3010

11: Auxiliary plants

Eucalyptus camaldulensis Dehnh.

Cat. pl. horti camald., 2nd ed.: 6, 20 (1832).

MYRTACEAE

2n = 22

Eucalyptus rostrata Schlechtendal (1847).

River red gum, Murray red gum, red gum (En). Indonesia: ekaliptus. Cambodia: pré:ng khchâl' slök sâ:. Thailand: yukhalip. Vietnam: b[aj]ch d[af]n [us]c.

Eucalyptus camaldulensis is the most widely distributed eucalypt. Its natural distribution area covers most of the Australian mainland, ranging from 12°48'S in the tropical Northern Territory to 38°15'S in cool, temperate Victoria. Eucalyptus camaldulensis is planted in many tropical and subtropical countries and is probably the world's most widely planted tree in arid and semi-arid lands. It is naturalized in many areas.

Eucalyptus camaldulensis is one of the main forestry species for seasonally dry sites in South-East Asia. It is widely planted for shade, shelter and amenity purposes and as a source of nectar to produce high quality honey. Wood of planted Eucalyptus camaldulensis is used mainly for firewood, charcoal, poles, posts and paper pulp. It is also used for hardboard, fibreboard and particle board. Logs may be sawn for construction timber (especially for bridges, wharves and ships), railway sleepers, furniture, flooring and packing cases, although the quality is sometimes poor. The bole has potential as a substrate for shiitake mushroom (Lentinus edodes) cultivation and yields a gum which can be used as a dye. Some tropical provenances produce eucalypt oil suitable for medicinal purposes.

In addition to extensive but largely unrecorded small-scale plantings worldwide for fuelwood, shade and shelter, over 500 000 ha of plantations had been established by the mid-1970s, mainly in the Mediterranean region using provenances from southern Australia. This figure has now probably doubled due to better adapted provenances from northern Australia being planted in tropical areas.
Wood production for domestic consumption is substantial. Wood chips for paper production are exported by several countries in South-East Asia, but statistics for domestic consumption and exports are lacking.

Some tropical provenances of Eucalyptus camaldulensis (e.g. 'Petford') are rich in 1,8-cineole leaf oil and are potential commercial sources of medicinal-grade eucalyptus oil.
The medium-weight to heavy timber is hard and durable. The heartwood has a handsome red colour, turning red-brown upon exposure, and is clearly demarcated from the paler sapwood, which is 50—75 mm wide. The texture is moderately coarse, the grain interlocked, straight or wavy, often producing an attractive figure. The density is 700—980 kg/m³ at 12% moisture content, with samples from natural forest having the higher densities. Density of plantation-grown Eucalyptus camaldulensis varies with age, the provenance used and planting site, but does not appear to be closely correlated with rate of growth. Density is positively correlated with charcoal and pulp yield. Provenances from tropical northern Queensland (e.g. 'Petford') produce wood with the highest density and thus the highest yields of charcoal and pulp.
Mechanical properties of samples from Australia at 12% moisture content are: modulus of rupture 101 N/mm², modulus of elasticity 11 180 N/mm², compression parallel to grain 55 N/mm², shear 15 N/mm², cleavage 89 N/mm radial and 98 N/mm tangential, Janka radial hardness 9745 N, Janka tangential hardness 9525 N and Janka end hardness 10 415 N. The timber is easy to saw despite its high density, and mature material can be seasoned with little degrade. The rates of shrinkage are high: from green to 12% moisture content: 4.4% radial and 8.9% tangential.
The heartwood is resistant to termites, but the sapwood is susceptible to attack by Lyctus borers. Preservation is necessary if the timber is to be used in contact with the ground; the heartwood is extremely resistant, the sapwood is permeable to preservatives.
The wood of plantation-grown Eucalyptus camaldulensis often has unfavourable characteristics such as growth stresses, shrinkage on drying, collapse, spiral grain and starch in the sapwood. Its durability is less than that of trees in natural stands in Australia. Careful post-harvest procedures can ameliorate this. The energy value of the wood is 21 000 kJ/kg. One kg of seed and chaff contains 700 000—800 000 viable seeds, the chaff being ten times heavier than the seed.

