At the end of 1990, Africa had an estimated 528 million hectares, or 30 percent of the world's tropical forests. In several Sub-Saharan African countries, the rate of deforestation exceeded the global annual average of 0.8 percent. While deforestation in other parts of the world is mainly caused by commercial logging or cattle ranching the leading causes in Africa are associated with human activity. Developing countries rely heavily on wood fuel, the major energy source for cooking and heating. In Africa, the statistics are striking: an estimated 90 percent of the entire continent's population uses fuelwood for cooking, and in Sub-Saharan Africa, firewood and brush supply approximately 52 percent of all energy sources. Land clearing by farmers may contribute as much as fuelwood gathering in the depletion of tree stocks. According to Porter and Brown, conversion of forests for subsistence and commercial agriculture may account for as much as 60 percent of world-wide deforestation. An estimated 20 to 25 percent of annual deforestation is thought to be due to commercial logging. The remaining 15 to 20 percent is attributed to other activities such as cattle ranching, cash crop plantations, and the construction of dams, roads, and mines. In Africa, governments invest substantially more in cash crops than in food crops as reflected in pricing and marketing policies. However, deforestation is primarily caused by the activities of the general population.