Brisbane Rainforest Action & Information Network


Natural regeneration of rainforests of the Brisbane region

Small Gaps
Large gaps

Natural regeneration processes of the dry rainforest communities typical of the Brisbane region have not been studied in the same depth as those of the wetter subtropical rainforest types. However the processes which have been identified as occurring within the latter communities can be applied to local dry rainforest remnants.

A mature rainforest community consists of large, long-lived trees with a closed canopy. In the local situation this would be a rainforest community with a relatively uneven, yet closed, canopy with numerous emergent trees (eg. Hoop Pine, Crows Ash) visible above the other tree layers.

Disturbance (natural or by human action) of the intact canopy of a rainforest community promotes a number of identifiable natural regeneration responses. The type and duration of this natural regeneration is dependent on the size of the gap that is created in the forest canopy.

Types of Natural Regeneration include:

Small Gaps

In small gaps, characteristically caused by individual trees falling within the forest, there are four main methods of regeneration.

When a small (low light) gap is created in the canopy of the mature forest there is an immediate growth response. Saplings of the mature canopy trees are usually waiting as suppressed individuals in the understorey. The seedlings of the slow growing canopy trees are able to exist for many years under the shade of the canopy just waiting for a large tree to fall. When this occurs, the immediate increase in light and space stimulates suppressed saplings out of their dormancy and they make rapid growth to reach the gap in the canopy.

Other growth responses occur in the existing canopy trees surrounding the gap. These trees respond to the increased light situation by sending out lateral growth shoots to take advantage of the new space. These individuals try to take advantage of the increased photosynthetic potential provided by the newly made gap.

Depending on the time of year that the gap has occurred there may also be strong germination of fresh seed from the surrounding canopy species. The slower growing canopy species require the lower light intensity and more protected situation, provided by these small gaps, for germination of their seed. Therefore if any individuals of certain species are fruiting at the time of the disturbance there is a good chance that some seedlings will germinate in the ideal conditions created by the gap.

There are many situations where trees that have fallen are able to regenerate from coppice shoots arising from the base of their broken trunks. Similarly many of the dry rainforest species are able to grow root suckers in response to death of their main stems and even damage to existing root systems. Trees with large existing root systems are at a distinct advantage over suppressed saplings and other juvenile trees in the race to reach the light and close the gap in the canopy.

Large gapsBack to top

Large gap disturbances can be associated with natural disasters (for example, cyclonic storms, landslips or fire) or can occur through human activities (for example, logging or clearing of forest for agriculture). These events are capable of creating large areas of bare soil within a formerly closed forest ecosystem. The destruction of the canopy of the forest and disturbance of the soil combined with the access of light to the forest floor stimulates a recognised regenerative process.

Long-lived seed that has lain dormant in the soil seed-bank responds to the light and warmth of the sun reaching the disturbed ground. There is usually a large-scale germination of pioneer species. These pioneer species are fast growing but short lived and produce large amounts of leaf litter. Their role is to create a shaded, stable micro-climate for the slower growing, longer lived species that make up the mature rainforest community. The shade-tolerant mature phase species germinate under the cover of the pioneers and eventually grow up through the pioneer canopy as the short lived pioneers die of old age. The pioneers are intolerant of shade and cannot regenerate under the canopy of the mature phase species that gradually replace them.

In the ideal situation, the forest will progress through the various stages of regeneration (known as secondary succession) until a mature stage structured rainforest ecosystem is re-established. This process may take over 800 years to complete.

The large gap regenerative process will only progress from the initial pioneer species mix if there is a ready seed source of mature phase species close by. Seed from the mature phase species is short lived and does not persist in the soil seed-bank. Therefore, seed of these species must come from a seed source forest that exists close by the regenerating gap otherwise the regeneration will stagnate at the pioneer stage. This is the scenario for many of the remnant rainforest patches around Brisbane.

INTERRUPTION BY WEED INVASION

Above we have examined the processes of natural regeneration occurring within the rainforest communities of Brisbane. So far we have looked at how rainforest recovers from disturbances (natural and human made). Now we need to understand how regenerative processes are interrupted by weed invasion.

All of the rainforest growing in the Brisbane region has to some extent been disturbed either by human activities (logging, clearing) or by more natural processes (storm damage, fire, flood etc.) or a combination of both. After disturbance rainforest regenerative processes take over. The type of regeneration that occurs at a site is dependent on the amount of disturbance that has occurred and a combination of site factors.

As we know, the process of regeneration (known as secondary succession) from disturbance to re-established mature rainforest is not one directional. The regenerative process is not wholly predictable and many outside factors can halt the successful regrowth of disturbed rainforest.

Weed invasion is one such blocking influence and is probably the number one ongoing management problem for anyone involved in regeneration of local rainforest.

A weed species is ‘a plant growing where it is not wanted’ and can be an exotic (non-Australian) or a native (for example, Umbrella tree, Schefflera actinophylla - a species native to central/north Qld). There are many weeds that invade and grow in and around rainforest communities and generally speaking these plants must be controlled if the rainforest is to progress to maturity and maintain a healthy diversity of native plant species.

It is important to consider how different weeds affect the regenerative process. Some of the weeds germinating in the early stages of secondary succession are mainly short lived, annual type species and will die out quickly in the shade of the establishing longer lived shrubs and trees of the later regenerative stages. These shade intolerant, short-lived weeds are the least threatening to forest development and can be easily removed (these include Lantana and Wild tobacco). Other weed species that germinate early in the succession are the real problem. Species, such as Camphor Laurel (Cinnamomum camphora), Chinese elm (Celtis sinensis), Broad-leafed pepper tree (Schinus terebinthifolia), Ochna (Ochna serrulata) and others, are shade tolerant and can survive for years under the temporary canopy that is produced by the native pioneer species. Because they have germinated early in the succession they have a distinct competitive advantage over the later germinating and slower growing mature phase native species. Natural gaps occur in the canopy as the native pioneers gradually die from old age and the already established weed saplings then grow up to take the place of the pioneer canopy.

Exotic weed tree species are mostly all long-lived and therefore successfully block the continued natural succession of the forest because they do not die off and allow the slow growing mature phase native species to reach the canopy. The exotics have the additional advantages of fewer natural pests, greater and more frequent seed production and superior dispersal mechanisms for their seed. (for example, 15-20 year old Camphor laurel trees can produce up to 113,000 viable seeds per year !!)

Exotic vine species are another weed threat to regenerating rainforest. Vines, such as Madeira vine (Anredera cordifolia), Cats claw creeper (Macfadyena unguis-cati), Balloon vine (Cardiospermum grandiflorum) , the Morning glorys (Ipomoea spp) and others, have the ability to smother regenerating native plants. These species favour the sunnier margins and larger gaps of the forest and are known rainforest killers.

Again they have the advantages of fewer natural pests and overwhelming seed and propagule production and dispersal.

Kenneth McClymont © 1996