Tree, commonly up to 20 m tall, occasionally reaching 50 m with a trunk diameter of 1(—2) m; in open formations with a short, thick bole and a large, spreading crown; in plantations, with a clear bole of 20 m with an erect, lightly-branched crown. Bark smooth, white, grey, yellow-green, grey-green or pinkish grey, shedding in strips or irregular flakes; rough bark may occupy the first 1—2 m of the trunk. Leaves alternate, petiolate, pendulous, (narrowly) lanceolate, 8—30 cm x 0.7—2.0 cm, acuminate, evenly green or grey-green; petiole terete or channelled, 12—15 mm long. Inflorescence an axillary, simple, umbelliform, condensed and reduced dichasium called a conflorescence; umbels solitary, 7—11-flowered; peduncle slender, terete or quadrangular, 6—15 mm long; pedicel slender, 5—12 mm long; flowers regular, bisexual; flowerbuds globular-rostrate or ovoid-conical, divided into a calyx tube or hypanthium (lower part) and the operculum (upper part) which is shed at anthesis; hypanthium hemispherical, 2—3 mm x 3—6 mm; operculum hemispherical, rostrate (northern provenances) to conical (southern provenances), obtuse, 4—6 mm long; stamens numerous, on a staminophore. Fruit a dry thin-walled capsule enclosed in a woody hypanthium, opening with 3—5 strongly exserted valves, hemispherical or ovoid, the hypanthium 3—6 mm x 4—10 mm; disk broad, ascending. Seed minute, about 15 per fruit, smooth, yellow-brown. Seedling with epigeal germination and bilobed cotyledons; first 4—6 pairs of leaves decussate; subsequent leaves alternate. Juvenile leaves alternate, petiolate, ovate to broadly lanceolate, 13—26 cm x 4.5—8 cm, green, grey-green or blue-green, slightly discolorous.

The germination rate is generally high and can reach almost 100%. Lignotubers develop early in the life of northern Australian provenances of Eucalyptus camaldulensis, but are mostly absent in those from southern Australia. Growth rates vary greatly between provenances and are heavily site-dependent. Seedling growth may exceed 3 m per year for well-adapted provenances on favourable sites. In trials in Peninsular Malaysia using a provenance from Ferguson River (Northern Territory) on three different sites, 4-year-old trees showed a mean annual height increment of 2.3—4.0 m and a mean annual diameter increment of 1.6—3.9 cm. In a trial over 8 years with 51 provenances conducted in Biñga, the Philippines, the survival rate ranged from 1—89%, and the average annual growth rate from 1.15—6.84 m in height and from 0.25—6.3 cm in diameter. In trials in Thailand, the mean annual increment was 1.7—4.1 m in height and 1.6—3.9 cm in diameter in the first two years after planting.
Time of flowering in natural stands depends on the geography of a given location. Flowering peaks in summer in southern Australia, in autumn in the far north-west and in winter—spring in the far north-east. In Thailand, some provenances flower almost throughout the year on a range of sites, although September—November is the peak period. Pollination is mainly by insects but also by birds and small mammals. Seeds ripen about six months later. In Thailand, peak flowering corresponds with seed ripening in April—May.
In South-East Asia, the period from planting to production of the first seed crop may be as short as three years. In Thailand, Eucalyptus camaldulensis may start flowering when 16—38 months old, but 24—28 months is common.
Eucalypts do not develop resting buds and grow whenever conditions are favourable.

There is considerable morphological variation within Eucalyptus camaldulensis, which is not surprising given its wide geographic distribution. Six varieties have been described, but this division has been largely ignored because of difficulties in identification. The northern and southern provenances are sometimes accommodated in two varieties: var. camaldulensis and var. obtusa Blakely, respectively. Var. camaldulensis has rostrate opercula, while var. obtusa has obtuse or rounded ones. However, the variation in this character seems to change gradually with the location.
Eucalyptus camaldulensis is closely related to Eucalyptus tereticornis Smith. The latter can be distinguished by its taller and more steeply branched habit, its acutely conical opercula and the black, rough-coated seeds. Where both species grow naturally, as in eastern Victoria and Queensland, hybridization and subsequent introgression occurs. Several populations in far northern Queensland, previously identified as Eucalyptus tereticornis, show several characteristics of Eucalyptus camaldulensis and are now considered a separate subspecies of the latter called subsp. simulata. Among them are the fast-growing provenances of 'Laura River', 'Palmer River' and 'Walsh River' that are widely used in South-East Asia. Natural hybrids between Eucalyptus camaldulensis and Eucalyptus alba Reinw. ex Blume are also reported, while in plantations hybridization with Eucalyptus grandis W. Hill ex Maiden occurs.

Under natural conditions, Eucalyptus camaldulensis occurs typically along watercourses and on floodplains, very occasionally in southern Australia extending to hills or ranges, usually in open forest and woodland, at 20—700 m altitude. It grows under a wide range of climatic conditions, from temperate to hot and from humid to arid. Annual rainfall in natural stands varies from 250—2500 mm, but planted trees can survive in areas with as little as 150 mm annually. In arid regions, it depends on the presence of a high water table or seasonal flooding for survival. The length of the dry season may vary from 0—8 months and the rainfall distribution varies from a winter maximum in southern regions to a monsoon type with summer rains in northern areas. Mean minimum temperature of the coldest month ranges from 3—22°C, mean maximum temperature of the hottest month from 21—40°C and mean annual temperatures from 13—28°C. In general, Eucalyptus camaldulensis tolerates up to 20 frosts per year, but does not tolerate temperatures below —10°C. The optimum temperature for germination is 32°C, but a wide range is tolerated. Eucalyptus camaldulensis occurs on a variety of soils, commonly on sandy and silty alluvial soils, but occasionally on heavy clays in southern Australia; it is also found along the borders of salt lakes. It is not adapted to calcareous soils, except for a few populations in southern and western Australia growing on shallow soils over limestone. Provenances may differ considerably in frost tolerance, fire resistance and salt tolerance.

