Pastoral Hill Slope Erosion in New Zealand and the Role of Poplar and Willow Trees in Its Reduction

Abstract

1.1 The geology of New Zealand and its contribution to erosion New Zealand is a predominantly hilly and mountainous country. An area of 18 million hectares (69% of the country) has slopes greater than 12°, and is commonly called ‘hill country’. This is further divided into ‘hill-land’ (12–28°) and ‘steepland’ (if slope exceeds 28°) (DSIR 1980). The range of slope and elevation, coupled with a wide latitudinal range, a mid-oceanic setting encompassing subtropical to cool temperate climates, and complex geologic and tectonic regimes means that New Zealand’s ‘hill country’ is physically diverse. As a consequence of this diversity, the productive potential of New Zealand’s’ hill country, and its response to climatic events, land use pressure and environmental change varies significantly across the country. Erosion is a significant environmental issue facing agricultural and forestry land uses in large parts of the hill country of New Zealand. It causes both on-site (loss of soil productive capacity and water holding capacity) and off-site (declining water quality, river aggradation, increased vulnerability of infrastructure to severe climatic events) effects. As well as the environmental costs there are major financial risks and social consequences, at every organisational level, of repeated erosion and flooding events. Of the 6.3 million hectares in the North Island, the majority is developed on soft rock and crushed soft rock terrain in the south-east and west of the island (2 825 000 ha, 45%). Volcanic ash and loess-mantled terrain comprises another 23% (1 456 000 ha), largely on the periphery of the Central Volcanic Zone. Hard rock hill country, exclusive of the igneous hard rock hill country, is largely concentrated on the margins of the axial ranges (919 000 ha, 14.5%), whereas the hill country developed on deeply weathered sedimentary and igneous rocks (863 000 ha, 13.6%) is located predominantly in Northland and on the Coromandel Peninsula. In the North Island approximately 200 000 ha has a mapped erosion severity of severe, very severe or extreme. The North Island hill country is dominated by mass movement erosion. Shallow landslide (soil slip) and sheet erosion are the most widely