Forests remove carbon dioxide from the air, conserve soil and water, and are home to a variety of species. (46) They are also repositories of potentially valuable new products, such as pharmaceuticals, and as a source of building material and firewood they provide employment for millions worldwide. In 1990 forests took up about a quarter of the planet"s land surface (not including an additional 13 percent of other woody vegetation, such as sparsely covered woodland and brush land). Russia accounts for perhaps a fifth of the globe"s forest. Brazil for about a seventh, and Canada and the U.S. each for 6 to 7 percent. (47) Historically, virtually all countries have experienced deforestation, mostly because of the need for new farmland, pasture, fuelwood and timber. In the U.S., forests now cover 22 percent of the land area, a decline of perhaps 40 percent since European colonization began. (48) Forest acreage, however, has remained about the same since 1920 as rising agricultural productivity moderated the need for new cropland. Among the most pressing concerns today in the U.S. are declining biodiversity of forests and stagnant or declining productivity of commercial timberland. In Europe, west of the former U.S.S.R., forests cover about 30 percent of the land, roughly half its original extent. A major problem there, particularly in Eastern Europe, is defoliation, apparently caused mostly by air pollution. Forests in the former U.S.S.R. once blanketed about half the land but now cover about a third. (49) Forest degradation is most serious there, not only because of air pollution but also because of a lack of effective conservation policies, such as replanting. Among other temperate regions, North Africa and the Middle East in 1990 had less than 2 percent forest cover, a decline since 1980. In contrast, China, through a massive tree-planting program, recently increased forest area, which now takes up 14 percent of its land. (50) The biggest changes have been in the tropics, where the natural forest dropped by a fifth from 1960 to 1990 as a result of population pressure, large-scale government development projects and commercial logging. The greatest decline was in tropical Asia, which lost a third of its forest. Almost all tropical countries lost ground in the 1980s except India, whose forest expanded by 5 percent. Brazil, which accounts for almost a third of the global tropical cover, suffered a 5 percent decline in the 1980s. There was a loss of 137 million hectares (338 million acres) of tropical forest worldwide, equal to the total land area of Spain, France and Germany. Agricultural expansion accounted for somewhat less than half the tropical contraction.