Selection of the proper genetic material for particular planting conditions is of paramount importance. Eucalyptus camaldulensis is usually propagated by seed. As a rule-of-thumb, 1 kg of seed is sufficient to provide plants for 100 ha at a spacing of 3 m x 2 m and the typical seedling recovery rate of 25%. Seed is best stored dry (5—8% moisture content) in airtight containers at 3—5°C. Viability will be maintained for several years and is still about 30% after being stored for seven years. No pre-sowing treatment is required. The fine yellow-brown seed and chaff are sown together under shade in a well-drained and sterilized medium and covered very sparingly with sand. After 4 days, seed has germinated and shade should be reduced. When 2 pairs of leaves have developed, seedlings are pricked out into containers such as polythene bags filled with a sterilized potting mix. Shading is needed for the first week after transplanting, thereafter plants should be fully exposed. A polythene bag size of 15 cm x 5 cm proved most economic in Nigeria. Direct sowing in polythene bags or in open nursery beds for the production of bare-rooted planting stock is also practised. Growth is fast under tropical conditions, and plants can be planted out after 3 months, occasionally after 6 weeks, when they are 30 cm tall. Excessive watering and shade often result in damping-off and in seedlings becoming too tall and weak for easy transplanting.
Eucalyptus camaldulensis is suited to mass vegetative propagation. Cuttings from juvenile shoots (i.e. below the 10^th node) root readily in about 30% of the genotypes. A major reforestation project in Morocco is based entirely on cuttings of Eucalyptus camaldulensis. In South-East Asia, propagation by cuttings is an integral component of breeding programmes. Elite trees are selected in young plantations (5 years old) and felled or girdled to promote coppicing. Coppice shoots of about 1 m long are collected and divided into pencil-sized cuttings with two leaf pairs. Half of the leaf blade is then trimmed and the cuttings are dipped into a hormone preparation and planted in pots under mist and shade. Rooted cuttings are usually planted in nurseries to provide further shoots. Methods of in vitro propagation have been developed.
Spacing varies with the management system — from community planting around homes, villages and roads to closely-spaced commercial plantations — and depends on the end-products required. For firewood, spacings as close as 2 m x 2 m are used; for pulpwood, a spacing of 3 m x 2 m is often applied. Wider spacings of 4 m x 2 m or 5 m x 2 m are recommended when larger trees are the objective. In plantations, Eucalyptus camaldulensis has a comparatively narrow crown and pendulous leaves which allows light to reach the forest floor. This is favourable for intercropping with food crops but also promotes weed growth. A spacing of 5 m x 2 m is recommended to allow intercropping during the first three years. Application of 100 g of NP or NPK (3:2:1) fertilizer to each tree at planting to assist establishment and early growth is common. In trials in Thailand, survival was 80—90% 12 months after planting.

Poor competition ability with weeds and the development of an open crown imply frequent weeding, up to 3 times per year, until the canopy closes 3—5 years after planting. Inadequate weed control may lead to complete failure of the plantation. Intercropping may facilitate proper weed control. A thinning to less than 700 stems/ha at 5 years provides posts, poles, fuelwood and pulpwood, leaving the better trees for the production of e.g. sawn timber after 10 years.
Crown dieback during the dry season as a result of boron deficiency is prevalent in parts of Africa, Asia and South America and must be corrected. A dosage of 10—20 g of borax per tree is recommended, depending on soil type.
All fast-growing provenances tested coppice well. The rotation may be as short as 3—5 years for small-sized pulpwood in Thailand and Vietnam, but is generally 8—10 years. In Israel, maintaining a plantation for 5 successive 10-year coppice rotations has been successful, but in general 2—3 coppice rotations of 10—12 years are feasible. Reduction of the number of coppice shoots on a stool is a most important and time-consuming operation in coppice management. In Nepal, a single reduction at 3—6 months to one shoot per stump is recommended. Competition from eucalypts can severely reduce yields of interplanted crops. In an experiment in northern Nigeria, pearl millet (Pennisetum glaucum (L.) R. Br.) yields were reduced up to at least 18 m from a eucalypt shelter-belt and pruning the roots of the eucalypts down to 1 m significantly increased millet yields.

