Eucalypts have many uses which have made them economically important trees, and they have become a cash crop in poor areas such as Timbuktu, Mali and the Peruvian Andes, despite concerns that the trees are invasive in some environments like those of South Africa. Best-known are perhaps the varieties karri and yellow box. Due to their fast growth, the foremost benefit of these trees is their wood. They can be chopped off at the root and grow back again. They provide many desirable characteristics for use as ornament, timber, firewood and pulpwood. Eucaplytus wood is also used in a number of industries, from fence posts (where the oil-rich wood's high resistance to decay is valued) and charcoal to cellulose extraction for biofuels. Fast growth also makes eucalypts suitable as windbreaks and to reduce erosion.