Effect of landslide erosion on Taranaki hill pasture production and composition

Abstract

Herbage accumulation, botanical composition, and selected soil properties were measured on hillslope pastures at three localities within eastern Taranaki hill country over 4 years, beginning in 1984. Measurement sites were either on uneroded soils representing top, middle, and bottom slope positions, or on landslide scars with ages ranging from 12 to 80 years. On uneroded sites, net annual herbage accumulation decreased with increasing slope angle from the bottom to the top of hillslopes. Net herbage accumulation was lower on landslide scars when compared with uneroded sites of similar slope. This was attributed to the presence of bare ground, lower soil water‐holding capacities, and lower ryegrass content when compared with uneroded sites. Results confirmed previous findings from Wairarapa and Wairoa hill country, but showed that pasture recovery on landslide scars in Taranaki hill country was slower. Pasture recovery on landslide scars was greatest during the first 40 years after slipping, followed by a more gradual increase. Annual herbage accumulation on 12‐ and 40‐year‐old scars, was 24 and 74%, respectively, of uneroded levels. Further recovery was related to soil moisture status. On scars where soil moisture conditions did not limit pasture growth, net herbage accumulation recovered to levels of uneroded soils after 80 years. In comparison, where soil moisture conditions limited pasture growth during late summer and autumn months, herbage accumulation was similar to 40‐year‐old scars. Results indicate that landslide erosion causes permanent reductions in mean herbage accumulation on hillslopes. These reductions increase from about 1 to 3% per decade with increasing slope angle from 28 to 42°, mainly because of increased landslide densities. Model simulations suggest that the rate of reduction will decrease over longer periods, corresponding to fewer fresh landslides being produced on hillsides.