Rhododendrons Can Be Dangerous

"We found that rhododendron had the shallowest, weakest roots suggesting that the recent expansion of this species may have lowered the cohesive strength of soil in some hollows," says Vose, research ecologist and project leader of the SRS Coweeta Hydrologic Laboratory located near Otto, NC. "Since debris flows usually start in the hollows, those dominated by rhododendron could represent a heightened hazard for landslides."

Research by U.S. Forest Service Southern Research Station (SRS) scientists and partners suggests that the expansion of Rosebay rhododendron (Rhododendron maximum) in Southern Appalachian mountain hollows may increase the likelihood of landslides during and after intense rain events. In an article recently published online in JGR-Earth Surface, SRS researchers Chelcy Ford and Jim Vose, along with T.C. Hales and Larry Band (University of North Carolina at Chapel Hill), examine how the interaction between topography and the species of tree or shrub present affects the ability of soil to hold together.

Landslides present a significant danger in the steep landscapes of the Southern Appalachians. Most of the recorded high rainfall events in the area occur in the fall and have been associated with tropical storms. In 1940, 1969, and 2004, intense rain from hurricanes caused landslides that together resulted in over 190 human casualties and $140 million in damage.

With accelerating land use change and more frequent storms predicted for the area under climate change scenarios, concern about landslides has grown. "Roots of trees and shrubs can represent up to 100 percent of what's holding soil together and keeping mountain slopes from sliding," says Vose, "For this study, we measured the root distribution and tensile strength -- roughly, the force required to pull a root to the point where it breaks apart -- of 15 southern Appalachian species in relation to topography and position on slopes."

The researchers dug pits down slope from 15 individual trees on the Coweeta site. The locations of trees varied from noses -- convex topographic positions -- to hollows. The trees included native species of oak, eastern hemlock, birch, tulip poplar, hickory, and other species. The researchers tested one woody shrub, Rhododendron maximum, a native species, which has come to dominate the forest understory in some areas of the Southern Appalachians.

"We found that root strength was similar among tree species, and root strength of trees was consistently greater than that of the native shrub rhododendron," says Vose. "Tree roots in nose positions were stronger compared to those in hollows, coincident with greater root cellulose content."