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

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ABSTRACT

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 (53 × Populus, 6 × Salix, 6 × Eucalyptus) 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 32–65 sph, height was 8–43 m, canopy radius was 1–10 m and trunk diameter was 18–99 cm. Trees reduced landslide occurrence by 95 per cent compared to paired pasture control sites (0·4 per cent vs. 7·9 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 <3·5 per cent, except at one site where it was 11·3 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 © 2011 John Wiley & Sons, Ltd.

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