Defining How Much of the World is Covered by Forests

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.

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​.

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.

Fonte : National Geographic