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

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 (see germination figures in Table 1). 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).

Table 1 and Figures 1 – 7 and the species descriptions that follow give summary details on the species and seedlots selected for inclusion in trials in the Mekong Delta of Vietnam. Excess seed is available for researchers for testing of this genetic material. Please write to Officer-in Charge, Australian Tree Seed Centre, CSIRO, P.O.Box 4008 QVT, Canberra ACT 2600, Australia.

Table 1. Details of a selection of seedlots of tropical melaleucas available for species/provenance trials from the Australian Tree Seed Centre.

¹ Q is Queensland;
NT is Northern Territory;
WA is Western Australia;
PNG is Papua New Guinea

Table 2 describes several other potentially interesting melaleuca species. These have not been sampled systematically throughout their ranges of natural occurrence. This will be done as the opportunity arises. However, a limited selection of research seedlots of all species is available on request.

Persons interested in obtaining seed from these collections should contact the Officer-in-Charge of the Australian Tree Seed Centre, CSIRO, P.O. Box 4008 QVT, Canberra ACT 2600, Australia.

Table 2. A summary of environmental characteristics and uses of other potentially interesting Melaleuca species

SPECIES DESCRIPTIONS

Priority species
The following species seem likely to have greatest potential in coping with the conditions of the Mekong Delta and providing useful products.

Melaleuca argentea                  (Silver-leaved Paperbark)
M. argentea usually occurs as a spreading tree to 25 m tall with slender pendulous branchlets, creamy white to grey papery bark and foliage that turns silvery-green seasonally. The approximate boundaries of the natural occurrence are given in Figure 1. The climate is mainly hot sub-humid with extension to the hot humid and hot semi-arid climatic zones. The range in latitude is 12 – 20°S and in altitude 5–200 m. Mean annual rainfall ranges from 600 – 1900 mm with a strongly defined monsoonal pattern. In Papua New Guinea, M. argentea occurs as scattered trees in open woodland with M. dealbata on acid soils that have a high clay content. In Australia, the species is found commonly along banks of fresh water creeks and rivers in deep sandy or sandy-loam soils that have a clay substrate, sometimes with M. leucadendra. It tolerates seasonally waterlogged conditions.

It does not appear to have been cultivated for forestry purposes, although it has been used in northern Australia as a graceful ornamental and shelterbelt tree. The wood is heavy with an air dry density of 1010 kg m-3 (Cause et al. 1974). It is a potential source of honey.

M. argentea was described by Blake (1968) and Byrnes (1986) and was illustrated by Brock (1988). In the taxonomic revision of Melaleuca presently in progress there are plans to divide M. argentea into two separate species: M. argentea as described and mapped (Fig. 1) in this treatment, and “M. fluviatilis” as shown in northern Queensland by Barlow (1988) (L. Craven pers. comm.).

Figure 1. Natural distribution and location of seed collection sites of M. argenta (map excludes distribution of “M. fluviatilis” (Barlow 1988).

Melaleuca cajuputi                  (Swamp Tea Tree)
This melaleuca is usually a tree up to 25 m tall with a single stem, although it may reach 40 m and 1.2 m in diameter in some situations. It displays dense erect dull green foliage with grey to white papery bark. The approximate boundaries of the natural occurrence in Australia and southern New Guinea are given in Figure 2. The species also has an extensive distribution in Southeast Asia: in Indonesia, Malaysia, Myanmar, Thailand and Vietnam. The boundaries of the natural range are not known as the species has been cultivated in southeast Asian countries for over 100 years. Range in latitude is 12°N – 18°S and in altitude 5 – 200 m. This is a species primarily of the hot humid climatic zone. Mean annual rainfall varies from 1300 – 1750 mm with a strong monsoonal pattern. The species grows in a wide range of situations but most stands are found on low swampy coastal plains often on heavy-textured black soils that are subject to flooding for six or more months each year.

The species tolerates waterlogged sites including those subject to brackish water. It regenerates successfully in Imperata grasslands, is fire resistant and has the ability to coppice and root sucker. It is moderately fast-growing. The wood is hard and resistant to rot. It is generally used in the round for posts, poles and piling. It makes good fuelwood. M. cajuputi makes an attractive ornamental tree, can be used for shade and shelter, and is a source of honey. It is also the source of Cajuput oil, produced by steam distillation of the leaves. This cineole-rich essential oil is used in local medicines and as an antiseptic and insect repellent.

