Geoscientist Sean Bemis, undergraduate student Madeleine Kronebusch, and Dhari Alharbi '22, who was a student at the time, review data collected by their ground-penetrating radar system. Photo courtesy of Sean Bemis

Virginia Tech scientists study how vegetation helps create soil on bare rock outcrops

Lon Wagner is senior director of communications and marketing in the Virginia Tech College of Science.

BLACKSBURG — In the forests of the southeastern United States, dense tree cover dominates most landscapes. That’s why the Appalachian Trail is sometimes nicknamed “The Green Tunnel.”

But avid hikers know that often in the Southeast, they’ll emerge from the green tunnel.

“When you walk out of the forest onto the rock, the contrast is immediate,” said Sean Bemis, a geoscientist and lead author of a study that uses these unique outcroppings as a natural laboratory to examine fundamental connections between rock weathering and vegetation. “You are standing on solid rock and you wonder where did all the trees go?”

Because of the lack of vegetation, these outcroppings are known as “balds.” But on geologic time scales, these balds have not always been devoid of vegetation. When bedrock is stripped of soil, it presents an extreme environment — hard, dry, and nutrient-poor — yet plants eventually take root. The study, published in the Journal of Geophysical Research-Earth Surface, reveals how this process unfolds during the earliest stages of what scientists call the critical zone: the life-supporting system of the Earth’s surface extending from bedrock to the tops of trees. 

The seismic data collection system then clocks the time it takes for the seismic waves from the blows to travel through the subsurface rock and arrive at each sensor.

The study site is part of the Panola Mountain Research Watershed, a long-standing U.S. Geological Survey research area used to study water movement through landscapes. While the site has a history of hydrologic research, it had not previously been the focus of subsurface geophysical imaging tied to vegetation growth.

Using hundreds of photographs collected systematically over the outcrop using a drone, the researchers created a very high resolution topographic model and photographic image of the outcrop surface. They used this to map patterns of erosion and the locations of patches of plants and soil. Geophysical methods — specifically ground-penetrating radar and seismic refraction — enabled them to peer beneath the surface. 

The team found that small changes in the bedrock cause portions of outcrop to erode at different rates, creating small depressions and linear troughs on the rock surface. These features collect water and sediment, becoming sites where vegetation can take root and survive. Moss and grasses appear first, and their presence begins a feedback cycle where they improve growing conditions by storing water and soil, supporting the growth of larger plants. A succession of shrubs and trees will then follow and, as these larger plants grow, their roots extend into fractures beneath the surface.

Geophysical data show clear differences between bare rock and vegetated areas. Beneath bare rock, seismic waves travel faster, indicating intact, unaltered bedrock. Beneath vegetation, seismic waves slow down, indicating fractured and altered rock. Radar data show shallow fractures filled with roots, sediment, and water beneath plant patches.

“As plants become established, they are not just responding to the rock,” Bemis said. “They are putting in the work to gradually change it.”

The researchers observed that larger plants correspond with deeper zones of altered rock. This suggests that vegetation plays an active role in expanding the critical zone after initial colonization begins.

This study is just one outcome from a project working at multiple sites around the U.S. that involved researchers from seven institutions and was supported by the National Science Foundation’s Critical Zone Collaborative Network program. This larger project was guided by a series of testable hypotheses to better understand the role of bedrock weathering in critical zone processes: Researchers point out that the new ideas presented in this study took shape from discussions while in the field, highlighting the value of collaborative, interdisciplinary fieldwork for motivating new, observation-based understanding of complex Earth systems. 

The findings help address a long-standing question in Earth science: whether plants simply occupy existing soil or actively contribute to soil formation. As demonstrated at Panola Mountain, the answer is both. 

“That really surprised us,” said Steve Holbrook, professor of geophysics and co-author of the study, “the way small changes in outcrop composition — for example, a thin dike intersecting the surface — create depressions and divots that collect rainwater and give seedlings a foothold on the outcrop. So the geology is providing a bottom-up control on the plant distribution, even as the plants are helping weather the underlying bedrock."