Forest Disturbances

Primary reference(s)

FAO, 2018. Global Forest Resources Assessment 2020. Terms and Definitions FRA 2020. Food and Agricultural Organisation of the United Nations (FAO), Forest Resources Assessment Working Paper No. 188. Accessed 24 October 2020.

FAO, 2020. Global Forest Resources Assessment 2020: Main report. Food and Agricultural Organisation of the United Nations (FAO). Accessed 25 October 2020.

Additional scientific description

Forest area is defined by the Food and Agricultural Organization of the United Nations (FAO) as: Land spanning more than 0.5 ha with trees higher than 5 m and a canopy cover of more than 10%, or trees able to reach these thresholds in situ. It does not include land that is predominantly under agricultural or urban land use (FAO, 2018).

Forest disturbances include areas of forest affected by: (i) insects: disturbance caused by insect pests; (ii) diseases: disturbance caused by diseases attributable to pathogens, such as bacteria, fungi, phytoplasma or viruses; (iii) severe weather events: disturbances caused by abiotic factors, such as snow, storm, droughts, etc; and (iv) fire: forest area affected by fire (FAO, 2018).

Forest degradation does not have a single commonly agreed definition, but forest degradation is considered to occur when forest ecosystems lose their capacity to provide important goods and services to people and nature (IUCN, 2017). In the context of the United Nations Framework Convention on Climate Change, forest degradation leads to a decline of carbon stocks (UNFCC, 2008).

The world’s total forest area is approximately 4.06 billion ha, which is 31% of the total land area. This area is equivalent to 0.52 ha per person. Forests are not distributed equally among the world’s people, or geographically. The tropical domain has 45% of the world’s forests. It should also be noted that more than half (54%) of the world’s forests are in only five countries (Russian Federation, Brazil, Canada, USA, China) (FAO, 2020).

Measuring forest degradation and disturbance is challenging. There is no commonly agreed definition or monitoring method. Degradation and disturbances can entail long-term negative changes in the structure, composition and other characteristics of the forest which can be difficult to detect.

Metrics and numeric limits

The Global Forest Resources Assessment 2020 noted that relatively few data were available for forest disturbance and that it was not possible to estimate accurately the total forest area affected by disturbances globally (FAO, 2020). Only 36 countries and territories, accounting for just 25% of the world’s forest area, reported complete time-series data for the forest area disturbed annually by insects (2000–2017). Therefore, further discussion is warranted on how best to obtain and analyse data and whether more information is needed to properly assess forest disturbance (FAO, 2020).

Examples of drivers, outcomes and risk management

Over 80% of the world’s terrestrial biodiversity can be found in forests. Forest disturbance can threaten the survival of many species and reduce the ability of forests to provide essential services such as clean air and water, healthy soils for agriculture, and climate regulation. Additionally, forests support the livelihoods of 1.6 billion people globally, one billion of whom are among the world’s poorest (IUCN, 2017).

Forests face many disturbances that can adversely affect their health and vitality and reduce their ability to provide a full range of goods and ecosystem services.

About 98 million ha of forest were affected by fire in 2015 (the latest available global data set) this was mainly in the tropical domain, where fire burned about 4% of the total forest area in that year. More than two-thirds of the total forest area affected was in Africa and South America (FAO, 2020). In Canada, since 1900, wildfires consume on average 2.5 million ha per year (NRC, no date). In Europe, in 2018, 58,016 fires consumed 350,180 ha compared to an average of 64,261 fires consuming 406,281 ha per year between 2008 and 2017 (JRC, 2018). The Global Forest Resources Assessment 2020 noted the extent of the different types of disturbance in the forests of the world in 2015 (FAO, 2020). Fifty-one countries representing 42% of the world’s forest area reported that an area of 6.6. million ha (0.4% of the forest area of the reporting countries) was affected by disease. Sixtytwo countries representing 52% of the world’s forest area, reported that an area of 30.2 million ha (1.4% of the forest area of the reporting countries) was affected by insects. Forty-eight countries representing 37% of the world’s forest area, reported that an area of 3.8 million ha (0.3% of the forest area of the reporting countries) was affected by severe weather events.

Air, water and soil pollution can also contribute to forest degradation. For example, acid rain, resulting from the interaction of sulphur dioxide and nitrogen oxides (mostly produced by the burning of fossil fuels, but can also be from volcanic eruptions) with water in the atmosphere, damages trees and forests. Acid rain leaches aluminium from the soil, and that aluminium can be toxic to plants, including trees. In addition, other nutrients are also removed from the soil by acid rain, limiting tree growth. At high elevations, acidic fog and clouds might strip nutrients from foliage, leaving trees with brown or dead leaves and needles. The trees are then less able to absorb sunlight, making them weak and less able to withstand freezing temperatures (US EPA, no date).

Many countries have developed policies and laws designed to promote the sustainable and multipurpose use of forests and trees. Such policies and laws serve as a foundation for sustainable forest management by, for example, promoting legal timber trade, involving stakeholders in forest management, addressing forest tenure and providing incentives (FAO, 2020).