M. cajuputi was described by Blake (1968) and Byrnes (1986) and was illustrated by Turnbull (1986). There are plans to recognise three subspecies within M. cajuputi: “subspecies cajuputi” from northwestern Australia and eastern Indonesia, “subspecies cumingiana” from Vietnam to western Indonesia and “subspecies platyphylla” from northern Queensland, southwestern Papua New Guinea and southeastern Irian Jaya (L. Craven pers. comm.).

Figure 2. Natural distribution and location of seed collection sites of M. cajuputi (map gives the geographic range in northern Australia and southern New Guinea only)

Melaleuca dealbata                  (Soapy Tea Tree)
M. dealbata is a tree to 25 m with blue-grey foliage, densely hairy branchlets and pale brown papery bark. The approximate boundaries of the natural occurrence are given in Figure 3. The range in latitude is 7 – 26°S and in altitude 5 – 550 m. The climate varies from hot humid to warm sub-humid. Mean annual rainfall ranges from 600 – 2500 mm. M. dealbata occurs chiefly along the banks of streams, on seasonally swampy ground and on the edges of lagoons that may be brackish. It frequently occurs close to the sea. In Papua New Guinea it extends to the dry savannah woodland of the Sogeri Plateau. Soils include silty sandy loams and clay soils.

The species has wood of high strength which is resistant to rot. In southeastern Queensland, one provenance (S 11935) showed excellent survival and fair coppicing ability in trials but only modest growth rate with the best plot averaging 3.6 m in 4.5 years (Ryan and Bell 1989 & 1991). M. dealbata may have potential for posts, small poles and fuelwood and should be well-adapted for coastal stabilisation work.

M. dealbata was described by Blake (1968) and Byrnes (1986) and was illustrated by Brock (1988).

Figure 3. Natural distribution and location of seed collection sites of M. dealbata.

Melaleuca leucadendra                  (Weeping Paperbark)
M. leucadendra is frequently a large tree 20 – 40 m tall with a diameter that may reach 1.5 m. Thin, shiny-green lanceolate leaves, attractive weeping habit and white papery bark are distinguishing features. The approximate boundaries of the natural occurrence in northern Australia and southern New Guinea are given in Figure 4. It is also found in the Maluku Provinces of Indonesia. Range in latitude is 3 – 23°S and in altitude 5 – 500 m. Climate varies from hot humid to warm sub-humid in coastal zones to hot and warm semi-arid inland. Mean annual rainfall ranges from 650 – 1500 mm with a strong monsoonal pattern to the north. Most stands occur on flat or very gentle topography, especially river flats, coastal plains or seasonal swamps. The soils are silty to loamy clays or sandy loams over clay.

M. leucadendra is relatively fast growing with height growth often averaging 1 – 2 m per year and has the ability to tolerate acidic, infertile, swampy conditions. In waterlogged and flooded areas it forms aerial adventitious roots. It is fire tolerant and coppices reasonably well. The wood is hard and durable in the ground and in water. Air dry density is 725 – 800 kg m-3. It is used mostly in the round for poles and posts, bush constructions, fuelwood and pulpwood. The species produces a good quality honey and has been used for shelterbelts, in coastal plantings and as a street or avenue tree.

This species was described by Blake (1968) and Byrnes (1986) and was illustrated by Boland et al. (1984) and Turnbull (1986).

Figure 4. Natural distribution and location of seed collection sites of M. leucadendra

Melaleuca quinquenervia                  (Broad-Leaved Paperbark)
M. quinquenervia is an erect, small to medium-sized tree, normally 8 – 12 m tall but sometimes reaching 25 m. It has stiff, leathery, lanceolate-elliptic leaves and white or greyish papery bark. The approximate boundaries of the natural occurrence in Australia and southern New Guinea are given in Figure 5. It also occurs in New Caledonia. The range in latitude is 8 – 34°S and in altitude 5 – 100 m (- 1000 m in New Caledonia). Climate varies from hot humid to warm sub-humid. Mean annual rainfall ranges from 900 – 1700 mm with a strong monsoonal pattern in the north. M. quinquenervia occurs mainly along streams and in swamps but in New Caledonia it forms extensive stands in upland areas. Excluding New Caledonia, the soils often have a high organic matter content and may be sandy on the surface but with silt and clay below promoting poor drainage. The water table is near or above the surface for much of the year.

