Forest owners say floods reduced by trees

25 Jul 2005

Forest owners say downstream damage and siltation during the recent Coromandel floods would have been much worse if there had been fewer pine plantations in the hills. 

NZ Forest Owners Association chief executive David Rhodes says there is a “mountain of research" to show that forestry is the most sustainable use of erosion-prone hillsides.

He was responding to concerned residents who have blamed forest owners for silt washing off recently harvested hills.

“Forest owners and workers are among those whose lives have been turned upside down by recent floods. But the reality is that the East Coast and Coromandel Peninsula are prone to high intensity storms which, coupled with steep terrain and unstable geology will inevitably result in landslips and other forms of erosion regardless of the land cover.???

He says rainfall, topography and geology are the main factors which determine soil erosion rates, but forests greatly increase the stability of slopes and soils.

“This was clearly demonstrated on the East Coast during cyclone Bola in 1988 and in the 2004 Manawatu floods, where pasture-covered hillsides were shown to have 5-10 times the erosion risk of hillsides under forest, scrub or bush.???

He says the highest erosion rates in the life cycle of a forest occur in clear-felled areas 2-8 years after felling as the old root system decays and the new one develops.

“But at all stages during the forest life-cycle the erosion risk in well-managed forest is less than from pasture, which is the land-use that these trees have replaced."

A proposed ban on clear-felling steeper hillsides, to further reduce erosion in forestry, is rejected by Mr Rhodes as self-defeating.

“If we did not harvest trees on steeper ground, they would become over-mature, top-heavy and prone to mass windthrow during severe storms. Accelerated erosion and unstable log dams in gullies would be the inevitable result.

“Mass windthrow and erosion even occurs in unmanaged native forests, so doing nothing is not a solution either."

Mr Rhodes says the best solution is to keep refining forest management systems so the erosion risk is minimised.

“New Zealand forest management practices are among the world’s best, thanks to a stringent Code of Practice which is constantly being updated on the basis of research findings."

He explains that significant research underway in Coromandel forests has helped forest researchers and foresters to get a better understanding of erosion and how to minimise it.

“For example, it is now normal practice to leave riparian strips of unplanted native vegetation along streams in forests. These help filter silt arising from logging operations and keep logs and trash out of waterways.

"However, it is not as simple as just leaving a strip of fixed width unplanted. There is now a highly technical decision support service set up in association with Scion research in Rotorua.

“The plantation forest industry takes great pride in being environmentally sustainable and takes the concerns of local communities very seriously. But issues like erosion and flooding have complex causes which need to be addressed calmly and rationally."

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For more information, please contact: David Rhodes, Chief Executive, NZ Forest Owners Association. Tel: 04 473 4769. Mobile: 0274 955 525