Demons live deep in the underworld. The underworld is a strange place: It is a cave system, composed mainly many extremely large caverns, with a few wide corridors connecting them. The ground of these caves is unusually soft and loamy, and has many relatively wide streams cutting through it. There are no trees, but instead the only plants are climbers, which stick to the high walls of the caverns, with the roots at the margins of the soil. There is also a clade of mosses, and several types of lichens. These plants feed on the radiation of the cavern walls. There are also many types of fungi and animals, including many domesticable animals with large bones, tusks, and other hard parts
While the demons have no shortage of stone, it is still expensive, and would not be suitable for vehicles, and various other things. There are also metals, which could be used in wagons and other such constructions, but are still generally quite dense, and it seems like they would generally be better used for weapons and other such things. Furthermore, neither stone nor metal are useful fuels
Out of all the materials that might be found in the underworld, what could replace wood, specifically in the realms of fuel, building construction, and things like vehicles and furniture?