Abstract
Clear-felling of beech-podocarp-hardwood forests and their replacement withP. radiata plantations on steep, dissected hill country underlain by massive, late-Tertiary sandstone and siltstone has resulted in an increase in denudation rate from c. 100 m3/km2/a to c. 4000 m3/km2/a. Landslide densities increased from 1/km2 to 20/km2. Forest operations of this nature on topographically similar areas underlain by late-Pleistocene and early-Pleistocene gravels have been followed by less-severe increases in mass-movement. Analyses of slope stability using the infinite slope model indicate that slopes greater than 25° underlain by late-Tertiary sandstone are unstable when saturated, unless additional shear strength is provided by a network of tree roots. Principal causes of this instability are 1) the impermeable sandstone promotes regolith saturation during moderatesized rainstorms (1 – 2 yr return period), 2) the sandstone surface is smooth, planar, coated with an allophane-rich organic clay layer, and is rarely pentrated by tree roots; shear strength at the regolith-sandstone interface is low.
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