How Forests Work

We tend to think of forests as being static, unchanging ecosystems. However, many of our forest remnants are undergoing a phase of re-establishment and repair. Many are in a transitional phase containing fast-growing colonising species and emerging slow-growing canopy species. Find out more about the process of change (succession) and its importance when planning a restoration project at your patch.

Natural succession

In their natural state, mature forests can remain in a stable state for long periods of time. However, even before the arrival of people to New Zealand, our forests had to develop strategies to re-colonise areas destroyed by large-scale events such as volcanic eruptions, fires or landslides.

The process of re-colonising a bare patch of land is called ‘succession’. It takes many years for the process of succession to transform an area from bare land to mature forest. The model of succession is outlined below:

Model of succession.Model of succession.

During the succession process each group of plants makes the site suitable for the next group of plants, but no longer suitable for themselves. These plants are called ‘colonising’ or ‘nursery’ plants. They are eventually replaced until mature canopy trees establish. The forest then stays in a period of relative stability until another large-scale event.

Colonising or ‘nursery’ species

Cabbage trees are a common colonising species in Southland.Cabbage trees are a common colonising species in Southland.Colonising or nursery species (also called primary or pioneer species) grow well on bare ground in open sites. They are hardy, easy to grow and will generally tolerate extremes in temperature, terrain, soil type and drainage. They provide the perfect habitat for forest canopy species to establish and grow.

Examples of Southland colonising species include manuka, mingimingi and cabbage trees.

Regenerating forests dominated by colonising species are often referred to as ‘scrub’ in New Zealand. Unfortunately ‘scrub’ areas are often misrepresented as wastelands or unproductive areas, rather than as an important stage in forest restoration and recovery. For this reason we will refer to scrub as ‘shrublands’.

See our plant lists for species suitable for replanting Southland’s forests and shrublands.

Mature forest species

Many of the forest trees that thrive in mature forests cannot survive on bare, exposed sites. In the forest, they are protected from wind, desiccation (drying out) and the extremes of temperature. They can only establish when protected by the more hardy colonising species. It’s important that this is taken into account when undertaking a restoration project.

Forest layers

Most of the forest remnants in Southland contain a mixture of podocarps and broadleafs and are referred to as podocarp/broadleaf forests. In their natural state these forests contain many layers of vegetation:

Emergent Trees – over 20 metres tall and tower above the forest. These are the podocarps, for example rimu, kahikatea and totara.

Canopy/Roof – up to 20 metres tall, forming a dense layer of foliage that filters rain and sunlight for the plants below. These are the broadleafs, for example, kamahi and pokaka.

Sub-canopy – 5 to 10 metres tall, for example, tree ferns, mahoe, wineberry and fuchsia.

Small Trees/Shrubs – no taller than about 5 metres. This layer is shaded by the trees above. Many of the trees in this layer are young; they are waiting for a gap to open up so they can grow tall.

Forest Floor – mostly mosses, ferns and seedlings.

Climbers and creepers – found growing from the forest floor to the canopy.

A healthy forest remnant should have many layers. However, many remnants have not been protected from stock or animal pests. Understorey vegetation is often absent and is unable to regenerate under pressure from browsing. Many restoration projects will involve fencing stock out and protection from animal pests. This allows the understorey to regenerate, forming a more natural forest structure.

The forest edge

Emergent canopy trees.Emergent canopy trees.In a natural forest situation there will be a gradual transition from grassland to tall forest. The plants growing in the transition zone or ‘edge’ will gradually increase in height, forming a protective buffer zone for the forest interior. This streamlined effect allows wind to flow over the forest, rather than through the understorey. The forest interior remains protected from the wind and is able to stay damp and humid, ensuring optimal growing conditions.

However, many forest remnants have lost their natural ‘edge’. Instead there is often an abrupt change from forest to farmland. Wind can blow directly into the remnant interior, producing dry, cool and generally unfavorable conditions.

An important step in protecting your forest remnant is to recreate a forest edge. You can do this by planting hardy colonizing species around the perimeter of your remnant. This will be particularly important on the side that’s exposed to the prevailing wind. For more information on what to plant, check out our information on Southland’s forests.

Planting your patch

Before planting at your patch, it’s important to remember that many of the weeds that thrive in Southland are expert colonisers. They thrive on bare sites and will outcompete your precious plantings if given the chance! See our information about protecting your patch.

To restore an area successfully takes planning, effort and time. A successful restoration planting should create conditions where native plants can regenerate themselves, so that eventually the planting can become self sustaining like a natural forest. However, seeds can only germinate if they reach bare soil. It’s important that dense grass swards are removed to allow open ground to be exposed. As plantings establish they will eventually shade out light-demanding grasses and weeds.

See how long it takes for a forest to establish in the diagram (by Lloyd Esler) below:

Stages of forest development over time (image by Lloyd Esler).Stages of forest development over time (image by Lloyd Esler).

Role of birds

With the right selection of plants, a successful planting should also attract and provide habitat for native birds and other wildlife. Birds play a vital role in spreading seeds, further helping the regeneration process.

Find out more about attracting birds to your patch.

Find out more

See our section on restoring your patch. For more information, come in and visit us at the nursery or contact us.