Late Quaternary variations in tree cover at the northern forest-tundra ecotone

1 Accurate land cover reconstructions are essential to understanding the past and present biogeochemical and biogeophysical interactions between the land surface and atmosphere and the impacts of these interactions on climate. Here we quantitatively reconstruct late Quaternary shifts in woody cover across the Northern Hemisphere forest-tundra ecotone, based on a synthesis of Northern Hemisphere pollen records and contemporary observations of woody cover from the advanced very high resolution radiometer sensor. Our reconstructions document the expansion of Northern Hemisphere forests following deglaciation and reveal significant hemispheric asymmetries in the Holocene position, steepness, and history of the forest-tundra ecotone. In western Canada, for example, forest expansion and infilling continued through the Holocene, while in much of northern Asia, forests reached their maximal expansion during the early Holocene, then retreated. The woody cover reconstructions are generally consistent with macrofossil-based reconstructions of northern tree line dynamics and complement them by extending study of the northern forest-tundra ecotone from the tree line limit (well mapped by macrofossils) to the entire ecotone. Using the Lund-Potsdam-Jena dynamic vegetation model, we estimate that changes in northern forest density resulted in at least a 47.7 Gt C increase in aboveground carbon sequestration between 21 and 9 ka, a 13.9 Gt C increase between 9 and 6 ka, and a 3.5 Gt C loss of aboveground carbon from northern forests after 6 ka. This trajectory is consistent with atmospheric carbon isotopic measurements for the Holocene, which suggest carbon uptake by the terrestrial biosphere until 6 ka and small carbon releases from the terrestrial biosphere afterward.