Relict Ice Age Microfauna of Georgia’s Boulderfield Forests

Spruce trees and northern pine species dominated the higher elevations of Georgia’s Blue Ridge Mountains during Ice Ages.  The highest peaks may have even been above the tree line and likely hosted grassy steppes or plants found today in the tundra region.  Severe freeze-thaw cycles of Ice Age winters broke big slabs of rock into boulders that  cover many north facing slopes of the southern Appalachians.  Following the most recent Ice Age, the climate warmed and northern species of hardwoods recolonized these boulderfield forests from refugia that probably existed adjacent to rivers and creeks.  Boulderfield forests have shallow rocky soils dominated by 20-30 foot tall stunted trees, and a great variety of beautiful shrubs.  Frequent high winds and ice storms keep boulderfield forests relatively open by knocking over trees and breaking limbs.  The most common trees include yellow birch, black birch, mountain maple, sugar maple, striped maple, beech, basswood, mountain ash, yellow buckeye, fire cherry, and serviceberry.  Northern red oak and white oak also occur.  Mountain laurel, rhododendron, witch hazel, climbing hydrangea, hazelnut, prickly gooseberry, rasberry, blackberry, strawberry bush, and elderberry grow in the understory,  Moss covers the boulders.  Some examples of boulderfield forests in Georgia are Brasstown Bald, Coosa Bald, Hightower Bald, Eagle Mountain, and Grassy Mountain.

Mountain Laurel growing alongside the trail on Brasstown Bald.  I hiked up and down this mountain 2 years ago but didn’t know about boulderfield forests then.  I do remember the stunted birch and beech trees and the abundant boulders. 

Birch trees are well adapted to boulderfield forests because they can germinate on top of rocks and logs.  The sugar in sugar maple sap lowers the freezing point, allowing this species to survive in colder climates.  Basswood sends lateral roots around boulders, helping them grow in rocky soils.

The abundant rocky crevices here provide shelter to many disjunct populations of small animals that formerly ranged into the piedmont and upper coastal plain during the Ice Age, but whose ranges have since contracted as the climate warmed.  Four species of northern shrews still live on the  peaks of the southern Appalachians which serve as islands of habitat surrounded by land that has become unsuitable for them.  Pygmy, long-tailed, smoky, and masked shrews thrive in boulderfield forests too cool for competing cold-blooded lizards.

Masked shrew.  Boulderfield forests on high peaks are too cool for their main ecological competitors–lizards.

The masked shrew (Sorex cinereus), like most insectivores, has a high metabolism and eats 3 times its own weight in insects everyday. (Imagine a 200 pound man eating 600 pounds of food everyday.)  They can hide in crevices under the abundant boulders and forage in the adjacent leaf litter.  The masked, pygmy, and smoky shrews all had a wider distribution during much of the Pleistocene than they do today.

Red backed vole

Fossils of the red backed vole (Myodes gapperi) have been found in central Georgia at Little Kettle Creek in Wilkes County.  Today, for the most part, this species ranges no farther south than Kentucky, but disjunct populations occur in some boulderfield forests in north Georgia.  These omnivorous mouse-like mammals eat green plants, fungi, seeds, roots, berries, snails, and insects.

Red squirrel.  They prefer forests dominated by spruce, pine, and hemlock.

The red squirrel (Tamiasciurus hudsonicus) reaches the southernmost limits of its range on the northfacing slopes of Georgia mountains.  Fossils of red squirrels have been found farther south in 2 Bartow County sites–Ladds and Kingston Saltpeter Cave, indicating this species too ranged farther south during the Ice Age.  They prefer seeds from conifers but will  also eat acorns, nuts, fungi, tree sap, and birds eggs.  They have an interesting habit of storing caches of food in hollow logs, tree snags, and crevices…far more than they need.  Native Americans use to utilize these caches to stave off starvation.

Red squirrel cache of pine cones.

Saw whet owl

The saw whet owl (Aegolis acadicus) nests in Georgia’s boulderfield forests, but they do not breed any farther south.  These tiny owls stay still to avoid detection and predation.  This defense mechanism misleads many people into thinking they are tame.  This species likely bred farther south during the Ice Age.

Winter wren.  I saw one of these for the first time last Thanksgiving.  They hop around on the ground like a mouse.

The winter wren (Troglodytes hiernalis) is a common inhabitant that breeds in boulderfield forests.  They do tend to migrate farther south during the winter.  This past Thanksgiving, I saw a winter wren hopping around like a mouse behind the Woodbridge Dam in Evans, Georgia.  They forage for insects in crevices and caves, hence the Latin name, troglodytes, which means cave dweller.

Other common animals with northern affinities that occur on boulderfield forests include deer mice, chipmunks, ruffed grouse, and yellow bellied sapsuckers.

See also: https://markgelbart.wordpress.com/2011/06/10/brasstown-bald-the-highest-mountain-in-georgia/

and https://markgelbart.wordpress.com/tag/miccosukee-gooseberry/

Tags: boulderfield forest, Brasstown bald, masked shrew, red backed vole, red squirrel, saw whet owl, winter wren