Management techniques

Wet rainforest

There are large differences in the costs of different restoration approaches for wet rainforest, and much has been learnt about the effectiveness of different techniques over the last 20 years. So it is worth spending some time determining what methods will be best for your site and situation.

Excluding fire

The risk of fire to a rainforest restoration site can be reduced by controlling weeds such as lantana and grasses within and adjacent to the site, and maintaining firebreaks. Closing the canopy and edges of rainforest (as much as possible—depending on the type of rainforest) will also create a moist microclimate within the patch which will protect against fire. A dense shady canopy will also help control weeds in rainforest restoration. A range of plant species have been suggested for creating buffers along wet rainforest edges and these can serve the dual purpose of blocking weed invasion and reducing the risk of fire.

Managing grazing pressure

Fencing around degraded remnant rainforest patches and excluding livestock may be all that is required to promote natural regeneration in the Wet Tropics. Temporary or moveable fencing (e.g. electric) is worth considering if restoration is to be undertaken in stages, and wallaby-proof fencing may be desirable if wallaby damage is a problem.

Vines

Native vines are an important and characteristic life form in established rainforests, but can be a major problem during early stages of forest restoration. Control of vines is recommended for most wet rainforest restoration sites, at least until trees are well established. It is preferable to cut vine stems close to the ground using loppers or a machete, and avoid the use of herbicide. In this way the growth of the vine is slowed, but the vine is not killed. This treatment can be repeated at intervals until the other rainforest plants are large and sturdy enough to support vines.

Vines may also be added to established plantings once the tree canopy is tall and well developed, and may be particularly useful around site edges to provide some protection from weed invasion, moisture loss, light penetration and temperature extremes.

Controlling weeds

Many weeds invade regenerating rainforest or plantings, but the impact of some weeds is worse than others. Therefore it is important to plan and prioritise weed control, based on the state and condition of the site, the availability of labour and resources, the level of weed infestation, the type and scale of restoration (e.g. are you restoring a remnant versus replanting a paddock? Is your site large or small?) and the best time and methods to control the weeds present.

General principles of weed spraying

When using herbicide, it is best to spray before or at the onset of weed flowering to ensure good uptake of the herbicide and to prevent seeding. It is preferable to use non-residual herbicides such as glyphosate to reduce long-term impacts. However, some native species are very susceptible to glyphosate so spray drift should be minimised. Weed spraying should be done by someone who can identify the target weeds and native species, or in tandem with an expert who tags the plants to be treated before spraying starts.

Weeds in remnants

Controlling weeds in rainforest remnants should exploit the natural resilience of the native vegetation, so that weeds are replaced by native species. To follow this strategy, weed control should be done in stages, with the weeds of one section of the site being removed and replaced with natives before the next section is tackled. This approach differs from the ‘target’ weed control strategy which focuses on the control of one species across the whole patch, and can be far less effective. However, the ‘target’ approach may be appropriate for the eradication of small weed outbreaks.

Margins of remnants can be ‘closed off’ with native vines and shrubs to discourage further invasion of weeds. Spraying margins of remnants infested with lantana or exotic brambles (Rubus spp.) can be helpful as this encourages the growth of native seeds and seedlings below these weeds (but note that wetting agents should not be used in these situations). However, the control of lantana is not recommended if it is likely that more flammable weeds may take its place (e.g. grasses).

Weeds in planted sites

Weed control is an important part of site preparation before planting, and this can be achieved by chemical or physical means. Blanket spraying with herbicide to kill all grasses and woody weeds is an effective way to remove competing plants and also allows dead material to act as mulch. The depth of the resultant mulch layer can be increased by slashing the area two to three times before spraying which also promotes new growth on the weeds thereby providing a better target for the herbicide. The final spraying can be completed one to two days before planting. Alternately, weeds should be removed within 500mm of where each seedling is to be planted. Retaining soft weeds around the plant may be beneficial in frost-prone areas.

If herbicides are not acceptable, physical removal can be an effective option for some weeds such as lantana, and in planting sites regular and heavy applications of mulch (following ripping, slashing, or other manual control) can also be used to control weeds. Hay bales, cardboard and sugar cane trash can all be used as mulch. Mowing or brushcutting may be useful in some circumstances but is generally not recommended because this encourages the growth and development of grasses, and because of the risk of cutting or damaging desirable plants.

The edges of restoration sites often provide the most weed problems, but these can be managed by maintaining a 3m buffer by slashing or spraying (which also provides a firebreak). Once planting has reached its full extent, margins can be ‘closed off’ with vines.