This species has been planted in tropical regions, especially in the dry tropical lowlands of the Philippines, India and West Indies. In the USA it is planted in Hawaii and Florida. In Florida it has become notorious for invading swampy ground and is regarded as a weed. Early growth is relatively fast (1 – 2 m per year) on sites where water is abundant and soils are deep. M. quinquenervia coppices well, tolerates fire and weed competition and prolonged flooding. The wood has moderate strength and is resistant to rot in the ground and in water. Air dry density is 700 – 750 kg m-3. Utilisation is mainly in the round for posts, poles and piling. It makes excellent fuelwood and charcoal. Non-wood uses include honey, shelter, erosion control and a cineole-rich foliar oil (Niaouli oil) which is produced in New Caledonia and has uses similar to Cajuput oil.

M. quinquenervia was described and illustrated by Blake (1968), Boland et al. (1984) and Turnbull (1986).

Figure 5. Natural distribution and seed collection sites of M. quinquenervia (map gives the geographic range in northern Australia (minus “M. clarksonii”) and southern New Guinea only).

Melaleuca saligna                  (Paperbark)
This species is a small to medium sized tree 10 – 20 m tall with a green pendulous crown and brownish papery bark. It has a limited distribution in far northern Queensland (Fig. 6). Range in latitude is 12°40' – 15°40'S and in altitude 5 – 150 m. Climate is hot humid and mean annual rainfall varies from 1700 – 1900 mm. M. saligna occurs on flats and levees beside fresh water streams, surrounding depressions and swampy ground and behind mangroves close to the coast. The soils are frequently sandy loams or clay soils.

M. saligna has not been widely planted and little is known of its silviculture and potential utilisation. In southeastern Queensland, one provenance (S 14149) showed excellent survival, fair coppicing ability and reasonable growth rate in trials with the best plot averaging 4.6 m in 4.5 years (Ryan and Bell 1989 & 1991). It is probable that the species would be a useful source of round timbers, honey and shade and shelter in suitable environments in the lowland tropics.

This species was described by Blake (1968) and Byrnes (1986) and illustrated by the former. M. saligna is a member of the M. leucadenara species complex.

Figure 6. Natural distribution and seed collection sites of M. saligna

Melaleuca viridiflora                  (Broad-leaved Paperbark)
This species is typically a small tree 5 – 10 m tall but may attain 25 m in height under favourable conditions. The leathery, dull-green leaves are borne on thick erect branchless while the bark is pale brown and papery. The approximate boundaries of the natural occurrence are given in Figure 7. Range in latitude is 8 – 26°S and in altitude 5 – 1000 m. Most of the distribution falls in the warm to hot sub-humid to humid climatic zones. Mean annual rainfall varies from 325 – 1750 mm. M. viridiflora occurs in lowland coastal marshes, seasonal swamps and estuarine plains where surface water may be present for several months of the year. It sometimes occurs on drier inland sites. Soils show great variation and may be sands, loams or clays.

M. viridiflora is tolerant of infertile seasonally waterlogged sites in the humid and sub-humid tropics. In southeastern Queensland, the species showed excellent survival, fair coppicing ability and reasonable growth rate in trials with the best plot averaging 6.3 m in 4.5 years (Ryan and Bell 1989 & 1991). There are no data on wood properties but it is likely that the species could be used for small posts and poles, fuelwood and charcoal. It has been recommended as a screening and shelter tree in coastal areas of northern Australia. Red-flowering types are available for horticultural use. It is a good source of pollen but the honey has a poor flavour.

M. viridiflora was described and illustrated by Blake (1968), Brock (1988) and Turnbull (1986).

Figure 7. Natural distribution and seed collection sites of M. viridiflora

Other potentially interesting species

The melaleucas described in table 2 are considered to have potential especially for windbreaks, erosion control and small round timbers. Additional information on these species can be found in various texts including Barlow 1986, Blake 1968, Byrnes 1984, 85, 86 and Turnbull 1986.

