Nearly all felling in New Zealand cable logging operations is mechanised and winch-assisted, which is a major change from just 10 years ago when the majority was still manual. More recently, winch assisted shovelling and skidder extraction are also being used more extensively.The safety and productivity benefits from winch assist on steep slopes are well known. What remains a concern after harvest in some locations is the level of soil disturbance and subsequent risk of erosion. While it is easy to see that operating machinery on steep slopes disturbs soil, the question is: How quickly does a site recover?
Answering this question as a Master’s project involved visiting a total of 14 sites around the country, from Lawrence in the South up to Whangarei in the North, and at each location completing a soil disturbance survey at a recently winch-assist harvested area, as well as at one-, two-, andthree-years post-harvest.
At each site these harvest areas were chosen to be close together to minimise variability in terrain, harvest methods, soil, and weather conditions. A line transect method was used to document disturbance types such as soil mixing, topsoil removal, rutting and changes in slash and vegetation cover. The process involved walking a contour across a site, recording one of 10 soil disturbance codes each metre. Across the 14 forests and four age classes, more than 52,000 geo-located observations were collected.
Expectations and differences
As one would expect, soil disturbance and slash cover decreased, while vegetation cover increased as time progressed. Slash cover averaged about 55% after harvest, this decreased to around 15% by year three. Vegetation covered just 2% of the site after harvest, but this increased to an average of 65% after three years. There was a big spike in year two, after release spraying in year one. Slash decreasing and vegetation increasing meant there was a slow increase in ground cover overall.
Overall, less than 2% of the harvest area was affected by deep rutting, which is the feature we should be most concerned about. This declined to less than half a percent over the three years, but this was because vegetation was growing over these features, making them invisible, as opposed to them disappearing from the soil profile. Ruts deeper than 15cm were most common.
Disturbance was very different between forests, expected to be mostly because of weather and soil differences. A big difference can also be made with different harvest systems and operator care, but there wasn’t enough data to separate this in this study. Progression over time was also very different. For example, at some sites, ruts and loose soil were far more likely to become erosion features. Areas with topsoil and areas with loose soil grew vegetation faster than areas where the subsoil was exposed and compacted. The disturbed soil area decreased by 50% over the three years after harvest.
Rills peaked one year after harvest, then decreased thereafter. Slips were the only feature that didn’t decrease in frequency over the three years after harvest. This aligned with the idea of...
