What causes the great biological diversity of the Blue Ridge?

The Blue Ridge Mountains, visible in this satellite photo as a copper-colored diagonal streak, are home to forest types found at lower elevations from North Carolina to Canada. (Satellite image provided by NASA. More about the photograph)

Although the distribution of forest types in the Blue Ridge is best explained by the relationship between elevation and temperature, the great diversity of these forests is not. To understand the underlying causes of this diversity requires some knowledge of geology, climatology, and evolution.

Natural history of the Blue Ridge

Geologically, the Blue Ridge Province formed when North America collided with Africa more than 300 million years ago. The continental collisions of this period compressed and melted preexisting rocks and transformed them into completely new minerals through the process of metamorphism. Shortly thereafter (in geologic time), the new rocks were thrust up and over their western neighbors to form the Blue Ridge.

These overthrust origins are reflected in the Blue Ridge of today. Its steep southeastern flank and western elevations were formed when the overthrust layers were forced up and over the original land surface. All of these rocks began eroding almost as soon as they were formed, but the topography created during the overthrust period led eroded fragments to be carried away from steep sides and high elevations of the new mountains. As a result, the soils of the high mountains remained thin while thick soils formed in the valleys. The deep soils were colonized by trees and forests while the thin soils supported only grasses and shrubs. This pattern remains with us today, with rich cove forests on thick streamside soils, and grasses and shrubs on the thin-soiled balds of the high ridges.

The varied exposure, elevation, and soils of the Blue Ridge have led to a wide range of habitat conditions within the province. In the moist, deep-soiled, densely-forested valleys the daily range of temperature, wind, and humidity remains relatively constant. On the thin-soiled and exposed balds, the daily range of climate conditions is much wider.

In addition, the overall climate of the Blue Ridge has been warming since the last glaciation cycle reached its peak about 20,000 years ago. The Blue Ridge itself was never covered by glaciers, but plants that now occur only in the Arctic were well established here when glaciers were as close as Pennsylvania. The relatively cold conditions of the high Blue Ridge have allowed many plants that are now generally found in higher latitudes to survive and add to the overall diversity of the province.

Finally, the geographic isolation of high latitude species in the Blue Ridge has allowed some of them to evolve into unique species of their own, adding further to the biological diversity of these mountains.

Diversity within forest types

Note that the forest types shown in this field trip are less uniform than they may seem! Several processes operate to create differences within each forest type.

• Some differences, such as exposure to sun and wind, are inherent to all sloping habitats — those that face south get more, and more direct, sun than those that face north.
• Wind exposure is greater at ridges and mountain tops than on the slopes below them.
• Other differences are linked to different rock types. Easily eroded rock disappears, leaving valleys between more resistant rock types. On upland slopes, small differences in soil depth are reflected in major differences in the size and type of trees that can live upon them.
• Finally, differences in recent habitat history are reflected in existing forest types. Almost all Blue Ridge forests have been cut for timber during the last century. Reforestation of these areas has followed different patterns that have left different forest types for us to see now. Some patterns had heavy human involvement in their early stages since the science of forestry was first practiced in the United States on the flanks of the Blue Ridge. Other sites were left to the processes of natural reforestation, a process called secondary succession by ecologists.

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