Indicators of Biodiversity and Ecological Services

Soil Stability and Water Regulation

This indicator aims to monitor the extent of tree cover globally that is critical for soil stability and water regulation as a proxy for measuring changes in these ecological functions directly. The indicator considers three categories of forest that are important for soil stability and water regulation. These three categories are not mutually exclusive (i.e., the same forest area may be included in more than one category) and are therefore estimated individually but not summed. 

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How much tree cover exists within areas of medium to high erosion risk?

As of 2010, when the most recent high-resolution map of global tree cover was produced, 47 percent of the world’s tree cover was located in areas defined as being at medium to high risk of erosion.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. Tree cover stabilizes soil and thereby reduces erosion. Erosion negatively impacts water quality—adding to the silt load—and water quantity—reducing reservoir capacity; both impacts drive up the costs of water treatment and supply. 

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Erosion Risk Calculation

The erosion risk calculation is based on an adjusted version of the Revised Universal Soil Loss Equation, which takes average annual precipitation, elevation, slope steepness, soil properties, and land cover into account. Qin et al. 2016, https://www.wri.org/publication/gfw-water-metadata . Areas of high erosion risk are identified by the presence of steep slopes, erodible soil, intense rainfall, and forest conversion.Qin et al. 2016, https://www.wri.org/publication/gfw-water-metadata .

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Forested areas with high erosion risk are geographically dispersed

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In 2020, 11.0 million hectares (Mha) of tree cover loss (43 percent of total global tree cover loss) occurred in areas of medium to high erosion risk, a 4 percent increase from 2019.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. Australia, Brazil, and Indonesia accounted for 33 percent of global tree cover loss in such areas in 2020, and Liberia, Sierra Leone, and Laos lost the most tree cover as a proportion of their medium to high erosion risk areas.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. When vegetation, such as forests, is cleared from these areas, soil is even less stable and more prone to entering waterways, resulting in reduced reservoir capacity, poorer drinking water quality, degraded aquatic habitat, and higher risk of landslides. 

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Tree cover loss in areas of medium to high erosion risk

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How much tree cover loss has occurred within watersheds that supply water to the world’s megacities?

Since 2001, 43 Mha of tree cover have been lost in watersheds that supply the world’s megacities, equivalent to 8 percent of tree cover in these areas in 2000.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. The watersheds serving the cities of Buenos Aires, Guangzhou, Istanbul, Lagos, and Sao Paulo have lost 9 percent or more of their tree cover since 2000.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. The dominant drivers of tree cover loss in these watersheds include forestry (in the Guangzhou, Istanbul, and Sao Paulo watersheds), commodity-driven deforestation (in the Buenos Aires watershed), and shifting cultivation (in the Lagos watershed).See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>.   

In most of the watersheds where tree cover loss was related to forestry activities (Beaumont, Concepción, Monroe, and Port Arthur), significant tree cover gain was also detected between 2001 and 2012, indicating cyclical patterns of tree harvesting and regrowth. More recent data on tree cover gain is not currently available. 

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Tree cover loss in urban watersheds closely correlates to global tree cover loss trends

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The source watersheds for the world’s megacities supply 7 percent of the world’s population with fresh water.Based on the United Nations global population estimate of 7.6 billion, https://www.un.org/development/desa/publications/world-population-prospects-the-2017-revision.html . Forests within these watersheds serve as “natural infrastructure,” supporting urban water supply by controlling erosion, purifying water, and regulating its flow to reduce the impacts of floods and droughts. Natural infrastructure yields financial savings by reducing sedimentation, which drives up the costs of water treatment, removal of silt deposits in reservoirs, and maintenance of built water infrastructure.Ozment et al. 2018, https://www.wri.org/publication/natural-infrastructure-sao-paulo . One study estimated that watershed degradation impacted treatment costs for 29 percent of cities worldwide during the study period, increasing operation and maintenance costs in these cities by 53 percent on average.McDonald et al. 2016, https://doi.org/10.1073/pnas.1605354113 . 

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How much tree cover loss has occurred in mangrove forests?

Since 2016, 144 Mha of tree cover loss has occurred within mangrove forests.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. Just under 80 percent of this loss occurred in Brazil, Cuba, Indonesia, Malaysia, and the United States.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. In Southeast Asia, the primary drivers of mangrove forest loss are related to the conversion to aquaculture and agriculture.Richards and Friess 2016, https://doi.org/10.1073/pnas.1510272113 . In countries such as Cuba and the United States, large storms can quickly destroy large swathes of mangrove forests, which Hurricane Irma did in 2017. The increasing frequency and intensity of storms caused by climate change compromises forest regeneration.Imbert 2018, https://doi.org/10.1002/ecs2.2231 .  

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Mangrove loss in selected countries, 2016–20

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Mangrove forests in the Everglades of Florida experienced massive damage from Hurricane Irma in 2017<fn>Staletovich 2018, <a href="https://www.miamiherald.com/news/local/environment/article209148999.html">https://www.miamiherald.com/news/local/environment/article209148999.html</a>.</fn>

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As of 2016, coastal mangrove forests covered 13.3 Mha across 106 countries, representing a tiny fraction of global tree cover—just 0.3 percent.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. One-fifth of coastal mangroves are located in Indonesia, and another quarter are located in Australia, Brazil, Mexico, and Nigeria.See <a href="/gfr/data-and-methods#soil-water">Data and Methods</a>. Mangrove forests reduce annual flooding for more than 18 million people worldwide and are important breeding grounds for numerous coastal species.Losada et al. 2018, https://doi.org/10.7291/V9DV1H2S . Their removal, usually to make way for aquaculture or agriculture, leaves coastal populations more vulnerable to natural disasters, such as flooding from storms or even tsunamis, and negatively impacts marine life.  

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Mangrove map

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Limitations and Future Prospects

Watersheds unaffected by humans are uniquely calibrated systems that generally maintain soil and regulate and filter water under local conditions. As such, the impacts of tree cover loss and gain are inconsistent across watersheds, and the ecological and geographic features most important to maintaining soil stability and regulating water vary. 

Future updates to this indicator will include additional areas critical for soil stability and water regulation, such as montane cloud forests, areas of high riverine and coastal flooding risk, and wetlands. The availability of annual tree cover gain data will also improve this indicator because existing data cover only 2001–12.

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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."},"97":{"name":"plantation","description":"An area in which trees have been planted, generally for commercial purposes.\u0026nbsp;\r\n"},"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"}}}