Story: Soil erosion and conservation
Page 2 – Natural and human causes of erosion
The landscape of New Zealand has been shaped by wind, water, volcanoes and glaciers over at least 25 million years. The forces that wear away at the land are considered part of natural erosion.
The rate of erosion can be increased by unusual events such as earthquakes and cyclones, or by human activity such as cutting down forests. This is known as accelerated erosion.
When only some of the topsoil is affected, it is difficult to distinguish between normal and accelerated erosion. But it is quite clear-cut when all the topsoil and some subsoil has been lost from the original location.
North Island forests: protecting the soil
In the North Island, much of the land was once forested up to altitudes of 1,500 metres. The forest had developed over a long period, and trees gave stability through their deep, extensive root systems. Forest canopies lessened the erosive impact of rainfall, and tree litter recycled nutrients and protected the soil from wind and water.
Clearing the forest
From the early 19th century, settlers felled the trees for timber. They also cleared the land for farming, by logging trees and setting fire to the understorey. They sowed English grasses on the forest ash deposits, which provided an initial source of nutrients. But the nutrient value of the ash soon declined, causing a fall in pasture productivity.
From forest to pasture
Pasture has a limited root system and gives the soil less protection from rain than trees. This created a big change in how water interacted with the landscape. The water yields of forested catchments were equal to 15–20% of the rainfall, but under pasture this increased to 30–40% of the rainfall. The increased water run-off increased the risk of erosion, especially on steep land.
In the 1950s artist Avis Acres produced a weekly comic strip featuring two pōhutukawa fairies, Hutu and Kawa, who lived in the bush. Her stories showed an understanding of ecology and a strong conservationist ethos. In Hutu and Kawa find an island, she described the terrible impact of possums on native trees and birds.
South Island tussock
In the South Island, early colonists found land covered in both tussock and forest. To transform it into farmland, they mostly cleared the land by burning, and then hand-sowed pasture seeds.
Later, burning tussockland became a regular practice. The tussock ecosystem, which had developed in the absence of fire, became weakened. Its protective mantle deteriorated, particularly in the harsh environment of the high country. The erosive forces of wind, frost and water accelerated soil erosion where vegetation was sparse.
Impact of animals
Both domestic and wild livestock have played a part in accelerating soil erosion.
Rabbits were recorded for the first time in New Zealand in 1838, and were causing major problems by the 1890s. Overrunning many parts of the country, especially Central Otago, they stripped the pasture cover, which led to wind and water erosion.
Deer and possums, introduced in the mid-19th century, upset the balance of forest environments. Adding to the impact was overgrazing by sheep and cattle on tussock land in the South Island, and hill country in the North Island.
In the South Island, grazing sheep damaged the plant cover on the fragile mountain lands. Deer, tahr and chamois, brought in for hunting, further exposed soil to erosion.
At first, cultivation of farmland was confined to terrain of easy contour, mainly on river terraces near the main towns. But as farming moved on to steeper land, the loss of soil during heavy rainstorms increased. Cultivated soils were also eroded in areas with strong prevailing westerly winds such as Canterbury, the Manawatū and the North Island’s East Coast.