Rockhouses shelter mysteries. Some cathedral-like and big as a house, some lowly and discreet, these openings at the base of sandstone bluffs hold the secrets of ancient and extinct animal species and the archaeological past of our human ancestors. For millions of years animals and, later, humans found refuge from predators and the elements in these rock shelters. Certain plants called filmy ferns have also found refuge there.

On the far southern edge of the Cumberland Plateau, as far west as the Southern Appalachian Mountains reach, rockhouses have provided habitat for filmy ferns for at least thirty-five million years. The canyons of the Warrior Mountains of north Alabama, which include the Bankhead National Forest and the Sipsey Wilderness, are among the few places on earth these ferns are found.

Filmy ferns are believed to be relics of fern species that thrived in the eastern part of the North American continent when the climate was much hotter and the land was covered by tropical and subtropical forests. Then the climate began to cool and most subtropical plants migrated south. The Carboniferous sandstone made it possible for some populations of fern to remain and evolve into filmy ferns. All of the filmy ferns’ modern relatives are tropical species. Many of them live on tree bark in the Amazon rain forest.

Filmy ferns’ requirements are extremely specific; they cannot tolerate long periods of dryness and, as a general rule, freezing temperatures. Sandstone absorbs a remarkable amount of water then slowly and constantly releases it through evaporation. The constant emission of moisture in the summer, thermal radiation of stored heat in the winter, and the shade of narrow, deep sandstone canyons provide the perfect environment for ferns that are hiding out from cold, dryness, and sunlight.

While the recesses of sandstone rock shelters provided the foundation for their survival, the ferns also had to make radical adaptations. They became very small and sacrificed their ability to produce spores. These translucent ferns are only a few cell layers thick and are extremely efficient at photosynthesis. Living in such dim sunlight, almost all the filmy fern’s energy is devoted to photosynthesis. No one knows how these mysterious ferns spread from one rock shelter to another.

There are very few places on earth that some species of filmy fern are found. In North Alabama, Bosch’s filmy fern (Trichomanes boschianum) is the most noticeable. It grows in small, shaggy colonies in the most shaded recesses of rock shelters in the Bankhead National Forest. Much more rare and secretive is Taylor’s filmy fern (Hymenophyllum tayloriae), a tiny wisp of fern that is known to exist in only two locations: a site in the Sipsey Wilderness and a site in South Carolina. It clings to life in narrow fissures in sandstone.

“Many people are aware that the canyons of the Bankhead National Forest are refuges for remnants of Ice Age forest plants such as Eastern hemlock and sweet birch trees that thrived here during colder parts of Earth’s history,” says Dr. Jim Lacefield, author of Lost Worlds in Alabama’s Rocks. “However, very few people are aware that these same canyons hide remnants of tropical forests from many millions of years farther back in time when Alabama was much warmer. Both of these plant groups hide away in these shady canyons waiting for Alabama’s landscape and climate to return to some ‘once or future stage’ that will release them from their austere refuge in the deep shade.”