Floating Islands and Flying Squirrels

When you hear the term “sky island” what do you imagine? Maybe something from a far-off fantasy land, or an alien planet? Well, what if I told you that sky islands aren’t just real, they are scattered along Southern Appalachia and house species who have called this land home for millions of years? Now, you may be thinking, “I have never seen a floating island while visiting the mountains in Southern Appalachia.” That’s because the term sky island doesn’t refer to landscapes resembling the world Pandora of the movie Avatar but rather refers to remnants of the Pleistocene Epoch that remain only in the high elevations of our eastern mountain range. 

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Kolin Toney

The Pleistocene Epoch lasted from 2.6 million years ago until 11,700 years ago. During this time, glaciers covered much of North America making the southeastern United States a boreal forest comprised of spruce/fir and northern hardwood species. As the Pleistocene Epoch ended and the climate began to warm, the glaciers and boreal forests retreated northward. But Canada wasn’t the only place to escape the heat; the Southern Appalachia mountains’ highest elevations also provided a safe harbor for boreal forests. Now, these Pleistocene relics sit only on the top the tallest mountains in Southern Appalachia - creating sky islands that host many species marooned as the ecosystems they need to survive vanished. 

Our Workshop & Walkabout this summer was focused on one of these marooned species, the federally endangered Carolina northern flying squirrel. The Carolina northern flying squirrel is a distinct subspecies of the northern flying squirrel due to the isolated nature of these mountain-top populations. At our workshop, Sue Cameron, a biologist with U.S. Fish and Wildlife Service, spoke to us about this unique species and why it needs sky islands to stay alive. 

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Corinne Diggins, Virginia Tech/USGS VCFWRU

The primary food source of the Carolina northern flying squirrel is the truffle, which is the fruiting body of fungal mycorrhizae. Fungal mycorrhizae attach to the roots of spruce/fir trees and aid in gathering water and nutrients for the tree while the fungus receives sugars from the roots. The squirrel in turn helps keep spruce/fir forests healthy by digesting and disbursing the truffle spores the trees need to thrive. Due to this fascinating symbiosis, the primary food source of the squirrel is thus restricted to patches of boreal spruce/fir forest. The squirrel requires not just the spruce/fir, but also northern hardwood species, specifically the yellow birch. The shaggy birch bark is easy to strip off the tree and makes warm nest insulation.    

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Gary Peeples/USFWS

Unfortunately, we also discussed how the sky islands are threatened and vanishing. Initially, natural warming after the Pleistocene Epoch caused a decline in habitat, but in the late 1800s/early 1900s extensive logging and slash burning caused a drastic drop in Carolina northern flying squirrel habitat. Our region’s soils were decimated by the slash fires and spruce seedlings could not re-establish. This led to as much as 50% of all Southern Appalachia spruce/fir forests being replaced by hardwood species that support the hard mast diets of the common southern flying squirrel, but not the truffle and lichen diet of the endangered Carolina northern flying squirrel. Introduced pests, like the balsam woolly adelgid, and acid rain also have been accelerating the decline of spruce/fir forests and habitat retractions are also predicted as a result of climate change.