Passing just south of Hudson Bay, then angling northwest to the mouth of the Mackenzie River and across the northern edge of Alaska, is the tree line, which identifies the transitional zone between the taiga and tundra in the Northlands. North of it, climatic conditions are too harsh for tree-like vegetation. Beyond lies the tundra, a region of lichens, grasses, mosses, and shrubs. It holds as much water as all the freshwater lakes in the world. In winter it extends southward to enshroud northern Alaska.
Summer melt briefly frees the area.
The ice limits ocean transport in the Arctic to a brief, often hectic period each summer. The ice also minimizes any moderating impact by the Arctic Ocean on the Northlands' climate. Nearly all parts of the Northlands are sparsely populated, with highest densities found along the southern margins. Inuit are the predominant population in most of the Arctic. American Indians are found mainly in the boreal forest area.