They have harvested wood for timber and charcoal. Wooden ceilings and timber beams were decorated. Where possible, an area of rotten wood surrounded by sound timber is used. Tropical plywood is made of mixed species of tropical timber. Most of the loggable timber is pine. Forest plantations are generally intended for the production of timber and pulpwood. The forests were heavily exploited over the centuries for firewood and timber. They provide many desirable characteristics for use as ornament, timber, firewood and pulpwood. There is a continuing threat to these forests from exploitation for timber, fuelwood and charcoal. Piles are made of a variety of material including steel, timber, and concrete. The hull is constructed of welded steel oversheathed with timber. Centering was made from timber and bamboo. Amusement parks often feature multiple roller coasters of primarily timber or steel construction. Germany would deliver factory equipment and machinery in return for Polish timber and agricultural produce. These diseases can be a serious problem on trees grown for timber or ornament.