Data & Methods

Key Terms and Definitions

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Talking about Tree Cover

The Global Forest Review (GFR) frequently refers to tree cover when talking about forest extent, loss and gain. Tree cover is a convenient metric for monitoring forest change because it is easily measurable from space using freely available, medium-resolution satellite imagery. This means that tree cover can be monitored frequently, at low cost, and over large geographic scales. However, the existence of tree cover does not always make a forest, tree cover loss does not always imply forest loss or deforestation, and tree cover gain does not always imply forest gain or restoration. Measuring these variables directly poses technical challenges, since most definitions of forest involve a combination of tree cover and land use. The latter is much more difficult, if not impossible in some cases, to monitor using satellite imagery. 

The tree cover data used in this report is based on Hansen et al. (2013)Hansen et al. 2013, https://doi.org/10.1126/science.1244693 and Potapov et al. (2022)Potapov et al. 2022, https://doi.org/10.3389/frsen.2022.856903 and defines tree cover as woody vegetation with a height of at least 5 meters (m) and a canopy density of at least 30 percent at 30-m resolution. This definition includes industrial tree plantations and commercial tree crops (such as oil palm or apple orchards) as well as managed and unmanaged natural forests. The definition excludes areas of sparse tree cover (less than 30% canopy density), such as in the North African Sahel or the Brazilian Cerrado, as well as individual trees in agricultural landscapes that may be critical for ecosystem functions.  

Tree Cover Loss and Deforestation

Tree cover loss is not always deforestation, which typically refers to human-caused, permanent removal of natural forest cover. Tree cover loss is defined as the complete removal of tree cover for any reason. It includes both human-caused loss and natural disturbances, and loss that is permanent or temporary. Examples of tree cover loss that may not meet the definition of deforestation include loss from logging within production forests, fire, disease or storm damage. 

This report uses tree cover loss dataHansen et al. 2013, https://doi.org/10.1126/science.1244693 to measure the complete removal of tree cover at a 30-m pixel scale because it can be measured consistently across the globe, but it also attempts to estimate forest loss (within primary forests) and deforestation where data allow. 

Tree Cover Gain and Restoration

Tree cover gain is defined in this report as areas that had tree cover at least five meters tall in 2020 but not in 2000 at a 30-m pixel scale. Tree cover gain is more difficult to detect from satellite imagery over short intervals due to the gradual process of tree growth compared to tree removal. As such, the tree cover gain dataPotapov et al. 2022, https://doi.org/10.3389/frsen.2022.856903 used in this report are only available as a cumulative estimate from 2000 to 2020. Annual data and updates for this data set will eventually be available.  

Tree cover gain does not perfectly represent “forest landscape restoration,” a key goal of international commitments such as the Bonn Challenge and the New York Declaration on Forests, for several reasons: 

  • The data aggregate many different causes of tree cover gain, including dynamics that may not always count towards restoration commitments (e.g., regrowth in tree plantations, regeneration after natural disturbances, etc.). In the future, once annual gain data are available, we anticipate that some of these dynamics could be distinguished by determining areas that have faced cycles of loss and gain. Ideally, a method similar to the Curtis et al. (2018)Curtis et al. 2018, https://doi.org/10.1126/science model for determining drivers of tree cover loss could be developed to distinguish the various drivers and dynamics of tree cover gain.  

  • The tree cover gain data do not cover other forms of restoration, such as enhancement of forest quality or planting of sparse trees in agricultural landscapes. Forthcoming annual height data has the potential to be used to determine areas that are becoming taller and denser over time, as a proxy for improving forest quality. Detecting sparse trees, on the other hand, likely requires higher-resolution imagery than that used in this dataset. The new Trees in Mosaic Landscapes data, which is an AI-derived map based on 10-m resolution Sentinel imagery, is more adept at picking up sparse tree cover outside of forests. Additionally, other tools such as Collect Earth allow local experts to interpret freely available high-resolution images within sample plots to detect individual trees outside forests and changes over time (see the Trees outside Forests Indicator).  

  • Most restoration initiatives have objectives beyond increasing forest cover that are not accounted for in current tree cover gain data, including carbon removal from the atmosphere, increased biodiversity habitat and connectivity, water filtration and flow regulation, improved soil fertility, and income generation from fuelwood sales or agroforestry activities. Although there is guidance on how to monitor these objectives at the landscape scale,Buckingham et al. 2019, https://www.wri.org/publication/restoration-monitoring-guide doing so at a global level remains difficult. 

