Ecology
Understanding the ecology of rainforest helps us to manage and restore these systems for carbon and wildlife.
Although rainforest can tolerate some forms of disturbance (e.g. individual tree death or storm damage), other forms of disturbance (such as clearing, grazing, weed invasion and fire) may delay the regeneration of rainforest.
Temperature, moisture, soil and drainage
The types of species and structure of rainforest are influenced by the temperature, rainfall, soil fertility and soil drainage of the site, e.g. the rainforest tree canopy tends to be tallest on sites that receive the most rainfall, if all other factors remain equal. This variation influences the carbon storage potential of rainforest.
Biology of rainforest trees
One of the most important features of mature wet rainforest is a closed tree canopy. This blocks most sunlight, creating a dark and humid atmosphere within the forest. This relative darkness prevents the growth of grasses and many weeds, which tend to need plenty of sun. The high humidity and lack of grasses on the forest floor also make it difficult for fires to ignite and spread within the rainforest. Achieving and maintaining a closed canopy is perhaps the most significant step in restoring wet rainforest.
The tree canopy of dry rainforest tends to be more open than that of wet rainforest, meaning that dry rainforest can be more vulnerable to weed invasion. The tree canopy still plays a major role in suppressing weeds and fire, so the restoration and maintenance of the tree canopy is also a major part of restoring dry rainforest.
The tree canopy of rainforest can quickly re-establish following some types of disturbance (e.g. individual tree death or cyclone damage), provided the disturbance is not too large or intense. If a mature tree falls and creates a gap in the canopy, the different plants in the rainforest respond to close the gap. The fallen tree may produce coppice or basal shoots. The canopies of standing trees may also expand into the gap. Seedlings or saplings that survived the disturbance may grow rapidly to fill the gap. The seeds of ‘pioneer’ plant species that are intolerant of shade may germinate from the soil, triggered by increased light and heat. Pioneer species tend to be short-lived, so other tree species tolerant of shade (‘climax’ or ‘mature phase’ species) may eventually grow up under the pioneer species, and replace them as upper canopy trees.
Mature phase tree species
Other features of mature phase tree species relevant to rainforest restoration are their short-lived seeds and how these disperse away from the parent tree. Many mature phase tree species have fleshy fruits and their seeds are dispersed by animals.
The seeds of many mature phase tree species tend to germinate immediately after dispersal and do not remain viable in the soil for long (less than 6 months). If mature phase tree species are removed from a site by clearing or fire, their replacement depends on the dispersal of seed from another site by animals. Seed dispersal to the site by native animals (e.g. birds, bats) will be unlikely or less frequent if the site is remote from an adequate seed source, or these animals are scarce or extinct in that area.
Monocultures
Many rainforest tree species do not grow well in monocultures. Exceptions to this are the hoop pine (Araucaria cunninghamii) and bunya pine (A. bidwillii), which can be grown successfully in monoculture plantations. A variety of rainforest tree species used for cabinet timber can also be grown in mixed-species plantations.
Nutrient cycling
Most nutrients in a rainforest are stored in the plants and topsoil, and are tightly cycled within the ecosystem (nutrient cycling). Clearing of rainforest, especially if combined with fire, can result in a loss of nutrients which may prevent the re-establishment of rainforest. Rainforest on more fertile surfaces (e.g. basalt) can regenerate more readily after disturbance.
Clearing
The capacity of a site to recover from clearing will depend on the intensity of the initial disturbance.
Small gaps produced by the removal of individual canopy trees tend to be rapidly filled by rainforest species. If fallen trees and tree stumps survive the disturbance, many of these will regenerate by producing root suckers and coppice shoots.
The removal of all canopy trees, seedlings and saplings will increase the opportunities for weed invasion and slow the recovery of the ecosystem. Restoration will be most difficult when disturbance also removes soil seed stores and the soil organisms.
