Little of Hawke's Bay woody debris is forestry slash

Little of Hawke's Bay woody debris is forestry slash

Friday 21 Apr 2023

Piles of wooded debris that dammed bridges, blocked riverways and littered beaches in the wake of Cyclone Gabrielle contain only a small portion of forestry slash, a report by the Hawke's Bay Regional Council has found. It’s a different story in neighbouring Tairāwhiti, where forestry slash has been widely blamed for much of the damage caused by the cyclone, with the Government ordering a ministerial inquiry into forestry practice and the slash.

The council report released last week found the make-up of the woody debris deposited at more than a dozen sites consisted of a mixture pine, willow, poplar and ‘other’ - native timber and debris that could not be identified.

“At all but one of the surveyed sites, there was little evidence of slash, indicating that the majority of pine came from erosion of hillsides and streambanks,” regional council group manager asset management Chris Dolley said in a statement accompanying the report. Pine plantations and wilding pine are found extensively across the East Coast – Tairāwhiti has 157,295ha in radiata pine plantation forestry, while Hawke’s Bay has 139,598ha.

“A major infrastructural risk was associated with [large woody debris] accumulating at road and rail bridges throughout the region,” the report stated. “A total of 18 bridges suffered some degree of damage during Cyclone Gabrielle within the Hawke’s Bay region alone.”

The survey used guidelines created and used in neighbouring Tairāwhiti, and looked at the woody debris composition of 17 sites including multiple sites along the Esk, Tutaekuri, Ngaruroro, Mohaka, Waikare and Wairoa rivers. Dolley said the debris composition differed from catchment to catchment and was largely the result of whatever tree species was dominant in the catchment upstream.

The regional council were particularly interested to see whether the pine debris included harvested logs, “slash”, or were whole or parts of trees sourced from erosion of hillsides and streambanks.

While the average pine presence was 35% higher compared with willow/poplar, the majority were pine pieces and didn’t show any cut marks indicating they were the result of forestry practices. Cut pine made up 3% of the large woody debris within the Hawke’s Bay region, compared with 15% found in Tairāwhiti during previous storm events. The site with the most cut pine was Mangaone at Rissington, where 9% of the timber had cut marks.

Wairoa mayor Craig Little said his district was a “different story from the rest of Hawke’s Bay”, where slash was thought to be more widespread.

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