Drylands, characterized by scarcity of water, globally support about two billion people. While most of these people live in developing nations, drylands in North America cover an extensive area and have a variety of uses. Drylands are experiencing noticeable stress and degradation from increasing populations and a changing climate, so it is important to know the current conditions and changes that have occurred over time for sustainable management and restoration of these areas.
This is the first global-level assessment of drylands. The global assessment is based on the interpretation of large-scale aerial photography by regional-based teams (imagery obtained through Google Earth Engine and Bing Maps). Dryland regions include North America, North Africa, the Middle East, and Southwest Asia. The U.S. Forest Service Forest Inventory and Analysis program coordinated the collection of data for the North America region with the Utah State University RS/GS Laboratory conducting the photo-interpretation.
Drylands are subdivided into aridity zones. These zones are based on ratio of mean annual precipitation and the mean annual potential evapotranspiration. The percentage of land covered by forest, grassland, crops, and rock, gravel and sand differ greatly by aridity zone (hyperarid, arid, semiarid, dry subhumid, with hyperarid the most dry and subhumid the least). Forest characteristics, such as canopy cover density, are not surprisingly dependent on aridity zone.
The Collect Earth unit of the Food and Agriculture Organization of the United Nations conducted the analysis of the data for all dryland regions and published the preliminary findings in July 2016. The full report, projected for publication in January 2017, will include additional analysis and interpretation by regional experts.
World’s drylands contain 1.11 billion hectares of forest. More than half are in the dry subhumid zone. At the other extreme, the hyperarid zone contains only 0.3 percent of the total forest area in drylands,
Croplands make up 25 percent of the dry subhumid zone but only two percent of the hyperarid zone.
FAO study provides the most detailed snapshot to date on trees, forests and land use in the world's drylands - Food and Agriculture Organization of the United Nations announcement of the preliminary findings.
Trees, forests and land use in drylands: The first global assessment. Preliminary findings. Food and Agriculture Organization of the United Nations. 2016