Mid Canterbury is at risk of future flooding, but there is no major risk from forestry slash (waste product from commercial forestry) causing the same devastation that occurred in the upper North Island.
The location of forestry blocks and the geography of the region meant there was a low risk from forestry slash build-up in the rivers, Ashburton District Council group manager business support, Leanne Macdonald, said.
“The recent rainfall events that resulted in the large-scale mobilisation of tree slash in the Tairāwhiti, Hawkes Bay, and Nelson region were related to large-scale forestry covering significant catchments characterised by steep terrain and erodible soils, which is quite different to Mid Canterbury’s landscape.
“It is very unlikely to have slash from the council’s plantations entering rivers during flood events.
“Most of the council plantations are on the plains away from rivers.”
In the forestry industry, slash is considered any vegetative debris generated by the forest – all the parts of the tree not utilised following harvesting including branches.
Macdonald said slash that is mobilised during a flood can include harvesting slash from a plantation, but can also include other trees involved in landslides and the vegetative material from the river berm including willows, poplars and various weed species.
That occurred in the Ashburton River in the 2021 floods.
Slash generated during a harvesting operation is ultimately the responsibility of the forest owner, Macdonald said.
“When engaging contractors to harvest plantations, the treatment of slash is a priority and can form part of the sign-off for operations.
“Ideally, slash piles will be avoided or at the very least, reduced [to] below three metres high.
“At the end of harvesting, the cut-over area will have the slash windrowed to allow access for replanting.
"The practice of burning slash was stopped many years ago.”
The treatment of slash is addressed in logging plans required by the National Environmental Standards for Plantation Forestry (NES-PF) and Macdonald said the plans are required to be submitted to both district and regional councils.
The council’s forestry plantations cover 1330 hectares, of which Macdonald said 1079ha is currently stocked, with 22 hectares waiting to be restocked this winter.
Forestry slash has become a major talking point following Cyclone Gabrielle, with the Government launching a ministerial inquiry that will make recommendations to improve land use, including changes needed to practices and regulations at central and local government levels.

• By Jonathan Leask