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How did glaciers change the distribution of forests and tree species?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

During the Pleistocene Epoch, the expansion of glaciers caused significant disruptions to global vegetation, forcing a southward displacement of tree species and climatic zones.1

In regions such as central Europe and the United States, areas that currently support deciduous forests were transformed into open, tundra-like landscapes dominated by grasses and herbs with very few trees. Meanwhile, boreal forests (taiga) were pushed as far south as 30° N latitude. These forests, composed of species like spruce and pine, extended toward the Mediterranean and northern Louisiana.12

As glaciers retreated, these species migrated northward. This process was generally gradual, though some species, such as white spruce, experienced rapid expansion facilitated by seed dispersal via strong northward winds. Additionally, lower sea levels during glacial periods created land bridges, such as the Bering Land Bridge, which allowed for the migration and mixing of plant species between continents.2