It seems that Erfworlders can build campfires. This also implies the existence of Wood, and presumably Trees as the source of wood. It also implies some way to start a fire, either magic or flint that can be struck to create sparks and catch kindling on fire. One clear difficulty with a fire is that most of Erfworld is very digital. Things are on or off, existing or not existing. Fire in the Real World is a continuous process, converting wood to heat and ashes. Especially if only magic of some sort starts a fire, it can probably be in only certain sizes. Since I seem to recall pictures of buildings on fire, one of those sizes should be a large as a house. I wonder if dwagons start the same kind of fire as is used to start a campfire ?
The Royal Crown Coalition and Gobwin Knob have had temporary fires in the field/Courtyard. Jillian noted the lack of a fire on her trip with the Transylvitians. Food has been seen cooking on some of them.