How much do trees reduce landsliding?

A method is presented for calculating the effectiveness of trees in reducing the incidence of shallow landslides on hillslopes. Computer processing of digital imagery is applied to aerial photographs of hillslopes on which widely spaced trees and landslide scars are both visible. A graph of average fraction of ground eroded versus distance from tree is derived. From this graph, the average radius of influence of trees and the area of ground saved from sliding by the trees are deduced. The area of ground that would have been saved from sliding had the trees been planted at other spacings also may be estimated. To demonstrate the method, it is applied to a hillslope that had experienced landsliding during a severe storm. The derived graph of average fraction of ground eroded versus distance from trees showed the landslide scars tending to occupy areas away from the trees. Because the trees had not been planted deliberately on areas that were less likely to fail, it is inferred that this preference is due to the influence of the trees for whatever reason. This influence decreased with distance from the trees, becoming undetectable at a radius of 11 m (36 feet). The storm caused 20.9% of the study am to become immediately unproductive for pasture growth, either through landsliding or deposition of debris. The method predicted that without the poplar trees this figure would have been 24.2%, that is, 3.3 % of the study area was saved from immediate production loss. The trees, therefore, reduced storm damage by 13.8 %. If the trees had been planted on a 10-m (32-foot) grid, with a 100 % establishment rate, storm damage would have been reduced at least 70 %.