Large areas of rainforest have been cleared for agriculture, including the establishment of pasture grasses. When these areas are abandoned and protected from fire and other major disturbances, secondary forest may eventually replace the grassland, which may contain a mixture of native and exotic tree species. In South East Queensland, areas that once supported wet rainforest are often dominated by the exotic camphor laurel (Cinnamomum camphora).
Seed rain
The rainforest tree seed naturally dispersed to a site (seed rain) is another factor that will determine if and how a rainforest will regrow after clearing. The nearness, location and composition of other rainforest patches will determine which tree species are likely to reach the site by wind, water or animal dispersal.
The types of animals present will also have a large influence. Different animals carry different seeds, and some can carry seed longer distances than others.
Seed rain tends to be concentrated around seed sources, and around food trees and perches.
Weeds
This section considers only the main weeds most likely to affect the restoration of rainforest—many other weed species invade rainforest.
Most weeds are unlikely to invade dense rainforest without some prior disturbance such as cyclones, clearing or burning, although some species can invade intact forest (e.g. Madeira vine Anredera cordifolia). The more open canopy of some dry rainforests makes it easier for some weeds to invade (e.g. lantana Lantana camara), green panic (Megathyrsus maximus var. pubiglumis) and rubber vine (Cryptostegia grandiflora).
Grasses
The presence of bulky pasture grasses (e.g. green panic or molasses grass Melinis minutiflora) in or near rainforest can increase the risk and severity of fire. Bulky pasture grasses can also suppress or prevent the regeneration of rainforest species by out-competing them in the race for space, light and water. The trampling of cattle and death of canopy trees help pasture grasses invade dry rainforest.
Lantana
Lantana (Lantana camara) is a bird-dispersed shrub native to tropical America, and is a serious weed of most rainforest types.
Lantana is absent from the drier forms of dry rainforest, and does not survive in shade. Dense infestations of lantana increase the likelihood of fires and compete with native plant species.
Lantana can be more easily controlled than many other weeds and provides habitat for native animals. The removal of lantana may also facilitate the establishment of grasses (which are more flammable than lantana) and other more serious weeds. In these situations it may be preferable to retain the lantana, and rely on the eventual closure of the rainforest canopy for its eradication from the site.
Camphor laurel
Camphor laurel (Cinnamomum camphora) is a fast-growing, bird-dispersed tree from subtropical Asia. Unlike many native rainforest trees, camphor laurel readily invades abandoned agricultural land. It often dominates the canopy in regrowth rainforest, and shades out and competes with other vegetation. Stands of camphor laurel also provide habitat for rainforest birds and create conditions (e.g. a moist and shady forest floor) suitable for the establishment and growth of native rainforest trees.
The staged removal of camphor laurel can be a much cheaper method of restoring rainforest than clearing and planting seedlings (between $0/ha and $10,000/ha compared to $30,000/ha). It is important to determine whether camphor laurel is helping or hindering the establishment of rainforest on your site before embarking on a control program.
Fire
Some rainforest plants can survive fire, but repeated fires eventually destroy most rainforest species. Weeds and more fire-tolerant native plants are likely to take over a rainforest site that has been repeatedly burnt. As a result, rainforests generally occur in areas that are protected from fires by topography, climate, or other vegetation that has low flammability.
Fire is generally not a threat to undisturbed rainforests, but fires can burn into rainforest after prolonged drought and spread through large areas. Rainforest becomes more vulnerable to fire following the invasion of weeds such as lantana and exotic grasses, which increase fuel loads.
Grazing
Grazing by livestock generally has a negative impact on both wet and dry rainforest, and high densities of wallabies may also damage rainforest. This is especially so where rainforest patches are surrounded by cleared land, and provide shelter for grazing animals. Moderate levels of cattle grazing may be compatible with the conservation of dry rainforest in north Queensland.
Pigs
Feral pigs are perhaps the worst animal pest in rainforest. The activity of pigs has been linked to reduced seedling density, litter cover and soil insects in wet rainforest, and to tree death in dry rainforest.