Stumpery - (also called rootery or rootree) begins with tree roots placed upside down in the landscape. The roots being the most dense part of the tree, these specimens will take longer to rot. Originally created in Victorian England as a structure for growing their prized ferns, stumperies have now been replicated in many parts of the world.
Some nice photos of the stumpery in Alaska's Mendenhall Gardens and Glacier Gardens.
These structures may have been the lemonade to a flood situation or of clearing land.