Story: Soil erosion and conservation
Page 7 – Biological control of erosion
A range of measures have been used to control or prevent erosion on New Zealand’s farmland. Some were adopted from other countries such as the USA, some were learnt by trial and error, and others developed from research.
Biological control is usually cheaper, but more risky, than mechanical methods.
Trees reduce the rate of erosion by:
- protecting the soil from the impact of rain
- transpiring large amounts of water, which counteracts very wet soil
- binding soil to sloping land with their roots.
Trees, commonly poplars, are planted strategically on slip-prone hill country such as the area around Taihape. Thousands have also been planted throughout the North Island hill country.
They are planted as poles, and protected from grazing animals until established. Poplars are fast growing and their root systems develop rapidly. But they are deciduous and so do not transpire as much water in winter as evergreen species.
Where trees can be easily harvested, pines (Pinus radiata) may be planted at a site where both pasture and trees for timber are managed together. Known as agroforestry, this can be successful on more productive sites and less steep terrain, where trees generally grow faster.
This method is used to control gully erosion, for both permanent and ephemeral streams. Willows or poplars are planted about 2–3 metres apart on each side of the gully floor. In time their roots create a damming effect which slows the water flow, stabilises the gully sides, and allows other plants to regrow.
Stream bank planting
Willows are planted at the edge of streams and rivers to stem the erosion of the banks. Traps or small dams may also be used to catch material dislodged from the edge.
This is a strip (10 or more metres wide) near a stream or river, which is fenced to exclude farm animals. It may be planted with a mix of trees, shrubs and pasture. The aim is to trap sediment, and nitrogen and phosphorus in water run-off, from nearby slopes before it enters waterways.
Pines and cedars planted for forestry production (around 400 stems per hectare) can control slips and gully erosion on hill country, if they cover an area larger than the gully. Some 320 square kilometres were reforested through the East Coast forestry project, begun in 1992.
Rows of trees, planted at right angles to the prevailing wind, help reduce wind erosion and protect livestock and crops. Radiata pine (Pinus radiata) and macrocarpa (Cupressus macrocarpa) are the most widely used species.
Shelter belts can have more than one row, and more than one species, and may be up to 10 metres wide. For best effect they should be as long as possible – at least 12 times as long as the height of the trees.
Protection forestry is managed on land that has little production value, but has considerable value as erosion protection if fenced off from grazing animals. Often these areas are the headwaters of large watersheds. They may consist of regenerating forest or be planted in trees (exotic or native).
Saved by trees
Research in the Gisborne region by Landcare scientist Mike Marden showed that reforestation of gullies successfully controlled erosion in all but the largest of them. Unplanted gullies continue to erode.
A dense and vigorous pasture will reduce sheet and wind erosion, but its shallow roots do not prevent slips, gullies and slumping. In the South Island high country, runholders are encouraged to retire the upper catchment areas from grazing and allow the snow tussock ecosystems to regenerate.
Animal pest control
Controlling animal pests is important in reducing erosion caused by farming. An example is Molesworth Station in Marlborough, once a sheep station ravaged by rabbits. Sheep were replaced by beef cattle, which do not graze as closely, so they expose the pasture to rabbit invasion. Controlling the rabbits helped in the revegetation of depleted hill country.