Glossary

Definitions for basic forest-related concepts and terms are often varied and controversial. For consistency and clarity, the Global Forest Review (GFR) uses the following definitions across the report: 

agricultural tree crop: Trees cultivated for their food, cultural, or economic values. These include oil palm, rubber, cocoa, cashew, mango, oranges (citrus), plantain, banana, and coconut. 

biodiversity intactness: The proportion and abundance of a location's original forest community (number of species and individuals) that remain.Hill et al. 2019, https://doi.org/10.3389/ffgc.2019.00070

biodiversity significance: The importance of an area for the persistence of forest-dependent species based on range rarity.Hill et al. 2019, https://doi.org/10.3389/ffgc.2019.00070

carbon dioxide equivalent: Carbon dioxide equivalent (CO2e) is a measure used to aggregate emissions from various greenhouse gases (GHGs) on the basis of their 100-year global warming potentials by equating non-CO2 GHGs to the equivalent amount of CO2. 

deforestation: The change from forest to another land cover or land use, such as forest to plantation or forest to urban area.   

forest: Tree cover greater than 30 percent tree canopy density and greater than 5 meters in height as mapped at a 30-meter Landsat pixel scale.  

forest concession: A legal agreement allowing an entity the right to manage a public forest for production purposes. 

forest degradation: The reduction in a forest’s ability to perform ecosystem services, such as carbon storage and water regulation, due to natural and anthropogenic changes.  

forest disturbance: A discrete event that changes the structure of a forest ecosystem. 

forest fragmentation: The breaking of large, contiguous forests into smaller pieces, with other land cover types interspersed.  

forest management plan: A plan that documents the stewardship and use of forests and other wooded land to meet environmental, economic, social, and cultural objectives. Such plans are typically implemented by companies in forest concessions.  

hectare: One hectare equals 100 square meters, 2.47 acres, or 0.01 square kilometers and is about the size of a rugby field. A football pitch is slightly smaller than a hectare (pitches are between 0.62 and 0.82 hectares).  

intact forest: A forest that contains no signs of human activity or habitat fragmentation as determined by remote sensing images and is large enough to maintain all native biological biodiversity.   

land and environmental defenders: People who peacefully promote and protect rights related to land and/or the environment. 

loss driver: The direct cause of forest disturbance. 

low tree canopy density: Less than 30 percent tree canopy density. 

managed forest concession: Areas where governments have given rights to private companies to harvest timber and other wood products from natural forests on public lands. 

managed natural forest: Naturally regenerated forests with signs of management, including logging, clear cuts, etc.Lesiv et al. 2022, https://doi.org/10.1038/s41597-022-01332-3

megacity: A city with more than 10 million people. 

persistent loss and gain: Forests that have experienced one loss or one gain event from 2001 to 2016.  

plantations: An area in which trees have been planted, generally for commercial purposes.  

planted forest: A forest composed of trees that have been deliberately planted and/or seeded by humans.  

primary forest: Old-growth forests that are typically high in carbon stock and rich in biodiversity. The GFR uses a humid tropical primary rainforest data set, representing forests in the humid tropics that have not been cleared in recent years.   

production forest: A forest where the primary management objective is to produce timber, pulp, fuelwood, and/or nonwood forest products. 

restoration: Interventions that aim to improve ecological functionality and enhance human well-being in degraded landscapes. Landscapes may be forested or non-forested. 

shifting agriculture: Temporary loss or permanent deforestation due to small- and medium-scale agriculture.  

tree cover: All vegetation greater than five meters in height and may take the form of natural forests or plantations across a range of canopy densities. Unless otherwise specified, the GFR uses greater than 30 percent tree canopy density for calculations.  

tree cover gain: The establishment of tree canopy in an area that previously had no tree cover. Tree cover gain may indicate a number of potential activities, including natural forest growth or the crop rotation cycle of tree plantations. As such, tree cover gain does not equate to restoration. 

tree cover loss: The removal or mortality of tree cover, which can be due to a variety of factors, including mechanical harvesting, fire, disease, or storm damage. As such, loss does not equate to deforestation.   

trees outside forests: Trees found in urban areas, alongside roads, or within agricultural land are often referred to as Trees Outside Forests (TOF). 

tree plantation: An agricultural plantation of fast-growing tree species on short rotations for the production of timber, pulp, or fruit. 

unmanaged natural forest: Naturally regenerated forests without any signs of management, including primary forest.Lesiv et al. 2022, https://doi.org/10.1038/s41597-022-01332-3

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