2026-02-26 バージニア工科大学（Virginia Tech）
＜関連情報＞
- https://news.vt.edu/articles/2026/02/science-trees-cover-rock.html
- https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JF008424
臨界地帯の創出：米国ジョージア州パノラ山の露出した岩盤表面における岩盤地質、保水性、植生間のフィードバック Creating a Critical Zone: Feedbacks Between Bedrock Geology, Water Retention, and Vegetation on an Exposed Bedrock Surface, Panola Mountain, Georgia, USA
Sean P. Bemis, W. Steven Holbrook, Brady Flinchum, Jorden Hayes, Russell Callahan, Ciaran Harman, Brad Carr, Cliff Riebe
Journal of Geophysical Research: Earth Surface Published: 25 January 2026
DOI:https://doi.org/10.1029/2025JF008424
Abstract
Most of Earth’s present-day terrestrial surface is covered by regolith—the layers of soil, saprolite, and weathered bedrock that together comprise the critical zone. Recent research has focused on understanding fluxes of minerals, water, and energy through the critical zone under steady state assumptions. However, in eroding landscapes, regolith and soil are produced from the bedrock as it is exhumed. Therefore, at some point in time, every location on the Earth’s surface currently mantled by regolith experienced an onset of weathering processes. This initial creation of a critical zone from rock is poorly understood. Here we study initial critical zone formation from exposed bedrock by combining surface and subsurface geophysical observations at a site where regolith appears to be forming from bedrock on a granodiorite outcrop in Panola Mountain State Park, Georgia, USA. Vegetation gains an initial foothold on the outcrop by colonizing microtopographic depressions created by differential weathering of contrasting bedrock compositions. We observe a range of colonization stages, from moss to grasses to small bushes and eventually to large trees. Subsurface signatures of the vegetation include enhanced radar reflectance and reduced seismic velocities, with larger vegetation associated with stronger subsurface signals. Using a space-for-time substitution approach, we propose an evolutionary sequence for critical zone development. While disentangling the chicken-and-egg questions that pervade this topic remains challenging, our results suggest that geological heterogeneity can provide the initial catalyst for colonization, but ultimately vegetation itself plays a strong role in producing subsurface structures associated with the critical zone.
Plain Language Summary
The uppermost layer of Earth’s land surface is known as the critical zone and includes the soil and weathered rock that support plant and animal life. If erosion removes soil at the ground surface, new soil must be continuously produced from the underlying bedrock. However, how this process begins—transformation of bare rock into a life-supporting surface—is not well understood. To investigate this, we studied a rock outcrop in Panola Mountain State Park, Georgia, where vegetation is gradually colonizing the surface, starting with moss and grasses and eventually supporting larger plants and trees. Examining the topography of the outcrop combined with subsurface images collected using geophysical tools, we found that vegetation first establishes where rock surface depressions hold water and sediment. As the first plants grow, they trap additional water and sediment, allowing larger plants to grow and altering the bedrock underneath. This process supports the growth of larger and larger plants over time, eventually becoming a mature forest. Our findings suggest that the beginning of soil development on a rock outcrop is controlled by features in the bedrock that create pockets where plants can take hold, but once vegetation is established, it becomes the major control on soil development.
Key Points
- Microtopography produced by differential weathering of bedrock heterogeneities form water and sediment traps on bedrock outcrops
- Trapped water and sediment initiate a vegetation and critical zone successional sequence
- Trees support critical zone expansion, but growth remains limited until a mature critical zone develops
