A forest is defined as an environment that is covered by trees at least five meters (16 feet) high over an area of at least 0.5 hectares (1.2 acres)—a bit smaller than the size of an American football field. Forests grow in cold, temperate, and tropical regions and cover about 30 percent of the land area around the globe. They provide resources for humans, including food, timber, energy, shelter, and medicine. The trees in forests help purify water by filtering pollutants from water in the soil before it reaches a waterway. In addition, trees store carbon from the atmosphere and provide supportive environments for plants and animals.

Because forest ecosystems are so valuable to the planet, scientists and policymakers are concerned about their maintenance and health. The Food and Agricultural Organization of the United Nations produces a report every five years on the state of forests around the globe. They use surveys from scientists on the ground and remote sensing of forests from space to calculate any losses or gains in forest coverage. They also use the data to monitor the overall health of forests.

Forests have long been under assault by loggers and people clearing land for agriculture. They have also been disappearing because of natural causes, such as climate change, forest fires, and the death of trees from disease or insects. Between 1990 and 2015, the area occupied by forests worldwide decreased by one percent, with most of the losses occurring in the tropics. Scientists predict that as human populations rise, deforestation to convert tropical forests to agricultural land will undoubtedly continue.

There have, however, been increases in the size of some forests, often because trees in those areas were replanted. Forests can also naturally replenish themselves if the land is nurtured and protected from any further timber harvesting.

Slowing the loss of forests, experts say, will require countries and communities to develop effective forest management plans. Such plans, they say, must strike a balance between environmental protection and the economic needs of human society.