Relic populations of Atlantic white cedar persist in 5 western and 1 eastern Georgia counties. At one time in the past, the range of the Atlantic white cedar must have been continuous.
Range map of the Atlantic white cedar. Note the gap between the eastern and western populations.
Today in Georgia, disjunct populations of the Atlantic white cedar are restricted to the banks of small streams that flow through the sandhills region in Peach, Marion, Talbot, Taylor, Schley, and Richmond Counties. In the latter county they’ve been reported at Fort Gordon. Unlike the more common and adaptible eastern red cedar, the white cedar needs specific conditions to grow. They require acidic wet soils. This type of environment is common in North Carolina and New Jersey where white cedar is a dominant or co-dominant tree in the Great Dismal Swamp and the Jersey pine barrens. They used to be abundant on Long Island, New York, and many stands still grow near the New England coast. In the south white cedar stands occupy land in the panhandle of Florida, southern Alabama, and coastal Mississippi. But there is a wide geographical gap between the eastern and western populations where the tree is completely absent. This means white cedar must have at one time grown in areas within this gap but habitat here is no longer hospitable for this species. I have a hypothesis that explains this ecological anomaly.
Though white cedar prefers moist wetlands, this tree can not survive prolonged flooding. And its seed will sprout in peat after their parents are killed by forest fire. But repeated fire and/or flood regimes will eliminate white cedar from a region, whereas oaks and pines are well adapted to survive in such environments. During stadials, the cool arid climate phases of the Ice Age, forest fires and flooding rivers were rare due to the decrease in lightning and precipitation. However, evapotranspiration rates were low because temperatures were colder. This meant soggy ground on poorly drained soils became bogs, the type of environment favored by white cedar. Bogs no longer exist in much of the southeast, explaining why white cedar occurs as just a relic, but they must have been fairly common during much of the Ice Age.
There is fossil evidence of Atlantic white cedar excavated from Cahaba Pond, St. Clair County, Alabama which is in the northeastern part of the state where white cedar is now absent.
Cahaba Pond is located in St. Clair County, Alabama. Ice Age plant macrofossils and pollen were excavated here. Atlantic white cedar no longer occurs here but it did during the Ice Age.
The fossil white cedar dates to about 14,000 calender years BP and was found associated with beech, hornbean, oak, hickory, elm, ash, striped maple, mountain maple, white pine, and hemlock. Most of these species are fire intolerant, suggesting the environment at this time and at this location did not have frequent fire.
Today, white cedar is unable to grow along the rich river bottomlands so common in the south because of flooding, and Indian-set fires long ago extirpated them from uplands where they were probably becoming rare anyway, since bogs began drying up in the warmer climate. In the rare relic environments where they can grow in the south, they’re associated with red maple, tupelo, loblolly pine, fetterbush, pitcher plants, blueberry, gallberry, sedges, and ferns.
Atlantic white cedar trees.
Great Dismal Swamp in North Carolina where Atlantic white cedar is a dominant tree on poccasins which are bogs with peat and sandy soils.
Pollen evidence suggests Atlantic white cedar was absent or rare in North Carolina during the Last Glacial Maximum. And, of course, the tree didn’t grow in New England when it was covered by the Laurentide Glacier. Thanks to southern refugia, where it is mostly absent today, this species was able to recolonize the northern parts of its range as the glacier retreated and left suitable habitat on outwash plains and morraine kettles.