Landslide occurrence as a response to land use change: a review of evidence from New Zealand

Vegetation cover is an important factor influencing the occurrence and movement of rainfall-triggered landslides, and changes to vegetation cover often result in modified landslide behaviour. However, it is difficult to relate the occurrence of landslides directly to variations in land use, especially in some European countries. In contrast, New Zealand provides a good opportunity to investigate geomorphic responses to anthropogenic land cover changes. Before European settlers first arrived in the 1840s, hilly regions were only marginally influenced by human activity. The Maoris, New Zealand's first settlers, lived largely on coastal plains or near lakes and rivers. They influenced general vegetation cover only through localized burning practices. In contrast, European settlers moved into the back country and converted extensive hill areas from native forest and bush to pasture. This reduced the strength of the regolith and rendered the slopes more susceptible to landslides. Pulses of natural sedimentation in the pre-European period have been related to volcanic activity, climatic variability, including changes in frequency of cyclonic storms and wind erosion and fluvial erosion following forest fires initiated either by volcanic eruptions or lightning strikes. Since European deforestation began, sediment production has largely been determined by landslide events. On unstable slopes, thousands of landslides were triggered by high-magnitude/low-frequency climatic events during storms with estimated return periods in excess of 50 years. In contrast, low-magnitude/high-frequency rainfall events have caused gully and channel erosion. Examples from different parts of New Zealand indicate changes in sediment-generating processes following land use modifications. After deforestation, landslides have contributed significantly to sediment sequences in depositional basins such as lakes, swamps, estuaries, coastal wetlands and the nearshore and offshore zones of continental platforms.