The Evolution of Abundance
Diversity is the biological keynote of the Great Smoky Mountains. Within the national park have been found about 1,500 species of flowering plants, among which are some 100 trees. There are around 2,000 fungi, 50 mammals, 200 birds, and 70 fishes, or more than in the fresh waters of any other national park on our continent. There are about 80 reptiles and amphibians, among which are 22 salamanders, which is probably as many as can be found in any similar-sized area in North America. Present conditions, such as warmth, abundant moisture, and a diversity of environments brought about by the height and dissection of the mountains, are partly responsible for this biotic wealth. But time, the many millions of years this land has been above the sea and south of the ice, has also been an important factor. It has been a span long enough for a great many species of plants and animals to get here and find a niche and for other species to evolve in the region. The story of the arrival and evolution of the present flora and fauna is intimately linked with the dramatic history of our continent.
We can only guess what life existed here during the 130 million years of the Mesozoic era, because no rocks from this period exist in the Smokies. But we can imagine that dinosaurs and primitive birds and mammals roamed the region, as they did other parts of the continent. Toward the close of the Mesozoic, flowering plants evolved and rapidly became the dominant type of vegetation. We can guess that some of these first magnolias, elms, and oaks grew right here in the ancestral Smokies. Newly evolved bees probably helped to pollinate some of the flowering plants.
The story becomes clearer and the life forms become more and more familiar to us during the 65 million years of the Cenozoic, the present era. In the first half of the Cenozoic, subtropical vegetation grew in the southern United States and temperate vegetation grew north to the Arctic. As these plants would indicate by their ability to grow here, this was a time of warm or mild climates throughout the Northern Hemisphere. Land bridges between North America and Eurasia, by way of the Bering Strait and perhaps Greenland, allowed the spread of a remarkably homogeneous flora throughout the then-temperate parts of these two continents. The Great Smokies, with their feet in the South and, as it were, their head climatically in the North, must have had both subtropical and temperate vegetation early in the Cenozoic era.
During the second half of the Cenozoic, a cooling trend set in. The widespread “Arctotertiary” vegetation of the northern latitudes moved southwards through North America and Eurasia. By the end of the Tertiary, which includes all but the past two to three million years of the Cenozoic, the vegetation zones of North America were probably very similar to those of the present. In the Smokies the trees probably ranged from southern types, such as sweetgum, at low elevations through the great mixture of cove forests and possibly to spruce and fir at the highest elevations. After a long period of gradual change in climate, the stage was set for the drastic events of the Pleistocene.
It is hard for us to imagine what an ice age must have been like in our country. Perhaps the only way to imagine it is to visit the Antarctic or one of the great glaciers in Alaska, and to watch giant slabs of ice fall from those towering walls. Then ... mentally transport the scene to the Hudson River valley or to the flatlands of Illinois, while magnifying the thickness of those glaciers several times over. Then imagine the surface of that great ice sheet stretching all the way to northern Canada.
If you had stood near the front of that massive ice sheet, you would have felt the cold air flowing off it. How far south that cold, dense air flowed and to what extent it affected temperatures in the southern states are unanswered questions. But undoubtedly temperatures were lowered throughout North America and perhaps farther south. Some scientists postulate a drop of 5.5 degrees Celsius (10 degrees Fahrenheit) in mean annual temperatures in southern United States. The high pressure that developed over the ice sheet would have pushed storm tracks southward, increasing precipitation in the South.
Such continental ice sheets advanced at least four times as climates cooled, and as many times they retreated during warmer intervals. With each advance and the consequent cooler, wetter climate, there was undoubtedly a southward shift of vegetation belts. In the mountains there would also have been a downward shift of forest types, particularly those of the higher elevations. That is, the higher elevation species would begin to grow down the slope. In sheltered coves temperatures probably did not drop as much as they did higher up or out in the open lowlands, and soils in coves were deeper and more fertile. The coves of the Southern Appalachians thus may have formed a refuge for many temperate species of plants, including some forced southward by the spreading ice. This is a factor in today’s biotic richness or abundance in the Smokies.
On top of the Smokies and other high mountains of the Southern Appalachians, tundra (treeless areas) may have developed as winter climates became too cold and windy even for spruce and fir, which is the situation today on high peaks of the Adirondacks and White Mountains in New England. Accumulations of blocky boulders in higher parts of the Smokies resemble block fields in the northern Appalachians that probably were formed above timberline in late stages of glaciation. From the location of block fields, geologists postulate a treeline in the Smokies somewhere between 900 and 1,500 meters (3,000 and 5,000 feet) elevation during the last glacial period, some 15,000 to 25,000 years ago. If islands of tundra did exist in the Southern Appalachians, it is not likely that tundra mammals would have migrated from the tundra bordering the ice front through the intervening forest to reach such Arctic pastures in the sky. But some birds might have. Water pipits, which today nest in the Arctic and above timberline on our Western mountains, might have bred on these patches of southern tundra. And the few snow buntings which have been seen wintering on Southern Appalachian balds may have been returning to ancestral nesting grounds of the species.
