By Using High-resolution Satellite Imagery to Survey Drylands, Researchers Boosted Estimates of Global Forest Cover by 9%

Two-fifths of the Earth’s surface is covered by drylands but the extent of forests in this biome was poorly understood until a group a researchers used high-resolution satellite imagery to remotely sense forest cover.

What are Drylands?

Drylands are areas defined by a scarcity of water.  Precipitation in these areas is offset by evaporation and evapotranspiration by plants.  Drylands are areas where the average annual rainfall is less than water loss through evaporation and transpiration.  The World Atlas of Desertification defines drylands as having a ratio of average annual precipitation (P) to potential evapotranspiration (PET) of less than 0.65.  While drylands are found on all continents but proportionately are highest in Africa (66% of the land) and Asia (40%).

Using Remote Sensing to Estimate Forest Cover in Dryland Biomes

The extent of forest cover in drylands biomes has been poorly researched until now.  Researchers analyzed high-resolution satellite imagery to determine the extent of forest cover in drylands areas. Using imagery from Google Earth and Collect Earth, a program developed by the Food and Agriculture Organization of the United Nations in Rome, the researchers identified and mapped out forest cover in drylands areas.  The study involved the remote sensing of 213,795 sample plots divided into 12 regions: Northern Africa; Horn of Africa; The Sahel (Africa); Southern Africa; North America; South America (East); South America (West); Central Asia; Southwest Asia; Europe and Russia; Middle East; and Oceania.  Those plots were assessed for forest cover by using “freely available and pre-processed remote sensing data, automatically “called”, gathered and geo-synchronized through Collect Earth to enable land assessment through “augmented visual interpretation (Bey et. al, 2016)””

Baobabs forests in Madagascar are an example of forests located in a drylands biome. Photo: Rod Waddington.

Baobabs forests in Madagascar are an example of forests located in a drylands biome. Photo: Rod Waddington.

Estimation of Forest Cover in Drylands

The team found that 1327 million hectares of drylands had more than 10% tree-cover, and 1079 million hectares comprised forest.  These new counts are 40 to 47% higher than previous estimates, revealing that drylands support a higher density of trees than previously thought.  This new estimate increases the global forest coverage estimates by 9%.

Map showing forest distribution in drylands. Yellow areas are plots without forests and green areas are plots with forest cover. Source: Bastin et. al, 2017).

Map showing forest distribution in drylands. Yellow areas are plots without forests and green areas are plots with forest cover. Source: Bastin et. al, 2017).

The study:

Bastin, J. F., Berrahmouni, N., Grainger, A., Maniatis, D., Mollicone, D., Moore, R., … & Aloui, K. (2017). The extent of forest in dryland biomesScience356(6338), 635-638.

Additional References

A. Bey, A. Sánchez-Paus Díaz, D. Maniatis, G. Marchi, D. Mollicone, S. Ricci, J.-F. Bastin, R. Moore, S. Federici, M. Rezende, C. Patriarca, R. Turia, G. Gamoga, H. Abe, E. Kaidong, G. Miceli, Collect Earth: Land use and land cover assessment through augmented visual interpretation. Remote Sens. 8, 807 (2016). doi:10.3390/rs8100807

Middleton, N. J., & Thomas, D. S. G. (1992). World Atlas of Desertification: United Nations Environmental Programme.

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