Causes a Mountain to “Go Bald?”
The Appalachians formed between 300 to 480 million years ago due to
upheaval caused by the collision of continental plates. They once reached
elevations similar to those of the Alps and Rockies before millions of
years of erosion brought them down to their present heights, leaving them
overlaid with thick vegetation.
Balds are mountain summits covered primarily by dense native grasses or shrubs that occur in areas where heavy forest growth would be expected. Balds are found primarily in the southern Appalachians where the climate is too warm and the elevations too low to support an alpine zone (upper areas where trees fail to grow due to short or non-existent growing seasons).
So why do these balds form? Actually, no one knows for certain how they came to be, or why they endured. One theory once held that the tops of the mountains were shaved off by gigantic ice sheets during the last ice age, but there are problems with this theory. The giant ice sheets capable of cutting off tops of mountains did not progress as far south as the southern Appalachians during the last ice age. Also, there are many places throughout the range where a bald mountain sits right beside a mountain that does not have a bald top, even though it is just as high in elevation.
Another theory says that perhaps the water table just does not rise high enough in the high summits to support large trees. But the fact that most of the mountains are not bald also discounts this thought, as well as the fact that natural springs can be found even near the very tops of these balds. Other theories that the tops of some mountains were cleared by natural fires or insect infestations have also been dismissed.
Scientists have tangled with this problem for decades, and some have developed a new idea involving a chain of events that may now explain the formation and continuation of the bald mountains.
The Appalachians are made up of some of the oldest mountains on earth, heavily forested far before the occurrence of the last ice age. Even though the thick ice sheets did not progress this far south, the weather would have still been horrendous by today’s standards. The frigid winds would have howled incessantly over the mountains, forcing trees to retreat from the mountaintops and inviting a profusion of grasses and plants more accustomed to living in tundra-like settings much farther north. This kind of weather condition can still be seen today in places where high winds prevent trees from thriving. Cadillac Mountain in Maine’s Acadia National Park is deserted of trees due to a constant strong ocean gale, even though it is only 1500 feet in elevation. However, depending on the specific topography of a particular region, some mountain crests would have more sheltered from the destructive winds than other more vulnerable ones. This would explain why some mountains remained “baldless” even though a nearby bald is comparable in elevation.
But why did the trees not return after the cold winds waned? The grasses and plants that supplanted the trees would have attracted the large grazing animals that still walked the continent at the time. Mastodons, tapirs, and mammoths acted as pre-historic weed eaters, keeping the mountain tops clear. When these creatures disappeared, bison and elk moved in. Some researchers also believe Native Americans may have kept the balds clear by fire, too, perhaps maintaining them in order to make easier hunting of the beasts that feasted there.
As with so many of our environmental histories, things changed when European settlers arrived. Within a century they killed off most of the bison and elk. Fortunately for the balds, the early farmers quickly capitalized on the waiting pastures, turning loose their goats, pigs, sheep, and cattle, which grazed contentedly, keeping up the natural history of treeless mountaintops.
But over the past few decades, most such grazing has vanished as mountain people have taken cash jobs and cut back on the size of their farming operations. The treeless balds that for millennia have supported a unique blend of native grasses and plants are now steadily shrinking beneath a steady advance of trees, shrubs, and bushes.