The natural vegetation of North America has been significantly modified by human activity, but its general nature is still apparent over much of the continent. The most notable forest is the taiga, or boreal forest, an enormous expanse of mostly coniferous trees (especially spruce, fir, hemlock, and larch) that covers most of southern and central Canada and extends into Alaska. In the eastern United States a mixed forest, dominated by deciduous trees in the north and by various species of yellow pine in the south-east, has mostly been cleared or cut over, but a considerable area has regrown since the 1940s. In the western portion of the continent, forests are primarily associated with mountain ranges, and coniferous trees are dominant.