On the dangers of Rhododendrons!

Rhododendrons are one of those plants that, when planted well, can create an amazing garden:

(from: http://www.kelleriisgaarden.dk/rhododendron-eng.html)

However, it might surprise you to hear that they can be a major cause of landslides. As the image below shows, rhododendrons are increasingly grown on the mountain slopes of the Appalachians:

(from: http://toursinthesmokymountains.com/SmokyMountainsInDepth.aspx)

As well as creating a somewhat beautiful landscape, rhododendrons have been grown in the Appalachians as a result of logging and fire suppression policies. Forest fires have long been perceived as a major hazard, and fire-exposed land is highly prone to landslides (a major fear in California given the fires in this El Nino year). The Appalachians have a long landslide history - in 1969 for example heavy rainfall associated with the passage of the remnants of a hurricane triggered 3700 debris flows, causing 150 fatalities and $116 million of economic losses.

In a recently published paper, Tristan Hales (now at Cardiff University) and colleagues (2009) looked at the role of roots in providing strength to the soil in the Appalachians. The results are quite interesting. It is clear that in many Appalachian slopes the key thing that prevents landslides is the strength provided to the soil by the roots of the trees and shrubs. Therefore, anything that causes a reduction in this strength will lead to an increased chance of landslides. Hales et al. (2009) set out to measure the strength provided by different plants and trees in plots on slopes in the Appalachians. They found that different varieties of trees had broadly similar root strengths, but that for the native rhododendron species was markedly lower. Furthermore, the roots tend to be concentrated in the upper layers of the soil (tree roots extend much deeper), the rhododendrons are less effective at removing and transpiring water from the soil than are trees, and the thick bushy vegetation starves the forest floor of light, which prevents tree sapling growth.

All of this is of course bad news in terms of landslides. Hales et al. (2009) are keen to stress that this should not be seen as a definitive indication that rhododendrons are responsible for landslide initiation in the Appalachians, but they do note that in the last large landslide event, in 2004, many of the landslides were initiated in thickets of rhododendrons.

Reference
Hales, T., Ford, C., Hwang, T., Vose, J., & Band, L. (2009). Topographic and ecologic controls on root reinforcement Journal of Geophysical Research, 114 (F3) DOI: 10.1029/2008JF001168