The theories behind the mystery of grassy balds
You may have seen grassy balds in areas like Roan Mountain and Carvers Gap in the western N.C. mountains. They’re areas of naturally occurring, treeless vegetation dominated by grasses, sedges and forbs on sites below the treeline in predominantly forested regions.
A variety of explanations for why they exist touch on human and environmental factors but according to Peter Weigl of Wake Forest University, the grassy balds are “natural and ancient and largely owe their origin and persistence to past climatic extremes and the activities of large mammalian herbivores.” This is known as the climate-herbivore hypothesis.
An alternative theory is that the balds are isolated artifacts, not natural landscapes, and a result of Native American activities or the European settlers’ agricultural practices. Weigl suggests instead that the balds are natural – a result of climatic changes and the feeding activities of large grazers and browsers over long periods of time. He recommended preservation of the whole landscape of the balds and the diverse species found there.
Grasslands, including Piedmont prairies and coastal plain savannas, are historically known to have existed in the Southeast, and so grasslands and unforested areas were known from this region. Forested areas on mountain peaks were believed to have been driven down by severely cold climates and were replaced by grasslands or tundra. Native Americans were not thought to use these high peaks for permanent settlement, so their influence there was limited. Certainly, European settlers helped maintain balds by grazing livestock, but the balds are known to predate European settlement. Descriptions of the balds date to the 17th century.
Balds are home to a variety of what Weigl calls “rare, endemic, relict and disjunct plant species.” These species are light-dependent and cannot exist in forested habitats. He says, further, that they must have existed over large enough distances and for long enough periods of time to speciate. These include such species as Gray’s lily and Roan mountain bluets. Balds that have grown over have a significant decrease in vertebrate diversity, as well.
Weigl began his talk by describing an item in his hand – the fossilized tooth of a tapir. Tapirs are one of a number of Pleistocene megafauna that once existed – species such as mammoth, mastodon, musk-ox, bison, elk, and ground sloths. Evidence of these large herbivores has been found at excavations at Saltville, Va. In addition, large grazers still play a role in other parts of the world in maintaining grasslands and savannas, such as elephants in Africa.
The loss of the North American megafauna came around 11,500 years ago. Early settlers found large populations of elk, bison and deer that had filled that niche on mountain grasslands. In the late 18th century, elk and bison were locally extinct, but by the 1830s domestic grazers took their place. None are thought to be as effective as the original megafauna.
Other grassland areas in other parts of the world are similar to the grassy balds of the Appalachians. The polonina in Poland are one such habitat that Weigl has studied. They also support rare plants and have large herbivores including European bison, red deer, wild horses, and wild cattle. In addition to fossil data, there is a record of megafauna left by drawings on the walls of caves in that region. There are also grass balds in the Oregon coast with a similar history and rare species.
Weigl makes a good case for the climate-herbivore hypothesis as an explanation for the grass balds. Whatever their origin, these balds are disappearing as woody plants invade. In the 1990s the U.S. Forest Service introduced sheep and Great Pyrenees dogs to control the blackberry vegetation, Weigl says, and this resulted in the reappearance of some rare animals. Restoration of these natural bald landscapes is necessary to protect rare biotas, scenic vistas, and historical places, and can be accomplished through herbivores, cutting, mowing and fire.
Thinking of the areas as complex landscapes that support rare species points to the need for management to protect them for generations to come.
For more information, see Peter D. Weigl and Travis W. Knowles, Temperate mountain grasslands: a climate-herbivore hypothesis for origins and persistence. Biological Reviews (2013).