In the mountains of Southern Appalachia, landslides sometimes follow major rainstorms. Increases in the frequency of heavy rainfall events predicted under climate change could ramp up the risk of landslides in an area where development and roads crawl up steep hillsides. In western North Carolina, the North Carolina Geological Survey documented over 2,700 landslides and landslide deposits since its landslide hazard-mapping program started in 2009.
Landslides aren’t just about blocked roads and property damage. Case in point: Peeks Creek, North Carolina. In late 2004, Hurricane Ivan dumped about 30 inches of rainfall on western North Carolina in 10 days, triggering over 140 landslides. The largest landslide at Peeks Creek in Macon County killed five people and injured two.
Through their work on the steep slopes of the Coweeta Hydrologic Laboratory (Coweeta) near Otto, North Carolina, Southern Research Station (SRS) scientists began to suspect that the dominant prescence on slopes of rhododendron and mountain laurel–woody shrubs common in the understories of Southern Appalachian forests–might play a role in destabilizing mountainside soil under certain conditions.
In 2008, SRS scientist Chelcy Ford (now project leader of the SRS Forest Watershed Science unit), former project leader Jim Vose, and University of North Carolina at Chapel Hill colleagues T.C. Hales and Larry Band set up an experiment at Coweeta to explore what part the type of vegetation on slopes plays in landslide risk.
In mountain landscapes such as those of the Southern Appalachians, plant roots help stabilize soils, reducing landslide potential, but root strength can vary greatly across plant varieties and by topographic position, which the researchers defined as a on a convex hilltop (nose), steep slope, or concave valley floor (hollow).
To study the relationship between root strength and soil stability, researchers dug pits downslope from 15 individual trees on the Coweeta site. The locations of trees varied from noses to hollows. Root and soil samples were taken below native oak, eastern hemlock, birch, tulip poplar, hickory, and other common tree species. The research team also tested areas below rhododendron thickets.
“We found that rhododendrons had the shallowest, weakest roots, suggesting that the presence of this species lowers the ability of soil to hold together under severe rain conditions in some hollows,” said Ford. “Since debris flows usually start in the hollows, those dominated by rhododendron could represent a heightened hazard for landslides.”
Since 2008, Coweeta scientists have extended their original experiment on root strength and are incorporating their findings and climate change factors into the Regional Hydro-Ecologic Simulation System (RHESSys) previously developed for Coweeta.