Indicators of Forest Extent

Trees outside Forests

The Trees outside Forests Indicator aims to monitor trees that are growing outside forests on farms, in orchards and tree plantations, in cities, along roads, and across other nonforest landscapes. Such trees provide critical products, including fertilizer, fruit, or firewood for local people. They are also important for ecosystem services such as erosion control and water retention, which make farms more productive, and they provide clean air, offset heat islands, reduce energy bills, and provide habitat for wildlife in cities. Spatial data on trees outside forests are not consistently available, so this indicator currently relies on other (nonspatially explicit) estimates as well as case studies from countries where spatial data are available. 

How much nonforest land has trees?

The Food and Agriculture Organization of the United Nations estimates that the world has at least 280 million hectares (Mha) of land that is not classified as forest but has a tree canopy cover of at least 10 percent.FAO 2020, https://doi.org/10.4060/ca8642en . This global estimate is based on data reported by national governments, although fewer than half of all countries reported data in this category during the most recent 2020 assessment. This estimate does not include a corresponding map showing where these lands are located.

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Why Do Trees outside Forests Matter?

Outside the world’s forests, trees thrive across pasturelands, along roads, and in urban areas. They also play a major role in agricultural landscapes, where they mark boundaries between farms or are directly planted with crops in a system called agroforestry. The special report on the impacts of global warming of 1.5°C above preindustrial levelsIPCC 2018, https://www.ipcc.ch/site/assets/uploads/sites/2/2019/05/SR15_SPM_version_report_LR.pdf . by the Intergovernmental Panel on Climate Change (IPCC) highlighted that all scenarios to reach this goal would involve significant carbon removal from the land sector (agriculture, forestry, and other land use, or AFOLU). The IPCC estimates a 9.5 million square kilometer increase in forested area by 2050 relative to 2010 is needed to limit global warming to 1.5°C. Trees outside forests will play a large role because there is limited land available and increasing forest area will increase the pressure on land to fulfill the various demands for human settlements, food, livestock feed, fiber, bioenergy, carbon storage, biodiversity, and other ecosystem services. However, trees in the agricultural landscape and agroforestry are no-regrets strategies with large benefits for the climate, people, and the planet. 

Even though trees outside forests benefit rural people directly, they have largely remained at the side lines of conversations about forest conservation and restoration—and climate change and productivity.  Trees outside forests contribute substantially to national biomass stocks.Schnell et al. 2015a, https://doi.org/10.1007/s10661-014-4197-4 . In some developing countries, as much as two-thirds of the fuelwood consumption is gathered from trees outside forests.Smeets and Faaij 2007, https://doi.org/10.1007/s10584-006-9163-x . In countries with low forest cover, trees outside forests may be the only wood resources available to local people and can contribute up to 20 percent of rural household income.Miller et al. 2016, https://www.profor.info/sites/profor.info/files/WPS7802_0.pdf . Trees outside forests can form a large part of a country’s biomass.Schnell et al. 2015b, https://doi.org/10.1007/s10661-015-4817-7 . 

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What is the density of trees on nonforest lands?

A global-scale map of trees on nonforest lands does not currently exist, making it impossible to monitor the density of trees outside forests. However, several studies suggest that tree density can be significant. Tree density in montane grassland, for example, can be a staggering 700 trees per hectare, which actually exceeds the average tree density of 300 trees per hectare in dry tropical forests.Crowther et al. 2015, https://www.ncbi.nlm.nih.gov/pubmed/26331545 .  

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Limitations and Future Prospects

Data on trees outside forests remain extremely limited. Most official government assessments of national forest resources do not inventory trees outside forests because these trees do not fall under the official definitions of forest typically used in policymaking.FAO 2018, http://www.fao.org/3/i8661en/i8661en.pdf . Furthermore, trees outside forests are difficult to detect using the medium-resolution satellite data (30-meter [m] resolution) on which current global-scale forest monitoring systems rely.

However, several advances in the fields of remote sensing and satellite imagery analysis may make it possible to monitor trees outside the forests as routinely as the world’s forests in the near future. New artificial intelligence methods developed in other scientific fields have now been successfully deployed in a research setting to detect individual trees on high- and very-high-resolution satellite data,Brandt and Stolle 2020, https://arxiv.org/abs/2005.08702 . which is becoming increasingly available on large scales. For example, the National Aeronautics and Space Administration recently published a study that deploys artificial intelligence on very-high-resolution images to automatically detect every tree in Africa’s Sahel.Brandt et al. 2020, https://www.nature.com/articles/s41586-020-2824-5  In addition, several countries have now demonstrated the use of such tools to map trees outside forests on a national scale (see the box below).  

