Is that a forest? Like mountains, there isn’t an exact definition of what forests are. Different organizations and countries use varying criteria to define a forest – often based on the percentage of land covered by trees (canopy cover), the total area of those trees, and the height of those trees. Those differences in defining a forest can result in large differences in mapping out forest cover.
What is a forest?
Canopy cover – the percentage of ground shaded by tree crowns – is the major criteria that varies among countries and agencies when calculating whether or not an area is a forest. The United Nations’ Food and Agriculture Organization (FAO) uses this definition: any land area over 0.5 hectares (about 1.2 acres) with trees taller than 5 meters and a canopy cover above 10% qualifies as “forest,” as long as it’s not chiefly under agricultural or urban use. Other agencies are more restrictive, only defining forests when the canopy cover is 30% or more.
This means that plant biomes such as savannah and oak woodlands that would be classified as a forest under the UN FAO’s definition, drop out of maps that use the higher density as a qualifier. As a result, two maps created from the same satellite imagery can produce noticeably different estimates of global forest cover simply by applying different forest definitions.
Comparison of forest definitions
How different organizations define a forest.
| Organization | Minimum canopy cover | Minimum tree height | Minimum area |
|---|---|---|---|
| United Nations Food and Agriculture Organization | 10% | 5 m | 0.5 ha |
| United Nations Climate Change (UNFCCC reporting) | 10–30% | 2–5 m | 0.05–1 ha |
| U.S. Fish and Wildlife Service | 10% | Not specified | 1 acre |
Examples of forest definitions
The European Forest Institute (EFI) has a table of forest definitions for each European country.
United Nations Framework Convention on Climate Change (UNFCCC)
Definition (excerpt from the Kyoto Protocol reporting guidelines): “Forest is a minimum area of land of 0.05–1.0 hectares with tree crown cover (or equivalent stocking level) of more than 10–30 percent, with trees having the potential to reach a minimum height of 2–5 meters at maturity.”
United States Fish and Wildlife Service (USFWS)
Definition: Forest land use is defined as a land area at least 1 acre in size, with at least 10 percent tree canopy cover, or can grow such canopy cover, and is not managed for other uses. Approximately 35 percent of land in the United States is forest land.
Using satellite imagery to map the extent of forests based on percentage canopy cover
Researchers found that global forest cover estimates can vary by as much as 6% to 13% of Earth’s land area – roughly the size of China – simply due to different forest definitions. In a study published in Nature Climate Change, scientists calculated global forest at 10% and 30% canopy coverage by analyzing Landsat satellite imagery. Under a 10% canopy cover rule, vast savanna regions with scattered trees are included as forests, whereas a 30% threshold would classify those same regions as non-forest or “other land”.
Mapping forest areas in the Uinta Mountain Range
An illustrative example of this was done by NASA researchers who mapped the extent of forests in the Uinta Mountain Range in Utah at 10% and 30% canopy cover. About half of the Uinta Mountain Range area is covered by temperate coniferous forests. Alpine tundra and meadows are found at the higher evaluations above the tree line.
A lack of a universal definition for a forest makes comparing forest data challenging
Such inconsistencies make it challenging to compare forest data across regions. As the World Resources Institute states, “there’s no international exchange rate” to convert forest figures from one definition to another. For example, in Indonesia, the official definition (for UNFCCC reporting) historically required a minimum area of 6.25 hectares, whereas Côte d’Ivoire’s definition used just 0.1 hectare.
Understanding how forests are defined is essential when comparing global forest statistics or evaluating changes in forest cover over time. Before comparing estimates from different organizations or countries, it’s worth checking exactly what each considers to be a forest.
Frequently asked questions
What is the difference between a forest and a woodland?
The distinction varies by country and organization. Some definitions distinguish forests from woodlands based on canopy cover or tree density, while others use the terms differently or interchangeably.
Why is there no universal definition of a forest?
Forests are defined for different purposes, such as conservation, timber management, land-use planning, and climate reporting. As a result, organizations and countries use different minimum thresholds for canopy cover, tree height, and land area.
What is canopy cover?
Canopy cover is the percentage of ground covered by the crowns of trees when viewed from above. It is one of the most common criteria used to determine whether an area qualifies as a forest.
Why do different maps show different amounts of forest?
Different forest maps often use different definitions of what counts as a forest. Changing the minimum canopy cover or tree height can significantly change the amount of land classified as forest.
Are savannas considered forests?
It depends on the definition being used. Under some definitions, savannas with scattered trees may qualify as forests if they meet minimum canopy cover and tree height requirements. Under stricter definitions, they are classified separately.
References
Sexton, J. O., Noojipady, P., Song, X. P., Feng, M., Song, D. X., Kim, D. H., … & Townshend, J. R. (2016). Conservation policy and the measurement of forests. Nature Climate Change, 6(2), 192-196.
