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THE DEVASTATING FLOODS THAT WASHED thousands of tonnes of forest debris down rivers and onto rural homes, farms, roads and beaches north of Gisborne last month has brought calls from forest leaders for everyone to lift their game when harvesting fragile steep slopes.
The industry has been sharply criticised by Forestry Minister Shane Jones and others for contributing to the estimated $10 million damage from the flooding in Tolaga Bay over Queen’s Birthday weekend.
“The forestry sector has enjoyed a laissezfaire set of rules and society’s attitudes are changing,” Mr Jones told media after witnessing scenes of logs and slash washing around bridges and surrounding rural homes.
As the clean-up continues from the event and the rains that followed a week later, foresters and the local council are seeking questions as to why so much debris was washed into rivers. And, more importantly, what can be done to prevent it happening again in future.
Peter Weir, President of the NZ Forest Owners Association and Environment Manager for Ernslaw One, which is harvesting forests on the East Coast, says the events will inevitably trigger a review of both harvesting practices and silvicultural strategies on land now zoned Red or Orange in the recently introduced National Environmental Standard (NES) for Plantation Forestry.
He told NZ Logger magazine that the risk of establishing short rotation plantation forests on some of the most erodible land in the country after Cyclone Bola may be “coming back to bite”.
But he adds that if everyone adheres strictly to the provisions of the new NES then some of the impacts in these sorts of events can be prevented.
“One of the provisions in the NES is for any slash to be moved to above the level of a 1-in-20-year flood, which would remove it from rising floodwaters in most (but not all) storms, which is where the danger lies. It’s not that slash is being washed down the hill – that doesn’t happen,” he says.
“These sorts of storm events are happening with much more frequency and more intensity, so it really is vital to get that material taken well away. The challenge is to estimate just where a 1-in-20-year flood might reach.”
Mr Weir suspects that some of the debris that ended up down river last month might have been logs that were left on steep slopes because they could not be retrieved in cable operations without achieving partial log suspension. The NES now effectively prohibits ground-leading because of the inevitable slope gouging and excessive soil disturbance. He says some could be the result of poor planning or poor harvesting methods from the past, as the square-ended stems appeared to be at least three-to-five years old.
“It goes right back to harvest planning and the need to prepare properly for bringing logs out of difficult areas,” he says.
Lidar now makes new high resolution Digital Terrain Models (DTM) available to assist with detailed harvest planning and “it’s especially useful for deflection and payload analysis for cable shows,” Mr Weir says, adding: “I wouldn’t harvest plan in Red Zones without the assistance of a Lidar DTM.
“It is equally vital for the fallers to make the right decision about whether to fall trees in locations that may be impossible to retrieve – if it can’t be pulled, then don’t drop it. Contractors need to be intimately involved in this decision-making process.”
Forest managers should also be checking harvested sites to install, maintain and clear well engineered woody debris traps for five or six years after the last trees are cut down, although these can sometimes be overwhelmed by the amount of rain in extreme events. And woodlot operators cannot walk away from this responsibility in Red Zoned or steep Orange Zoned terrain.
Mr Weir also says the practice of leaving slash over the side of hill-top landings will also need to be risk managed and says his company is already pulling it back with long reach diggers and working towards burning it once the harvesting operation has completed, as happens in some other parts of the country. There is always the potential for poorly compacted fill on the edge of landings to collapse in a really big storm, taking debris down the hill, which has happened in the past.
“Burning slash on landings takes that issue out of the equation,” he adds, though resource consent conditions prevent all slash on the slopes being burned.
Mr Weir says forest owners and contractors working on Red and steep Orange Zoned land will need to be much more aware of the likelihood of severe weather events happening in the future. It is predicted that large intensity storms like the one that hit Gisborne will be four times more likely to occur.
That’s a major issue for the 140,000 hectares of pine forests identified as being planted on highly erodible Red-Zoned land, one-third of which is on the East Coast.
Peter Clark, the soon-to-retire CEO of PF Olsen, says Red Zone land requires a resource consent, involving a Land Management Plan to establish a plantation forest.
“Conditions may be imposed by Regional Councils, even for a replant, that seek to avoid excess erosion or debris flows at harvest time,” he says in the company’s latest newsletter.
“The conditions may cover species, timing of harvest, harvest coup size and unplantable reserve or retirement areas. Riparian or other land deemed too risky to harvest would be excluded from planting, other than in permanent species.”
He predicts that the productive area of much of the East Coast hill country will shrink as more land gets retired from harvesting and could go into Mānuka, planted for bees to harvest honey
He adds: “Land values for such land will likely fall and some investors will simply avoid the region, rather than risk extremely costly mitigation measures or community backlash against forestry.”