Much of the innovation in this field has been driven by the global restoration movement, which focuses on restoring trees across all kinds of forest and nonforest landscapes. To this end, two new global monitoring initiatives began in 2019. The United Nations Decade of Ecosystem Restoration has formed a working group on monitoring trees outside forests,See the United Nations Decade on Ecosystem Restoration, https://www.decadeonrestoration.org/ . and the Global Restoration Observatory has established a special team for the same purpose. These initiatives will further develop techniques and data to better monitor the size, distribution, and dynamic nature of trees outside forests. 
 

Although global maps of trees outside forests do not yet exist, a small number of countries have begun utilizing new tools such as Collect Earth,Lyons et al. 2017, https://www.wri.org/blog/2017/07/scientists-use-google-earth-and-crowdsourcing-map-uncharted-forests . which makes it easy to systematically interpret high-resolution satellite data (10 m resolution or better), to measure the extent of trees outside forests at the national or landscape level. These national studies provide insight into the importance of trees outside forests in a country and demonstrate that trees can occur in every land use and not exclusively in forests. The tree density varies with land use and/or location. The Rwanda case study shows the density and variation of tree density in different areas and types of land use, which are governed by many factors, including climate, topography, local preferences, firewood needs, and so on. Trees can be abundant on nonforest land, ranging from 0 to 26 trees per hectare on grassland and from 0 to 14 trees per hectare on cropland. Though these tree densities are much lower than in forests (41–89 trees per hectare), these nonforest land-use types are often more expansive than forests and can therefore account for a significant share of the total number of tree biomass in a region.Schnell et al. 2015, https://doi.org/10.1007/s10661-014-4197-4 .  This study clearly shows the presence and spatial variation of trees in different land uses and administrative units and the overall contribution to the tree cover of an administrative unit.

In Rwanda’s Gatsibo district, almost as many trees can be found in cropland, grassland, and wetland as in areas officially designated as “forest”

In Rwanda’s Gatsibo district, almost as many trees can be found in cropland, grassland, and wetland as in areas officially designated as “forest”

Tree densities in Rwanda’s croplands and grasslands are lower than in forests, but these land-use types are far more expansive than forests

Tree densities in Rwanda’s croplands and grasslands are lower than in forests, but these land-use types are far more expansive than forests

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A study in the Mekong region showed the time dynamic for trees outside forests. The study looked at changes in tree density for trees outside forests between 2010 and 2018.New York Declaration on Forests Assessment Partners 2019, https://www.climatefocus.com/sites/default/files/2019NYDFReport.pdf .  All countries in this region showed large areas with an increase in trees outside forests; however, this gain was sometimes negated by losses in other areas. However, the net increase of trees outside forests in the region was significant (4.7 Mha). 

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Tree cover on nonforest land in Mekong countries has changed rapidly but experienced a net gain between 2010 and 2018<fn>New York Declaration on Forests 2019, <a href="https://forestdeclaration.org/images/uploads/resource/2019NYDFGoal5.pdf">https://forestdeclaration.org/images/uploads/resource/2019NYDFGoal5.pdf</a>.</fn>

