Expert witness under scrutiny

The ecological value of willow wetlands alongside the Wairau River and the expertise of TrustPower expert witness David Slaven came under scrutiny at the Environment Court in Blenheim yesterday.

At a hearing on TrustPower's proposed hydro scheme in the Wairau Valley, Boffa Miskell ecology section head David Slaven said the canals, spillways and power stations would not directly affect any significant native vegetation. However, the regulation pond below the Wairau Power Station would flood a remnant stand of kanuka forest.

Mr Slaven said the health of riverside willow wetlands could be affected by a drop in groundwater levels due to drawdown by the scheme, but the wetlands contained mostly exotic weed species. He believed the willows, some 70 years old, would survive the change as their roots aggressively sought available water. Total dieback of the wetlands was unlikely, he said.

Fish & Game lawyer Marie Baker asked Mr Slaven whether he was aware of wider ecological values of the wetlands, such as supporting the native plants they sheltered and bird species.

Ms Baker quoted several mentions of significant native plant species found in the riverside wetlands in a report by ecologist Philip Simpson, contracted by Boffa Miskell to rate vegetation along the route of the scheme.

Mr Simpson's report concluded that "the willows are an important habitat and their health should be maintained. However, they are very weedy and reduced water flowing through or beneath them is likely to increase their weediness."

Mr Slaven didn't dispute there was a native flora component to the wetlands but, using standard conservation values, he ranked their botanical value as low.

As to other conservation and landscape values, Mr Slaven insisted that he was a botanist and not qualified to assess these.

This was later challenged by commissioners Alex Sutherland and Helen Beaumont, who pointed out that in his CV, Mr Slaven described himself as "a professional ecologist for the past 20 years".

Ms Beaumont said that she had relied on the ecological expertise of Mr Slaven to draw together the findings of all terrestrial investigations.

Rather than just presenting data, she expected him to evaluate its significance.

Asked what quantity of wetland could be lost as a result of the scheme, Mr Slaven said he did not know.

"If that is so, on what basis do you assess the loss as not significant," asked Ms Beaumont. "I am not talking about the loss of willow trees ... let's talk about wetlands as a system rather than willows."

Mr Slaven remained adamant that his assessment was based on few native plants being present, although they might have amenity or gamebird values, for example.

He said that wetlands on terraces of the Wairau River continued to be drained, cleared and destroyed by stock and would likely disappear in 10 years.

Mr Slaven anticipated a 0.3 to 0.4-metre drop in groundwater levels in the vicinity of the willow wetlands when the scheme was at full capacity and river flows were low.

Mr Sutherland revisited the evidence and suggested that this was wrong. Along the Hillersden stretch – 75 per cent of the river length affected by the scheme – groundwater levels would actually drop 0.6 metres.

Mr Slaven said this drop was greater than he had suggested and there was an increased threat to the health of the willow wetlands.

"But from a purely botanical perspective, I do not see their demise as being significant."

Ms Baker asked why the botany of the braided riverbed had not been surveyed, but Mr Slaven said it would be too difficult because it was dominated by exotic weeds and changed with every fresh. The Department of Conservation backed this.

Asked whether he had considered using riparian transects, which recorded species growing on representative areas of the riverbed, Mr Slaven said that he was not familiar with this technique, a claim that was later challenged by Mr Sutherland.

"We were relying on the expertise of Mr Simpson," Mr Slaven said.

TrustPower plans to mitigate vegetation loss caused by the scheme by restoring the Traverse Spur area at the top of the scheme.

It would include open water and a wetland around the margin of the 13 hectare regulating pond to keep the wetland wet as water levels dropped 1.7 metres as often as twice in 24 hours.

Areas of native vegetation would be protected and about three hectares of native bush would be revegetated.

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