ACKNOWLEDGEMENTS

Financial support of the Australian Centre for International Agricultural Research and the assistance of Mr Khongsak Pinyopusarerk, officer coordinating the ACIAR project at CSIRO Div. of Forestry is gratefully acknowledged. We thank the Papua New Guinea Forest Authority for permission to collect in PNG forests. Mr David Lea and Mr Andrew Griffiths assisted with the most recent collections and Ms Suzette Searle and Mr Jim Moriarty were responsible for earlier seedlot numbers available from the Australian Tree Seed Centre. Mr Lyn Craven and Mr Brendan Lepschi of the Australian National Herbarium and Mr John Clarkson of the Queensland Herbarium have been of great assistance in identifying species with potential, authenticating voucher specimens and providing comments on this manuscript.

REFERENCES

Barlow, B.A. (1986). Contributions to a revision of Melaleuca (Myrtaceae): 1–3. Brunonia 9: 163–177.

Barlow, B.A. (1988). Patterns of differentiation in tropical species of Melaleuca L. (Myrtaceae). Proc. Ecol. Soc. Aust. 15: 239–247.

Blake, S.T. (1968). A revision of Melaleuca leucadendron and its allies (Myrtaceae). Contributions from the Queensland Herbarium No. 1: 1–114.

Boland, D.J., Brooker, M.I.H., Chippendale, G.M., Hall, N., Hyland, B.P.M., Johnston, R.D., Kleinig, D.A. and Turner, J.D. (1984). Forest trees of Australia. Nelson - CSIRO: Melbourne.

Brock, J. (1988). Top end native plants. J. Brock, Darwin, Australia.

Brophy, J.J., Boland, D.J. and Lassak, E.V. (1989). Leaf essential oils of Melaleuca and Leptospermum species from tropical Australia. Pages 193–203. In: (Boland, D.J. ed.) Trees for the tropics: growing Australian multipurpose trees and shrubs in developing countries. ACIAR Monograph No. 10.

Butcher, P.A., Bell, J.C. and Moran, G.F. (1992). Patterns of genetic diversity and nature of the breeding system in Melaleuca alternifolia (Myrtaceae). Australian Journal of Botany 40, 365–375.

Byrnes, N.B. (1884). A revision of Melaleuca L. (Myrtaceae) in northern and eastern Australia, 1. Austrobaileya 1: 65–76.

Byrnes, N.B. (1985). A revision of Melaleuca L. (Myrtaceae) in northern and eastern Australia, 2. Austrobaileya 2: 131–146.

Byrnes, N.B. (1986). A revision of Melaleuca L. (Myrtaceae) in northern and eastern Australia, 3. Austrobaileya 2: 254–273.

Cause, M.L., Weatherhead, T.F. and Kynaston, W.T. (1974). The nomenclature, density and Lyctus-susceptibility of Queensland timbers. Queensland Department of Forestry Pamphlet No 13.

Geiger, R.K. (ed.) (1981). Proceedings of Melaleuca symposium, September 23–24, 1980. Division of Forestry, Florida Department of Agriculture and Consumer Sevices.

NAS (1983). Firewood crops: shrub and tree species for energy production. Vol. 2. National Academy Press: Washington D.C.

Ryan, P.A. and Bell, R.E. (1989). Growth, coppicing and flowering of Australian tree species in trials in southeast Queensland, Australia. Pages 49–68. In: (Boland, D.J. ed.) Trees for the tropics: growing Australian multipurpose trees and shrubs in developing countries. ACIAR Monograph No. 10.

Ryan, P. and Bell, R. (1991). Review report on ACIAR Project 8809: Australian hardwoods for fuelwood and agroforestry. Queensland Forest Service, Gympie.

Searle, S.D. (1989). Seed collections of lesser-known trees and shrubs in Queensland, Australia. Pages 27–34. In: (Boland, D.J. ed.) Trees for the tropics: growing Australian multipurpose trees and shrubs in developing countries. ACIAR Monograph No. 10.

Turnbull, J.W. (1986). Multipurpose Australian trees and shrubs: lesser-known species for fuelwood and agroforestry. ACIAR Monograph No. 1.2

Forest Genetic Resources No. 22. FAO, Rome (1994)
Manuscript received July 1994