Would it be too much of a stretch to allow druids, rangers or oath of the ancients paladins to use tree stride to move between giant mushrooms in the underdark? Mushrooms are the trees of the underdark providing the equivalent of lumber and they are natural.
My campaign is in the underdark at the moment. The lack of trees means that wooden objects are seen as a luxury and command premium prices down there. It's something we surface dwellers take for granted.