The global Forest Resource Assessment (FRA), which is provided regularly by the United Nations Food and Agricultural Organization (UN FAO), uses three major terms related to tree cover. Forest is defined as a land class with tree cover more than 10%, and the trees should be able to reach a minimum height of 5 m. Other wooded land (OWL) has either a tree canopy cover of 5-10%, or is presented by a combined cover of shrubs, bushes, and trees above 10%. Other land with tree cover (OLTC) should meet the above criteria for forest but is related to land classified as 'other land' (e.g., groups of trees on agricultural land, parks, gardens, etc.). All three definitions require a minimum area of 0.5 ha. The last FRA (2005) estimated the world's forest area as 3952 x 106ha of which the Tropics (~46%) and the boreal domain (~29%) are major forest biomes. About 30% of land surface is covered by forest. In addition, 1376 x 106ha was classified as OWL. Global data on OLTC are incomplete - the area of this land-cover category was estimated at 76 x 106ha in 61 countries out of 229. These estimates are mostly based on national forest statistics and individual country's reports. Another information source on the world's forests is observation of the Earth from satellites. Four different global remote sensing (RS) land-cover products, which have been reported during the two last decades, indicated the area of the world's forests on average as ^20% smaller than the FAO estimate. Reasons for this inconsistency stem from coarse resolution of the imagery used (1 km), not completely compatible definitions of forest, differences in classification, fragmentation of forests in many regions, long-period cloudiness in some (particularly tropical) regions, and lack of satisfactory ground truth data for proper validation of RS imagery. On the other side, national forest inventories are neither complete nor reliable for many developing countries.
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