At the onset of the Late Glacial Interstadial (15,000–11,000 BP), global warming resulted in shrub and dwarf birch in northeastern Siberia, which was then colonized by open woodland with birch and spruce during the Younger Dryas (12,900–11,700 YBP). By the Holocene (10,000 YBP), patches of closed larch and pine forests developed.[22] Past researchers had once assumed that the mammoth steppe was very unproductive because they had assumed that its soils had a very low carbon content; however, these soils (yedoma) were preserved in the permafrost of Siberia and Alaska and are the largest reservoir of organic carbon known.