A report commissioned by the NZ Farm Forestry Association and the Forest Owners Association provides guidance to landowners preparing to plant trees on steep, erosion-prone land now zoned red under the new National Environmental Standards for Plantation Forestry (NES-PF) which came into effect on May 1.

The report was written by Dean Satchell, of Kerikeri environmental services firm Sustainable Forest Solutions. He is also immediate past president of the Farm Forestry Association.

It follows major devastation at Tolaga Bay on the East Coast when a storm in early June hit recently harvested and replanted sites causing slash mobilisation which made news headlines.

Satchell says owners of red-zoned land who wish to clearfell need to provide regional councils with evidence that significant adverse environmental effects can be minimised.

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"This report provides information on best practice, identifies the gaps in knowledge and sets the scope for the future to improve environmental outcomes from plantation forestry on steep lands," he says.

The report includes details about a wide variety of trees suited to steep erosion prone land because they have root structures that may better resist landsliding after harvest. Four most suitable "alternatives" to radiata pine on steep lands are eucalyptus (stringbark/ash), redwood, cypress and poplar, with totara showing the most promise among native tress in terms of overall potential as a profitable and productive plantation species.

Current Farm Forestry Association chairman Neil Cullen said more forests were needed on steep hill country to mitigate erosion where pastoral cover was not enough.

"However, we need the right species, the right rotation lengths and the right harvesting strategies for the best environmental outcomes. Forestry is the best land use for erodible hill country, but best practice changes over time to meet the expectations of society and increasing severity of storms."

National Environmental Standards

Standard-practice radiata pine clearfell regimes are no longer a permitted activity on steeper (red zoned) slopes greater than 2ha. The NES-PF specifies red zone land can now only be planted or replanted with a territorial authority resource consent and the application will be subject to detailed risk assessment. Although the assessment aims to mitigate adverse environmental effects, such as storm-initiated slope failures with the potential to form debris flows that could result in damage to downstream infrastructure and property, this requirement may potentially increase compliance costs, particularly for growers operating in steeper terrain.

Measures available to reduce erosion in a clearfell regime include:

-Undertaking only best-practice earthworks (eg, as per NZ forest road engineering manual)

- Minimising soil disturbance and compaction when harvesting

- Managing slash to minimise risk for entrainment in debris flows

- Providing buffers between productive areas and water courses that act as slash traps

- Identifying areas with excessive risk of erosion and retiring these from productive use

- Replanting or planting at high initial stocking rates and reinstating vegetative cover as soon as possible to mitigate erosion after clearfell harvesting.

Erosion mitigation practices

A national erosion susceptibility classification (ESC) has been developed to support the NES-PF. Current limitations of the ESC include the scale of mapping, the quality of underlying data and misclassification of land.

For red-zoned land, in the absence of evidence supporting risk mitigation measures, the adequacy of such measures would be at the discretion of territorial authorities. Although the risk level itself can be assumed from the ESC; setting conditions for consent to meet specific performance thresholds such as estimated sediment yields would need to somehow match the measure to the risk. This could potentially be achieved by factoring in tree stocking rates, rotation length for alternative commercially viable species and measures restricting the likelihood of debris flows.

The report says more research is needed to quantify the erosion-control effectiveness of "alternative" species to radiata pine. There was also a need to better understand relationships between alternative species restocking rates on forest cutover, and their level of effectiveness in mitigating post-harvest sediment generation rates, relative to that measured for radiata pine planted at the same densities.