Hunua Forest: Landscape-scale restoration

Watercare purchased Hūnua Forest to protect Auckland’s water supply. Since 2018, Whirka has supported managers to undertake one of the largest native forest restoration projects in Aotearoa – to revert 2,300 ha of pine plantation to native forest.

Why replace a productive plantation? Soil erosion associated with wood harvesting on the steep slopes threatens Auckland’s supply of clean water.

Adding to the complexity of the project, weeds are a problem on cleared sites and herbicide use is forbidden within the reservoir catchments to ensure there is no contamination of drinking water.

Our challenge is to find affordable and effective methods of native regeneration, which work quickly to minimise the potential for soil erosion and weed invasion. 

Working with Watercare staff, our role is to plan and advise on the best pathways for native forest restoration across the area.

In 2017, key findings of baseline vegetation surveys showed that there is great potential for pine canopy thinning to be an effective and efficient method for native forest restoration. On sites too steep to clearfell, we’re monitoring a trial of different thinning densities, where trees are felled and left on site to rot, allowing native plants to regenerate under a gradually opening canopy.

On flatter sites, we’re monitoring different ways of planting cutover areas, to see which site preparation technique, planting density and maintenance method promotes the fastest growth and canopy closure, without the use of herbicides. 

From two years of permanent sample plot monitoring, results show that preparing the site by mulching forestry debris is the most effective method for establishing native forest and weed control.

Exploring all options, we’re collaborating with a nearby nursery evaluating the cost/benefit of using different sizes of native seedlings in plantings. 

Monitoring revegetation is important, so we’ve introduced the use of multispectral drone imagery to evaluate vegetation cover over time. Correlating this to our permanent sampling plot data, we’re building a rich dataset that will help with large-scale reforestation implementation as the project steps up in scale.

The results of these trials will help plan for the planting of one million trees per annum, beginning in 2024.

The vision is that the regenerating forest can be transferred to the people of Auckland as an extension of Hūnua Regional Park, home to kākā, kokako, kiwi, and for everyone to enjoy.