Weeds as wildlife habitat

Some weed species which are a threat to rainforest (e.g. camphor laurel Cinnamomum camphora, lantana Lantana camara and privet Ligustrum spp.) also provide important habitat (including food resources) for native animals. So any removal of these weed species should be done in stages, and the weeds immediately replaced with native rainforest species that provide similar resources to wildlife.

Control of camphor laurel

Stands of camphor laurel can function as ‘nurse’ trees for native rainforest trees, and facilitate the conversion of pasture to rainforest, and this can be a relatively economic method of restoring rainforest. However it is not known whether stands of camphor laurel will eventually revert to native rainforest without intervention and this is probably unlikely if most seed rain is composed of exotic species. Instead, a strategy of selective, progressive culling of camphor laurel trees is recommended, where trees are poisoned but left in place to provide perches for seed dispersers, some shade for seedlings, and extra carbon for the soil. Removal of trees can be staged, or in patches, as both methods have been found to successfully convert stands of camphor laurel to regenerating rainforest. Both methods require intensive control of camphor laurel recruits and ongoing maintenance.

Other weed control guidelines for individual species

Detailed weed control information is available on the following Weeds of National Significant (WONS):

• Lantana
• Parthenium
• Rubber vine.

Controlling non-rainforest trees

Acacia species can establish as canopy dominants in areas that once supported rainforest, and delay the recruitment of rainforest trees. The lifespan of these acacias can be 50–70 years, and during this time the conversion of the stand to rainforest may be delayed. To restore rainforest in these situations, we suggest that the staged removal method is used (as described for camphor laurel above), by poisoning trees and leaving them in place. But note that the effectiveness of this approach is uncertain and needs to be tested. In theory, rainforest seedlings should grow through the gaps and eventually shade out neighbouring acacias. Care should be taken that tree removal does not facilitate the invasion of grasses and other weed species, and follow-up monitoring and control of weeds (including acacias) may be necessary.

Encouraging natural regeneration

Natural regeneration of rainforest uses seed dispersal by animals and wind to re-establish native plants on a site. Therefore the success of this method relies on an abundant and diverse source of rainforest plant seeds on-site or nearby, and this method is most suited to expanding remnant rainforest patches or stimulating regeneration close to existing rainforest stands. As with other rainforest restoration techniques, natural regeneration requires the ongoing management of threats such as grazing, fire and weeds to be successful.

Seed-dispersing birds can be attracted to the site by providing perches, temporarily retaining fleshy-fruited exotics (e.g. camphor laurel) or planting bird-attracting native species. It may be worth encouraging a wide range of seed-dispersers (e.g. including birds that mainly eat insects or nectar, but occasionally eat fruit), as specialist fruit eaters may not be the best seed dispersers.

Staged removal of camphor laurel

Encouraging natural regeneration using existing camphor laurel stands can be a much cheaper method of restoring rainforest than clearing and planting seedlings ($0 – $10,000/ha vs. $30,000/ha). Camphor laurel trees are progressively culled from a site by poisoning, and dead are trees left in place. These provide perches for seed dispersers, some shade for seedlings, and extra carbon for the soil. Removal of trees can be staged, or in patches, as both methods have been found to successfully convert stands of camphor laurel to regenerating rainforest. Both methods require intensive control of camphor laurel recruits and ongoing maintenance.

Because natural regeneration relies on the seed rain brought to the site by natural dispersal, problems can arise through the overabundance of weeds in the seed rain, or lack of certain native seeds. These issues can be addressed by ongoing and vigilant weed control, and the supplementary planting, or direct seeding of native species that do not arrive naturally.

Revegetation

A number of methods have been proposed for revegetating rainforest, and these can be grouped into two main categories. The ‘pioneer’, ‘early succession’ and ‘framework species’ models all use hardy pioneer and early to mid-successional tree species to create a canopy and ‘bait crop’ for birds, and rely on natural dispersal for the recruitment of mature phase (or ‘climax’) tree species. The ‘late succession’, ‘maximum diversity’ and ‘mixed species’ models include mostly secondary and mature phase species, and aim to rapidly establish a multi-layered canopy with low light conditions in the understorey and the original (pre-clearing) diversity of trees.

These early revegetation methods were based on observations of intact rainforest and did not address all factors influencing the success of restored sites. After two decades of rainforest revegetation it has now become clear that the ‘maximum diversity’ or ‘mixed species’ models generally perform better than the ‘framework species’ and similar models. This is because the diverse planting models can rapidly achieve canopy closure and suppress grasses and weeds, whereas the more open-canopied ‘early succession’ models are more vulnerable to weed invasion as planted pioneers senesce and therefore require much more maintenance. In addition, mature phase species may fail to recruit even if there is an adequate seed source nearby. As a result, ‘Maximum diversity’ models are now recommended for wet rainforest revegetation in almost all situations.

