M¯anuka replacing pines on erosion-prone M¯arahau hills

The first 50 hectares of ma¯ nuka seedlings have taken well on erosion-prone land behind Ma¯ rahau.

Planted at a density of 1000 stems per hectare over the winter of 2019, the trees are permanent replacements for commercial radiata pine trees on land in the Ma¯ rahau catchment that is at high risk of slips and erosion.

They are the first 50,000 of 114,000 ma¯nuka trees to be planted in the area over four years – a programme of work known as the Ma¯ rahau-Otu¯ whero ma¯ nuka reversion project.

Tasman District mayor Tim King said it was good that the initial plantings had taken well. It was a case now of finishing the planting and monitoring the effectiveness of the switch from pine to ma¯ nuka.

‘‘The key thing is what happens when it rains, and particularly when it rains heavily,’’ King said.

Intense rain that arrived with extropical cyclone Gita in February 2018 caused multiple slips across the district, including on the forestry land behind Ma¯ rahau, which was returned to iwi in 2014 as part of a Treaty settlement.

The replanting project has been designed to help mitigate the effects of adverse weather in areas with highly erodible soils known as Separation Point granite, a strip of granitic bedrock that extends from Separation Point in Abel Tasman National Park to Mt Murchison.

A partnership between iwi and the council is driving the replanting in the Ma¯ rahau catchment, in an effort to manage sediment and land disturbance.

Nga¯ ti Tama ki Te Waipounamu Trust, Nga¯ti Ra¯rua Settlement Trust and the council attracted $144,000 from the Ministry for Primary Industries’ Hill Country Erosion Fund as a grant for the project. That grant is to be topped up by $140,000 each from the council and iwi. Tasman Forest Management is managing the planting operation.

King said the information gained from the project, which he described as a trial, would be shared.

Replanting ma¯nuka instead of pine could be ‘‘another tool in the toolbox’’ for landowners and forestry companies to manage erosion, he said.

Another 22ha of the land in the Ma¯ rahau catchment is due to be replanted in ma¯ nuka over winter as part of the project, followed by 32ha during winter 2021 and the final 10ha over winter 2022.