Conservation Comment: Let's look at possibility pine trees cause of ruinous slips

The June flood and slips were a severe setback to my native forest project here in Aramoho. The greatest loss, conservation wise, was a miniature mature forest that slid off the hillside down on to Roberts Ave.

Now it is just a papa face with 4000 cu meters of soil at the bottom.

Lying among the mud are mature lacebarks, mahoe, pigeonwood, coprosma, mamaku ferns and kawakawa with less mature species like Fuchsia excorticata, rimu and tawa which I planted over the last decade.

Running from this into the stormwater drain on Roberts Ave and on down to our river are vast quantities of yellow muddy water, silt and dead biota.

I cut a disc out of a large lacebark tree that had slipped, to find its age. It was 63 years old and had a girth of 420mm. The root ball was 1800 deep by 4000 diameter. So this little forest had been there at least 70 years, probably many more. What caused its demise?

I believe I know the answer to this. Glancing up to the top of the slip, I can see three large, decaying pine tree stumps. Their decaying roots can be seen embedded in the surface of the papa as far as 20m down the slip face.

The slip can be clearly seen to start from these three stumps.

These 80-year-old Pinus radiata trees were cut down several years ago because they endangered overhead power lines.

I believe rain on these decaying pine tree roots would have created a "slip plane" on the papa base underneath the established vegetation.

As an engineer, I am familiar with Newton's "Forces on an inclined plane" and that the record rain we received could have provided the force required to cause this disaster.

Cutting down large pine trees on hilltops for infrastructure "needs" is common practice. Is this why many of our hillsides are slipping away?

These pine trees are a northern hemisphere species that has different behaviour patterns to our native conifers, which reach maturity after thousands of years. Pinus radiata are mature at 70 years in our climate, with vigorous growth that threatens local stability.

The slips on my 10-acre property are only one tiny part of our river catchment area. Imagine the amount of degradation over our whole Wanganui catchment.

The yellow run-off from these slips make our river toxic, I have seen flounder jumping out of the estuary to die on the riverbank when the water starts to discolour. Our river are the "heart, lungs and organs" of our atmosphere. We must help Nature conserve them. We could analyse each slip to find the cause and evolve a future code of practice.

- Rob Butcher is a retired engineer and conservationist