Passive forest regeneration is where native plants grow on your land without you having to plant them. It occurs if there's an existing native forest (a seed source) nearby and the conditions on your land support the regeneration of native plants.
Despite the name, you might need to encourage passive forest regeneration by:
Active forest regeneration is where you must plant native trees to grow the forest. Other active regeneration methods can include:
In many areas, native forests will regenerate by themselves if given enough time. But that could be many decades or centuries. In this article, we talk about regeneration that would take about 2 or 3 decades.
When assessing your land for passive or active forest regeneration, the first questions to ask are:
Before humans arrived, New Zealand featured 85–90% forest cover. So, there’s a good chance your land can support native forests.
There are areas where native forests won’t regrow, such as on cliffs and sand dunes. Native regeneration will also struggle in drylands east of the main mountain ranges in the North and South Islands.
There are other factors that make it hard to regrow native forests. These include:
If native trees are already regenerating on your land in adequate numbers, you can opt for passive methods of regeneration. You want enough native regeneration to create a closed forest canopy when the trees are mature.
Look for signs of native trees that are growing near your planting site. Locations that are more likely to support the regeneration of native trees, include:
There are 5 factors that suit passive forest regeneration, even if you can’t see native regeneration on your land. These factors are:
You’re more likely to succeed with passive forest regeneration methods if your land has a:
Native forests can still regrow in areas with less rain and colder temperatures. But you’ll find the active growing season is shorter.
See NIWA’s climate maps for the mean annual rainfall and temperature in your region.
National and regional climate maps – NIWA
Native trees are more likely to regrow by themselves if there’s a native seed source close to your planting area. The closer the seed source, the better.
You should also investigate the makeup of the seed source. Is there a good mix of native plants in the existing forest? For passive forest regeneration, it’s best to have a diverse seed source. If necessary, you can plant other native seedlings to bolster natural regeneration.
Consider the terrain on your land:
You’ll find passive regeneration works best in gullies and on lower slopes compared to ridges and upper slopes. Gullies and lower slopes have greater soil moisture and provide more shelter for growing plants.
Native forests also regrow more quickly on south-facing slopes compared to north-facing slopes. This is because south-facing slopes are cooler and wetter.
It’s also useful if the terrain shelters growing plants from the prevailing wind and limits the direct sunlight they get.
Shade-tolerant native seedlings grow quicker if there’s existing woody cover (a nurse crop) to provide shade and shelter. Suitable native woody cover includes mānuka and kānuka. But exotic species like gorse and scotch broom also work.
As well as providing shade and shelter, woody cover reduces competition from grasses and other exotic plants that need light.
The methods you need to use to encourage regeneration will most likely sit on a continuum between fully passive and fully active.
If you have all 5 factors above on your land, there’s a good chance passive forest regeneration will work for you. You might only need to monitor and protect native regeneration from pest plants or animals.
Where you only have some favourable factors, consider what interventions will help passive forest regeneration. For example, you might need to plant woody species to block out light to grass and kick-start native regeneration.
If you have no favourable factors on your land, you need to rely on active forest regeneration methods.
It’s a good idea to get expert advice on what native species to plant, and where and how to plant them.
There are different types of interventions to encourage passive forest regeneration:
You can plant native trees into existing native regeneration to:
You need a thorough plan to introduce native plants into an existing forest. Seek expert advice here as native seedlings will struggle if you plant them in the wrong place or at the wrong time. You can also damage the existing forest if you plant the wrong species or plant seedlings with disease.
To encourage passive forest regeneration, remove shade-tolerant weeds or vine weeds that compete with native plants for light.
If you plant native seedlings into an existing forest, you might need to increase the light they get by creating gaps in the forest canopy.
For passive forest regeneration to work, you need to plan to:
The forestry advisory team at the Ministry for Primary Industries (MPI) can help you make decisions about growing trees and establishing forests.
To contact a forestry adviser, email forestserviceadvice@mpi.govt.nz
For more information about the forestry advisory service, see the MPI website.
There are things you need to consider when planning to revert land back to native forest, if you want to register the forest land in the ETS. Find out more in this guide:
Native forest reversion and the Emissions Trading Scheme (ETS) — Guide
For further reading on native regeneration, see the article by Adam Forbes et al in the New Zealand Journal of Ecology.