REDUCING SHALLOW LANDSLIDE OCCURRENCE IN PASTORAL HILL COUNTRY USING WIDE-SPACED TREES

Shallow landslides occur globally on soil-mantled hilly and mountainous terrain. In New Zealand, they are a nation-wide problem, particularly on pastoral hill country grazed by livestock. On these landscapes, trees are planted at low densities, often <70 stems per hectare (sph), to reduce landslide occurrence, but there has been limited quantification of their effectiveness in this role. This study determined the reduction in landslide occurrence at 65 sites planted with spaced trees (53xPopulus, 6xSalix, 6xEucalyptus) following rainstorm events. Sites had a mean slope angle of 27 degrees and soils were predominantly silt or sand-loams. Tree density across all sites was 3265sph, height was 843m, canopy radius was 110m and trunk diameter was 1899cm. Trees reduced landslide occurrence by 95 per cent compared to paired pasture control sites (0 center dot 4 per cent vs. 7 center dot 9 per cent scar area, respectively), and scars occurred on fewer sites with trees than pasture (10 vs. 45). For the 10 tree sites with scars, their area was <3 center dot 5 per cent, except at one site where it was 11 center dot 3 per cent. There were no significant differences between species in their effectiveness in reducing landslide occurrence. Analyses were partially successful in discriminating between sites with and without shallow landslides and identified some attributes with potentially useful discriminatory power. Aspect, mean slope angle and tree density did not feature significantly in the analyses because they were homogeneous across site groups. The study verified the large benefit from wide-spaced tree planting on sites susceptible to shallow landslides. Copyright (c) 2011 John Wiley & Sons, Ltd.