plants, or for animals where long distances separate habitat patches, habitat fragmentation can hinder reproduction by reducing the chances that males and females of reproductive age meet each other. Habitat fragmentation can make it difficult for organisms to reproduce without genetic bottlenecks, and the forest fragments may be too small to support large or territorial animals.

In some places deforested lands that are planted with commercially important trees lack biodiversity and do not serve as habitats for native plants and animals, many of which are endangered species.