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{"Glossary":{"51":{"name":"agricultural tree crops","description":"Trees cultivated for their food, cultural, or economic values. These include oil palm, rubber, cocoa, cashew, mango, oranges (citrus), plantain, banana, and coconut.\r\n"},"94":{"name":"biodiversity intactness","description":"The proportion and abundance of a location\u0027s original forest community (number of species and individuals) that remain.\u0026nbsp;\r\n"},"95":{"name":"biodiversity significance","description":"The importance of an area for the persistence of forest-dependent species based on range rarity.\r\n"},"1":{"name":"deforestation","description":"The change from forest to another land cover or land use, such as forest to plantation or forest to urban area.\r\n"},"77":{"name":"deforested","description":"The change from forest to another land cover or land use, such as forest to plantation or forest to urban area.\r\n"},"76":{"name":"degradation","description":"The reduction in a forest\u2019s ability to perform ecosystem services, such as carbon storage and water regulation, due to natural and anthropogenic changes.\r\n"},"75":{"name":"degraded","description":"The reduction in a forest\u2019s ability to perform ecosystem services, such as carbon storage and water regulation, due to natural and anthropogenic changes.\r\n"},"79":{"name":"disturbances","description":"A discrete event that changes the structure of a forest ecosystem.\r\n"},"68":{"name":"disturbed","description":"A discrete event that changes the structure of a forest ecosystem.\r\n"},"65":{"name":"driver of tree cover loss","description":"The direct cause of forest disturbance.\r\n"},"70":{"name":"drivers of loss","description":"The direct cause of forest disturbance.\r\n"},"81":{"name":"drivers of tree cover loss","description":"The direct cause of forest disturbance.\r\n"},"2":{"name":"forest","description":"Forests include 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.\r\n"},"3":{"name":"forest concession","description":"A legal agreement allowing an entity the right to manage a public forest for production purposes.\r\n"},"90":{"name":"forest concessions","description":"A legal agreement allowing an entity the right to manage a public forest for production purposes.\r\n"},"53":{"name":"forest degradation","description":"The reduction in a forest\u2019s ability to perform ecosystem services, such as carbon storage and water regulation, due to natural and anthropogenic changes.\r\n"},"54":{"name":"forest disturbance","description":"A discrete event that changes the structure of a forest ecosystem.\r\n"},"5":{"name":"forest fragmentation","description":"The breaking of large, contiguous forests into smaller pieces, with other land cover types interspersed.\r\n"},"6":{"name":"forest management plan","description":"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.\r\n"},"62":{"name":"forests","description":"Forests include 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.\r\n"},"69":{"name":"fragmentation","description":"The breaking of large, contiguous forests into smaller pieces, with other land cover types interspersed.\r\n"},"80":{"name":"fragmented","description":"The breaking of large, contiguous forests into smaller pieces, with other land cover types interspersed.\r\n"},"74":{"name":"gain","description":"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.\r\n"},"7":{"name":"hectare","description":"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).\r\n"},"66":{"name":"hectares","description":"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).\r\n"},"67":{"name":"intact","description":"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.\r\n"},"78":{"name":"intact forest","description":"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.\r\n"},"8":{"name":"intact forests","description":"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.\r\n"},"55":{"name":"land and environmental defenders","description":"People who peacefully promote and protect rights related to land and\/or the environment.\r\n"},"9":{"name":"loss driver","description":"The direct cause of forest disturbance.\r\n"},"10":{"name":"low tree canopy density","description":"Less than 30 percent tree canopy density.\r\n"},"84":{"name":"managed forest concession","description":"Areas where governments have given rights to private companies to harvest timber and other wood products from natural forests on public lands.\r\n"},"83":{"name":"managed forest concession maps for nine countries","description":"Cameroon, Canada, Central African Republic, Democratic Republic of the Congo, Equatorial Guinea, Gabon, Indonesia, Liberia, and the Republic of the Congo\r\n"},"91":{"name":"megacities","description":"A city with more than 10 million people.\r\n"},"57":{"name":"megacity","description":"A city with more than 10 million people."},"56":{"name":"mosaic restoration","description":"Restoration that integrates trees into mixed-use landscapes, such as agricultural lands and settlements, where trees can support people through improved water quality, increased soil fertility, and other ecosystem services. This type of restoration is more likely in deforested or degraded forest landscapes with moderate population density (10\u2013100 people per square kilometer). "},"86":{"name":"natural","description":"A forest that is grown without human intervention.\r\n"},"12":{"name":"natural forest","description":"A forest that is grown without human intervention.\r\n"},"63":{"name":"natural forests","description":"A forest that is grown without human intervention.\r\n"},"82":{"name":"persistent gain","description":"Forests that have experienced one gain event from 2001 to 2016.\r\n"},"13":{"name":"persistent loss and gain","description":"Forests that have experienced one loss or one gain event from 2001 to 2016."},"93":{"name":"plantations","description":"An area in which trees have been planted, generally for commercial purposes.\u0026nbsp;\r\n"},"88":{"name":"planted","description":"A forest composed of trees that have been deliberately planted and\/or seeded by humans.\r\n"},"14":{"name":"planted forest","description":"A forest composed of trees that have been deliberately planted and\/or seeded by humans.\r\n"},"73":{"name":"planted forests","description":"A forest composed of trees that have been deliberately planted and\/or seeded by humans.\r\n"},"15":{"name":"primary forest","description":"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.\r\n"},"64":{"name":"primary forests","description":"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.\r\n"},"58":{"name":"production forest","description":"A forest where the primary management objective is to produce timber, pulp, fuelwood, and\/or nonwood forest products."},"89":{"name":"production forests","description":"A forest where the primary management objective is to produce timber, pulp, fuelwood, and\/or nonwood forest products.\r\n"},"87":{"name":"seminatural","description":"A managed forest modified by humans, which can have a different species composition from surrounding natural forests.\r\n"},"59":{"name":"seminatural forests","description":"A managed forest modified by humans, which can have a different species composition from surrounding natural forests. "},"96":{"name":"shifting agriculture","description":"Temporary loss or permanent deforestation due to small- and medium-scale agriculture.\r\n"},"17":{"name":"tree cover","description":"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.\r\n"},"71":{"name":"tree cover canopy density is low","description":"Less than 30 percent tree canopy density.\r\n"},"60":{"name":"tree cover gain","description":"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."},"18":{"name":"tree cover loss","description":"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. "},"19":{"name":"tree plantation","description":"An agricultural plantation of fast-growing tree species on short rotations for the production of timber, pulp, or fruit.\r\n"},"72":{"name":"tree plantations","description":"An agricultural plantation of fast-growing tree species on short rotations for the production of timber, pulp, or fruit.\r\n"},"85":{"name":"trees outside forests","description":"Trees found in urban areas, alongside roads, or within agricultural land\u0026nbsp;are often referred to as Trees Outside Forests (TOF).\u202f\r\n"}}}