Just south of the tundra that rings the Arctic Circle lie vast, cold-adapted,
evergreen forests. Spruce and pine predominate over a smaller contingent of
hardy deciduous trees such as aspen and birch, all eking a living out of frigid
winter temperatures, frozen soils, minimal moisture, and frequent fires. These
vast tracts of uninterrupted, spruce-dominated forest create a sense of
uniformity, even changelessness—a stillness of time.
Uniformity and changelessness are illusions, however—a product of the
shortness of our lives compared to geologic time. In their studies of the
Earth’s climate history, scientists have accumulated evidence that far
from being staid and static, the tree species we think of today as belonging to
the boreal forest have done a bit of globe trotting, migrating back and forth
over entire continents, heading south with advancing ice age glaciers on their
heels, and then north as climate warmed and glaciers retreated.
|Hardy evergreens such as spruce, pines, and firs dominate the boreal forests. Along with larch, aspens, and birch, these trees are able to withstand the long, dark winters of the far north. Moss and lichen cover the forest floor. (Photographs copyright David P. Shorthouse, University of Alberta, Image 1450078, and William M. Ciesla, Forest Health Management International, Image 3948095. ForestryImages.org)|
Changes in the distribution of trees associated with the boreal forest have been studied extensively in North America. The evidence of all that cross-continental tree traveling is recorded in fossilized plant parts, including macrofossils, like cones, leaves, and stems, as well as microfossils like pollen grains. Linking the appearance and disappearance of tree remains in different regions of the continent with the Earth’s temperature and carbon dioxide record, scientists have put together a history of the travels of boreal forest species that highlights life’s remarkable capacity to fill whatever niche the Earth provides, but adds a cautionary tale about the rate at which species can adapt, and the consequences when they can’t. This history may help scientists trying to predict how the boreal forest of today might fare in a world much warmer than the one in which we now live.