Collapse-scar bogs at Innoko National Wildlife Refuge, Alaska, USA. September, 2012.When permafrost thaws, the ground surface subsides and becomes inundated and the black spruce forest growing on the permafrost can no longer survive. Trees teetering and falling over in thawing permafrost are often referred to as "drunken trees." The black spruce forest is replaced by a saturated bog, dominated by sedges and mosses. This wholesale ecological transformation has large impacts on carbon cycling.