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Forest revegetation - selecting a method

Selecting a revegetation method

The main forest revegetation options are:

  • Assisting natural regeneration
  • Direct seeding
  • Establishing a nurse crop
  • Planting a mixture of quick growing species.

Assisting natural regeneration

This method can be effective in converting areas to native plants.

For success, this method requires:

  • Sufficient adjacent seed sources of quick-growing colonising species.
  • An absence of grazing animals and control of animal pests.
  • Control of competing grasses and weeds.

Natural regeneration in healthy forests can be prolific. By contrast, at the edge of a remnant or planted area, a dense grass sward may prevent native trees and shrubs from germinating and establishing.

To follow this method:

1.  Remove the dense grass sward at the site by chipping off the surface vegetation to expose the soil or by spraying with knockdown herbicide.

This is best done in mid to late summer in time for the annual native seed crop.

Seed can be left to drop on to the prepared site from adjacent seeding native species or can be spread by hand.

Or small seed-laden branches can be brought in from other sites and laid on the area.

2.  12 to 18 months later, a mixture of weeds, grasses and native seedlings will have germinated.

3.  Select 4 to 6 of the most vigorous native seedlings in each square metre, hand weed around them, and mark their positions with small stakes.

4.  While protecting these seedlings with a shield of some form e.g. a 2 litre plastic bottle with the bottom cut out, spray out or chip away all the other germinating plants.

The surplus native seedlings can be carefully dug out for potting up and growing on.

5.  Once the area has been cleared, it can be useful to mulch the area e.g. with shredded bark, to prevent the growth of further competing vegetation and to conserve moisture around the selected plants.

6.  The selected plants will grow quickly, shading the ground and making it less favourable for the growth of grass and weeds, and effectively extending the forest.

Other shade-tolerant plants can be introduced under the colonising species.

Direct seeding

This is a relatively inexpensive supplement to hand planting of native trees and shrubs.

For success, this method requires:

  • Sufficiently large quantities of viable seed.
  • Time to do the work in the right season.
  • That optimum conditions are maintained during germination.
  • That competition from invading, unwanted exotic species can be controlled.

Direct seeding involves the broadcasting or placing of seed directly into a prepared site where plants will germinate and grow.

Because the conditions are inevitably less suitable for germination and early growth than a nursery situation, only a small percentage of seed will ‘take’.

With many seeds, especially larger ones, there is a risk of damage by insects, birds and rodents, or by drying out.

However, where seed is plentiful or easily brought to the site, the method can be useful.

The most critical factors in direct seeding are the elimination of competing plants, especially grasses, and the maintenance of a microclimate suitable for seed germination and growth.

Grasses compete for available moisture in the soil and often outgrow and smother native seedlings.

The ground can be suitably prepared by spraying with knockdown herbicide, chipping off the vegetation, rotary hoeing, ripping, discing or ploughing.

Ploughing has the advantage of putting the surface layer of vegetation under the soil.

Establishing a nurse crop

Often existing vegetation, whether native or exotic, is useful as shelter for establishing plants.

Where there is no existing vegetation, it is often advisable to establish a nurse crop to provide this initial shelter, especially on exposed sites.

A commonly used nurse crop is manuka.

Succession to tall forest can be left to proceed slowly from wind or bird dispersed seed, or can be hastened by artificial manipulation of the stand by, for example, thinning out some of the nurse crop plants and underplanting with native species.

1.  Manuka

In many parts of New Zealand, the native species that most often acts as a nurse plant in natural situations is the familiar manuka Leptospermum scoparium.

Manuka cover provides ideal conditions for the establishment of native trees and shrubs.

Plants that can tolerate shade will grow up, overtop, and eventually eliminate the light-demanding manuka.

Manuka goes through a very dense thicket stage when lack of light and competition for nutrients and water limits the growth of other species.

When used as a nurse crop, a manuka stand may have to be thinned to let in more light.

Characteristics that ensure the early and rapid establishment of manuka on a wide range of sites include:

  • Its wide ecological tolerance.
  • Vigorous growth.
  • The ability to colonise inhospitable sites, especially those of low fertility and with low temperatures.
  • Prolific production of light wind-borne seed. Whole capsules are readily entangled or water borne so that people, animals and rivers also carry the seed.
  • Manuka seed fall occurs throughout the year, with a major peak in late winter/spring and a minor peak in autumn.
  • Seed germinates over a wide temperature range.

Manuka forms a very dense ground cover with more than 400 plants per square metre in the beginning of a succession.

As the stand develops and ages, the density declines so that when the manuka is 5m high, there are about 10 to 20 plants per square metre.

The time it takes for a stand of manuka to regenerate to forest depends on the site and individual circumstances.

Successions through manuka usually take less than 100 years and on fertile sites manuka can die out within 50 years.

However, on some sites, succession may be prolonged indefinitely. Stands of manuka can be permanent.

2.  Laying manuka brush

It is possible to establish high densities of manuka seedlings by laying branches of manuka laden with ripe seed (brush) and bearing semi-mature seed capsules over bare or cultivated ground.

If there is other vegetation such as grass it should first be cleared.

The brush is laid over the cultivated ground in several layers in a criss-cross fashion. It should not be laid too densely or it will shade out the germinating manuka.

On windy slopes or steep areas, brush should be pegged down e.g. with No 8 wire.

As the cut brush dries out, the semi-mature seed capsules split open and release large quantities of seed. The leaves from the brush fall, covering the seeds and forming an excellent germination bed.

The matrix of cut stems will improve the growing environment of the young seedlings, providing shelter, increased humidity and warmth.

This site improvement greatly increases the chances of both the survival of manuka seedlings and the establishment of other species.

Manuka is light-demanding and forms even-aged stands with a closed canopy and more open conditions below.

The uniform canopy height means that the younger or less vigorous manuka seedlings and seedlings of less vigorous and light-demanding species are quickly suppressed. These may be weeds that are undesirable anyway.

3.  Direct seeding of manuka

This is another method of establishing a nurse cover.

Ripe seed capsules are collected from trees and scattered over the newly cultivated ground.

This method does not offer the same improved microclimate and growing conditions as the brush method.

The less fertile the site, the better are the chances that manuka will germinate and survive.

Without further assistance it will take up to 50 years before broadleaved species emerge through the manuka canopy.

You can hasten the development of the stand by thinning and planting under the manuka.

Planting a mixture of manuka and other quick growing species

The crown spread of many other quick growing species such as Coprosma and Pittosporum is potentially greater than that of manuka.

Canopy closure may be attained more rapidly if a mixture of these species as well manuka is used.

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Open SpaceTM Magazine No. 39, June 1997 © QEII National Trust

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