Retiring forests an option
The biggest forestry business in the top of South Island is considering retiring pine plantations on steep land at high risk of landslides, which threaten communities downstream.
Nelson Forests Ltd said it had identified around 5500 hectares, of its 60,000ha of productive forest in Nelson, Marlborough and Tasman district, that was in an area ‘‘considered to be at the highest risk’’ of landslide.
Forests estate value manager, Andrew Karalus, said research that the company commissioned Landcare Research to carry out, divided the estate, into ‘‘very high landslide susceptibility on the basis of slope and geology’’, down to low susceptibility.
Rainfall data would be used to assess the likelihood of a rain event that would trigger a landslide in those areas, to decide which catchments to examine first, to see what was at risk downstream.
‘‘What we’re trying to establish is, where have we got that risk to our downstream neighbours.
‘‘Immediately we can use history and say the areas we’ve experienced this problem is in what we call the Wangapeka Glenrae area [near Tapawera], and Onomalutu [near Blenheim].’’
It was perhaps unsurprising that the research identified Wangapeka as one of the highest risk areas, he said.
The land was ‘‘granite country’’, similar to that near Abel Tasman national park where multiple slips occurred around homes during excyclone Gita in February 2018.
Residents in the Motueka catchment have been calling for tighter rules around forestry for decades due to damage caused by flooding and debris in heavy rain events.
A company facilitator had talked to people living in the Tapawera area, and was about to start discussions with them about how to manage the risk.
‘‘That might be us retreating from some areas, it might be houses building structures around them,’’ Karalus said.
‘‘We might be able to get most of the gain with actually not a lot of retreat, but a lot of different practice.’’
Such changes could include pulling less forestry material over gulleys, or having higher standard of removing forestry slash, he said.
Karalus questioned whether, if the forestry company left high risk areas, it would solve the problem.
‘‘Even relatively stable [non forestry] land can landslide . . . if you had a big enough storm.’’
The community needed to consider, as well as the landowner, how best to use the land if it was retired from forestry, Karalus said.
Letting it revegetate into native bush would expose the land to landslides for longer than if replanted, and there was a question mark over who would pay deforestation liability, if the land was not going to be replanted, he said.
‘‘An analogy is ... roads and houses on the coastal fringe.
‘‘Somewhere, someone is going to make a decision that retreat is the right answer.
‘‘As much as you might have the view that, well I chose to live in a low risk area, so why should I pay? I think
you’ll find that we’ll all end up paying, because it’s the only practical way that we’re going to implement that.’’
The research was part of a $500,000 sedimentation research fund, announced by Australian owners of Nelson Forests Ltd, OneFortyOne, in September 2018.
It came after a Niwa study in April that year showed a link between pine forests and damaging fine sediment in Waimea, Moutere estuaries.
That study relied on a qualitative method, that was ‘‘open to error and overlap’’, Karalus maintained.
Information from Landcare Research showed landslides were considered to be the biggest contributor of sediment, he said.
Nelson Forests had been ‘‘more specific’’ however about how it controlled water that left its harvested sites, Karalus said.
‘‘This year, we’re setting up a paired catchment study, two catchments side by side, one with no [such] work in it, one with harvesting under our new controls, to measure what amount of sediment leaves.’’
Lees Seymour, managing director of Nelson Forest’s management company, Nelson Management Ltd, said clear felling (the practice of harvesting blocks of plantation forests in one go) in smaller areas, wouldn’t necessarily reduce the risk of landslides, and therefore sediment.
‘‘You can have multiple small blocks harvested across your whole estate, or you can have larger blocks in less parts of your estate.
‘‘The single biggest determinant of landslide is the rainfall, so if you increase your exposure to rainfall events, then you might have more landslides.’’