Although Pleistocene tundra in the Smokies is a rather speculative notion, it seems certain that spruce-fir forest existed below today’s 1,400-meter (4,600-foot) limit. This supposition is supported by the fact that fossil pollen and other fragments of spruce and fir have been found in several lowland bog deposits of the South.
During the last Ice Age the Southern Appalachian spruce-fir forests and their animals must have been a richer version of the plant-animal community that exists in this zone today, for at the peak of the ice advance northern plants and animals probably could migrate along a continuous avenue of this boreal forest in the Appalachians. Bones from cave deposits at Natural Chimneys in Virginia’s Shenandoah Valley indicate that such northern animals as porcupines, snowshoe hares, pine martens, fishers, spruce grouse, and gray jays, as well as the now extinct longnosed peccary and giant beaver, roamed that area 10,000 to 15,000 years ago. The still existing species mentioned above now live farther north in the forests of New England and Canada. If such animals could live during the late Pleistocene at 450 meters (1,500 feet) in Virginia, many and perhaps all of them might well have lived at higher elevations in the Smokies. In the case of porcupines, archeological records from nearby regions in fact support this idea.
After the retreat of the last ice sheet a warm, dry period set in and caused the development of grasslands as far east as Ohio. To what extent this change in climate may have affected the Smokies is not known. But it may have been responsible for the development of the beech gaps: as the spruce-fir forests were forced ever higher, beeches and yellow birches followed in their wake. The once continuous band of spruce-fir forest through the Southern Appalachians would then have been broken into patches as it migrated to higher elevations—and disappeared entirely on the lower mountains. Today such forest is restricted in the Southern Appalachians to the highest parts of eastern West Virginia, southwestern Virginia, western North Carolina, and areas in and just north of the Great Smokies. During the warmer, drier period following glaciation, boreal forest must have been even smaller in extent.
Another consequence of warming was the northward migration of plants and animals into territory vacated by the ice sheet (north of the Ohio River and Long Island). The result today in northeastern United States is a broad patchwork of forest types, each type dominated by a few species, as in beech-maple or beech-birch-white pine forests. This stands in contrast to the diversity of the cove forests from which the migrants extended. Cove forests still harbor individuals of all these species.
What happens next? Has the Pleistocene epoch really ended or are we merely between glaciers, awaiting the next invasion of ice? For the Smokies the question implies others: Will the forest zones move up or down the mountainsides? How will this affect animal life?
While terrestrial life in the mountains flourished during the continent’s climatic swings, aquatic life fared equally well. Within Great Smoky Mountains National Park live some 70 species of fish. Contrast this with the number in Shenandoah National Park in the Blue Ridge of Virginia, which has only about 25 species. Why so many in the Smokies? The answer parallels the situation for plants and terrestrial animals: diversity of environments and plenty of time. All the streams of the Smokies lie within the Tennessee River drainage, which is part of the Mississippi River drainage. The Tennessee River has more species of fish than any other river in North America, because of its many environments (lowland, plateau, and mountain); the vastness of the Mississippi drainage; its existence for many millions of years; and its Pleistocene history. During glacial periods many species of fish were forced southward by ice and cold glacial water. The Tennessee River system offered them a refuge just as Southern Appalachian coves offered a refuge for plants. Even the Mississippi itself was a less favorable haven because it received most of the meltwater. In the headwaters of the Tennessee, the streams of the Smokies thus benefit from their contact with an ancient, relatively undisturbed river system. Within the park, stream environments range from cold and fast to comparatively cool and slow, with large, deep pools.
While most of the present plant and animal species of the Smokies have ranges that extend far beyond these mountains, and while many of these have spread here from other areas of origin, a few are restricted solely to the Smokies. This suggests that they may have evolved here. One such plant is Rugel’s groundsel, a member of the Composite family that grows to about 28 centimeters (11 inches) high and bears large, cylindrical clusters of tiny golden flowers. This plant, abundant in the park’s spruce-fir forests, has not been found outside the Smokies. This suggests that it evolved in the Southern Appalachians and that after the last glacial period, when connection with other sections of spruce-fir forest was broken, it persisted or survived only in the Smokies. By comparison, the Fraser fir, though most abundant in the Smokies, also occurs north to southwestern Virginia on the highest mountains, indicating that it evolved at some earlier, colder time when spruce-fir forest was more nearly continuous. The red-cheeked salamander, a striking creature, is probably the sole vertebrate found exclusively in the Smokies. Many other species of salamanders, however, are restricted to parts of the southern end of the Appalachians and probably evolved there, where the cool, wet climate and diverse topography provide ideal conditions for this group of animals. How many other species of plants and animals evolved in the region and subsequently spread far beyond this point of origin we can only conjecture.