Good site preparation before planting is important as it can equate to three to six months extra growth. Mechanical site preparation such as deep ripping may be necessary on cleared sites with compacted or poorly drained soils, or if the area has been heavily grazed by stock over a long period, or mowed weekly with a ride-on mower. Mulching is a good way of suppressing weed growth and improving the soil but can be expensive.

Plant species should suit the site conditions (including soil type, aspect, exposure) and be native to the local area. Contact your local rainforest plant expert for advice about which species to plant at your site, and where to get them.

In the Wet Tropics, all site preparation should ideally take place in winter or early spring so that planting may begin when summer rain arrives. For South East Queensland, the best time to plant is after the wet season begins (late February to late April). Earlier planting may be desirable in frost-prone areas, but avoid the long hot days between November and January. Close spacing of trees (1.5–1.8m apart) are likely to result in taller canopy tree heights, which may have significant implications for carbon storage. However, an assessment of planting models in the Wet Tropics recommended a planting density of 1.8m, as planting at higher densities was more costly and did not achieve better results.

Ongoing maintenance (especially weed control) is very important to ensure that the planted site develops into a rainforest and not a weed-dominated stand.

Direct seeding can also be used to revegetate rainforest and is most successful under a canopy where there is little or no weed competition. Seed should be buried at a depth roughly equal to the diameter of the seed, and the site protected from indiscriminate trampling or weeding. Soil can also be disturbed in cleared areas adjacent or downwind of remnants as seed ripens nearby, and large seeds (e.g. black bean or bangalow palm) can be cast directly into dense weedy areas with some success.

Improving soil conditions

If your site includes badly eroded areas where the humus layer has been lost and the mineral soil is exposed, it may benefit from heavy mulching, and the planting of pioneer species to rebuild soil structure and microbe levels. However, many rainforest tree species are tolerant of mineral soil exposure and site conditions caused by erosion. Areas of severe soil compaction may benefit from deep ripping and/or cultivation, but this should be done at least 6 months prior to planting (preferably earlier) to allow the soil to settle and reconsolidate.

Dry rainforest—restoration principles and techniques

The restoration of dry rainforest is problematic. Natural regeneration after clearing seldom occurs, as dry rainforest species are usually eradicated by post-clearing fire. Revegetation of completely cleared sites is rarely feasible because of slow plant growth rates and the ongoing management costs of weed, fire and grazing control.

However, the restoration potential of some dry rainforest types is greater than others, and these differences are outlined below. There is also far less known about the restoration of dry rainforest than wet rainforest, as most rainforest restoration has focussed on the latter. It is hoped that ongoing attempts to restore dry rainforest will yield more specific and detailed information about the restoration of this vegetation type.

Dry rainforest on rocky substrates, or on better soils where annual rainfall < 900mm

This type of dry rainforest is the most difficult to restore because of slow plant growth rates. If a cleared site was replanted with dry rainforest species, it is estimated that about 50 years would be required to develop a closed canopy. This is due to the typically slow growth rates of these dry rainforest plant species. Until canopy closure is achieved, ongoing management of grazing, fire and weeds would be required, demanding substantial cost and effort.

For these reasons, broadscale restoration of this type of rainforest is unlikely to be feasible. But there may be situations where small areas can be restored more easily. For example, a small area could be protected from fire if surrounded by a buffer of fire-retarding vegetation (e.g. brigalow forest without exotic grasses). If fencing or weed eradication of the revegetation site is required there would be high initial costs, but ongoing costs may be lower if the surrounding buffer vegetation is weed free. There may also be some natural dispersal of rainforest plants into the site if they are present in the surrounding brigalow vegetation.

Dry rainforest on better soils where annual rainfall is 900–1400 mm

Restoration of this type of dry rainforest is possible, but it requires intensive planting, weed control, fire management, and watering in establishment phase. Hoop pine (Araucaria cunninghamii) is a common species of this vegetation type and is a relatively hardy, and fire-resistant species. Species mixes with a substantial proportion of hoop pine may have a better chance of success than those consisting of more delicate species.

Revegetation

Techniques for the planting and maintenance of dry rainforest species are largely consistent with those recommended for wet rainforest, although the plant species will vary. But as dry rainforest occurs in more seasonally dry conditions, more water and mulch may be required to assist the survival of planted trees.