EXPLORING THE GENETIC RESOURCES
OF TROPICAL MELALEUCAS
by J.C. Doran and B.V. Gunn
CSIRO Division of Forestry
PO Box 4008 QVT,
Canberra ACT 2600
Australia
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SUMMARY
Tropical melaleucas are being used to reforest the inundated, acid sulphate lands of the Mekong Delta of Vietnam. M. cajuputi grows naturally in the Delta, this species and a number of other melaleucas with potential for the Mekong Delta are described. Melaleuca spp. seed collections undertaken in northern Australia and Papua New Guinea are summarised. The problems of weediness and successful propagation associated with melaleucas are discussed. Excess seed is available for interested research institutions.
INTRODUCTION
Dense forests of Melaleuca cajuputi (known locally as M. leucadendra) once dominated the seasonally inundated, acid sulphate soils of the Mekong Delta. These forests have now largely disappeared as have the associated benefits that the forests brought to the local communities (fuelwood, posts and piling, honey, cajuput oil) and other services like soil conservation and wildlife habitat. Much of the land is unsuitable for agriculture. As a consequence the Vietnamese government has identified the Mekong Delta as one of its priority areas for reforestation. |
The environment of the Mekong delta is difficult for successful tree establishment and growth. Sites are frequently inundated with up to 2 m of water for months at a time, the soils are acid sulphate, competition from Imperata grass and other weeds is severe and there is a high risk of fire during the dry season. To establish eucalypt and acacia plantations deep drainage channels must be dug by hand. This requires relatively high capital investment and is impractical on the broad scale. M. cajuputi, on the other hand, is well adapted to the conditions and can be successfully established without major drainage works and its cultivation is favoured. Work on exploring and assessing the limited natural resources of M. cajuputi still available in Vietnam is in progress. A tree improvement program with this species is planned along with the introduction of new genetic material of various exotic melaleucas to assess their potential on the difficult sites in the Delta.
Several species of the genus Melaleuca occur naturally on poorly drained or seasonally waterlogged, heavy textured or organic acid soils in tropical Australia and Papua New Guinea (PNG). Some of these species have a reputation for fast growth and good form when conditions are favourable. They also have the potential to yield other products like essential oils and high quality honey.
The seed collecting program described in this paper has its origins in a collaborative forestry project between the Forest Science Institute of Vietnam, Australian Centre for International Agricultural Research and CSIRO Division of Forestry based in the Mekong Delta region of southern Vietnam. It was the aim of the collaborative project to collect seed from a range of provenances of several well-adapted Melaleuca species native of northern Australian and PNG to allow their potential to be assessed in the Mekong Delta and elsewhere. This paper describes the species selected for initial screening and identifies several others of future interest. Summary details of the seedlots available are given.
TROPICAL MELALEUCAS WITH POTENTIAL
The genus Melaleuca is part of the family Myrtaceae. It is a predominantly Australian genus, where it occurs throughout the continent in many forms ranging from small, sometimes prostrate to semiprostrate shrubs common in the sandy heaths of the southwest to large forest trees over 40 m tall in the tropical swamps of the north. Only about nine species have been recorded outside Australia (Barlow 1988). Within Australia some 250 species will be recognised on completion of a taxonomic revision of the entire genus presently in progress (L. Craven pers. comm.).
About 30 species are tropical in origin, out of which the following 7 species are highlighted in this paper: M. argentea, M. cajuputi, M. dealbata, M. leucadendra, M. quinquenervia, M. saligna, and M. viridiflora. These species are mainly medium to large trees with a marked preference for damp or wet depositional landforms which may partially dry out seasonally; such sites are often coastal and include areas where there is water of varying salinity. The wood of these species is mostly hard, moderately dense, resistant to rot, often of high silica content and mainly used as posts and poles, piling and fuelwood. Most species are good sources of honey and are useful for amenity planting, shelterbelts and erosion control on difficult sites where few other trees thrive (e.g. on swampy ground; sites subject to salt spray). Essential oils are extracted commercially by steam distillation of the leaves of two species (M. cajuputi and M. quinquenervia) and others contain oils with potential for commercial use (Brophy et al. 1989).
POTENTIAL PROBLEMS
Weediness
When assessing the desirability of introducing and testing Melaleuca species in areas where they have not been grown previously, it is extremely important to consider the potential for their spread from cultivation to become noxious weeds. Characteristics of members of the genus that promote such events are early and heavy seeding, fast growth, adaptation to swampy environments including long periods of inundation, resistance to damage by fire and the ability to regenerate by coppicing and/or root suckering. In Florida, conditions prevail that have allowed M. quinquenervia to invade natural wetlands and become a serious pest, crowding out regeneration of native plants and destroying wildlife habitats (Geiger 1981, NAS 1983).
Propagation
Melaleucas produce extremely small seed. Inexperience in handling such seed often leads to failure in the nursery. The seed often germinates readily but the tiny seedlings are easily damaged by overhead watering or rain, or may be killed if the sowing mix dries.
A method of watering that avoids these problems is the “bog” technique. Here the germination tray stands permanently in water so that moisture soaks up to the surface which is constantly moist but not flooded. Seed is sown evenly over the surface at the recommended density. An inflated plastic bag may be fitted over the container to maintain a moist environment. Once the germinants are sturdy enough to withstand overhead watering (ca. 4 weeks), the container should be removed from the water and handled normally. The risk of fungal disease is high, so good hygiene is essential.
After germination the tiny seedlings can be slow initially to develop, presumably while the roots establish. Once underway, however, they grow quickly and their total nursery period is similar to other fast growing species like eucalypts.
SEED COLLECTIONS
Seed of a range of provenances of the priority species (see Table 1) were collected in Australia (Northern Territory and Western Australia) and Papua New Guinea during November and December 1993. These new collections complement collections already available from previous work in northern Queensland (Searle 1989). This material is considered to provide an adequate sample for first-stage species introduction trials. Melaleucas appear to be strongly outcrossing (e.g. 93% in M. alternifolia (Butcher, Bell and Moran 1992)) so it is reasonable to assume that many males are contributing to the genetic diversity of each population even when seed-tree numbers are relative small (e.g. 3–5).
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