A "wall of water" filled with forestry slash laid waste to parts of a Kaiteriteri holiday park on Friday, while Ngātīmoti residents were faced with cleaning up extensive damage from logs and silt from a forestry block on steep land about two kilometres upstream.

Forestry practices and climate change have combined to create a ‘perfect storm’.

Nathanael Melia, climate scientist and founding director of Climate Prescience, says  a recent jump in temperatures has seemingly correlated with increases in extreme weather events worldwide, from heatwaves and wildfires to floods.

“In New Zealand, this period has also coincided with plantation pine trees reaching maturity and the land subsequently undergoing clear-fell harvesting."

“I would argue that in this decade-long period of mass clear-fell harvesting, climate change has weaponised plantation forestry with flashier flooding and mobilised timber torpedos.”

Melia explains that a mature forest provides free flood mitigation for its catchment, leaves intercept rainfall, roots stabilise soil, and trees take up water. “I suspect that clear-fell harvesting has the opposite effect, with all the rain reaching the ground, hitting bare soil, and running off faster.

“The other hazard is that of forestry slash, the bits of tree that have been left post-harvest. When these now vulnerable slopes fail, the slash falls down into the streams and rivers and is then transported with the flood.”

He says there is a tragic irony in this for forestry. “Pine plantation forestry essentially provides two benefits for New Zealand, economic and climate mitigation. Let’s run some back-of-the-envelope numbers on NZ forestry and climate. Over the last ten years, global temperatures have jumped ~0.4°C, if I’m feeling really conservative, we can say it’s 0.2°C.

“Meanwhile, if New Zealand forestry removes ~200Mt CO 2 per decade, that is about equating to 0.0001°C of cooling, reducing the impacts of climate change by that amount. Now, when a flood event hits, climate change is typically calculated to make the event 10% worse than it would have been.

“So the question then becomes, which is better, a 0.0001°C of reduction on 10% of the impact of a flood event, or whatever the effect of clear-fell harvesting is on worsening the impact of the flood event?”

Melia wants to see changes to clear-fell harvesting practices. “For me, clearly the answer is to transition from clear-fell harvesting on vulnerable slopes. However, I’ve said this before in the Tairawhiti slash investigations, and essentially nothing changed. This issue is that the pine model is only economically viable with clear-fell harvesting, and people have a right to employment.”

Melia says it’s also a concern that high-density housing has been approved on the Maitai floodplains. “Harvesting these catchments will lead to higher and faster floods until mature trees return.”

The government is currently rushing through legislation that is intended to stop farm-to-forestry conversions in the Emissions Trading Scheme, but which sustainable land-use campaigners say will force more forestry onto high erosion risk land, meaning that the worst land will become the only land left for “the most intensive and destructive land use.”

Last week the Independent Reference Group on Climate Adaptation published its report intended to advise the government on policy. However a climate adaptation expert has slammed its approach to managed retreat, saying it is misguided, morally questionable, administratively inept, and politically naïve.