The forested area of 23.9 million hectares (58.9 million acres) occupies about 51% of the land area. Forestry is mostly conducted in the Littoral, Center, South, and South West provinces. Of the 300 commercially valuable species, the principal types of trees felled are assié, azobe, dussil, eloorba, mahogany, sapele, sipo, illomba, ayus, iroko, dibetu, and silk cotton. Timber exports in 2000 included about 575,000 cu m (20.3 million cu ft) of logs (valued at $97 million) and 540,000 cu m (19 million cu ft) of sawn timber ($222 million). In 2000, roundwood production was estimated at 12.1 million cu m (427 million cu ft). Wood sales make up the fourth-largest source of foreign revenue, but infrastructural problems and weak demand for lower-quality wood limits the development of the forestry sector.