In the nursery, Eucalyptus camaldulensis is susceptible to various fungi causing damping-off and leaf diseases. Proper hygiene and watering sparingly minimize damage. Insects (e.g. termites and aphids) and rodents may be troublesome, and both physical and chemical control measures are used.
In Australia, natural stands and plantations are affected by many fungi and insects. On suitable sites outside Australia, Eucalyptus camaldulensis is relatively free of diseases and pests. Stem canker and leaf diseases proliferate where rainfall and humidity are much higher than encountered in the natural habitat. In South-East Asia, Eucalyptus camaldulensis may be defoliated by fungi including Cylindrocladium spp. during the rainy season. The most susceptible provenances suffer mortality and general decline, but well adapted provenances (e.g. 'Katherine') are little affected.
In parts of Africa and Asia, termites attack seedlings and young trees and must be chemically controlled. In Africa, the Eucalyptus snout beetle (Gonipterus scutellatus), of Australian origin, feeds on young shoots but is controlled biologically; moribund or newly-felled trees may become infested with an Australian stem borer or the longicorn beetle (Phoracantha semipunctata).

Eucalyptus camaldulensis is usually grown on a short rotation and clear-felled at an age that maximizes production for a particular end-use. Generally, this is small-diameter material for fuelwood or pulp. The felling season affects coppice regeneration. Felling during the dry season delays sprouting and increases the risk of the stump drying out. Felling by saw to give a clean-cut short stump with minimum bark damage is best. In coppice systems e.g. in Nepal, some stems are sometimes left uncut as standards. This practice is recommended to produce wood of a range of diameters suitable for various products.

Very high productivity is possible under favourable conditions: a mean annual increment of 70 m³/ha of four-year-old trees planted at 3 m x 2 m on a fertile site with high water availability has been recorded in Israel. However, such conditions are seldom met. In the drier tropics, yields of 5—10 m³/ha per year on a 10—20-year rotation are common, whereas in moister regions up to 30 m³/ha per year may be achieved on 7—20-year rotations. In southern Vietnam, mean annual increments of 12 m³/ha over 4 years have been recorded, which can reach 20 m³/ha for the best adapted provenances. Coppice rotations give higher yields than the initial seedling rotation (e.g. 25—30 m³/ha per year versus 17—20 m³/ha per year in Turkey) and the length of the rotation may be adjusted accordingly.

End-splitting of roundwood may be reduced by felling during winter months. For sawn timber production in Pakistan, it is recommended to fell in October, convert immediately into 70 mm quarter-sawn planks, carefully stack in a well-ventilated room and then top load each stack, in order to reduce defects.

Both primary and secondary centres of diversity hold vast genetic resources of Eucalyptus camaldulensis. It is often impossible to trace the origin of seed used for plantations, so the extent of genetic variation available in various areas is uncertain. Systematic introduction of appropriate seedlots from native Australian stands is highly recommended to ensure that a wide genetic variation is used for selection and breeding.
In Australia two groups of provenances are distinguished: a northern tropical group and a southern temperate group. The better-performing tropical provenances, such as 'Petford' and 'Katherine' are generally the most sought-after for breeding programmes in South-East Asia. The Australian Tree Seed Centre (ATSC) provides both single-tree and bulk provenance collections of Eucalyptus camaldulensis for breeding programmes. A well-documented, 400-tree collection from the Petford region is presently available at ATSC along with accessions from many other areas.

The ideal commercial tree should have good vigour and resistance to diseases and pests, a straight single bole, drought tolerance, good coppicing ability, high pulp yield (light-coloured timber), thin branches and good self-pruning ability, and a thin bark.
Although seed availability of climatically adapted northern Australian provenances has increased, supplies are still insufficient to meet demand in South-East Asia. Consequently, a number of countries in the region support selection and breeding programmes, for instance the comprehensive programme in Thailand is based on seed from local trees and 308 seedlots from northern Australia including 200 from Petford. These accessions have been planted in orchards in 4 locations with different environments; they will be thinned progressively and supply improved seed and coppice material.

Eucalyptus camaldulensis is one of the best performing trees in the seasonally dry tropics for an impressive array of end-products. The success of Eucalyptus camaldulensis as an exotic is attributed to its superiority to other trees in the production of wood for firewood, charcoal and other purposes on infertile dry sites, its tolerance of drought and high temperature combined with rapid growth when water is available, its tolerance of periodic waterlogging and soil salinity and its fair tolerance of fire and frost. Its productivity and versatility can be enhanced by breeding programmes now under way in South-East Asia and elsewhere.
With careful selection of provenances for specific sites, Eucalyptus camaldulensis is expected to gain importance in South-East Asia.

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Jacobs, M.R., 1981. Eucalypts for planting. 2nd Edition. FAO Forestry Series No 11. Food and Agriculture Organization of the United Nations, Rome, Italy. pp. 369-381.
Kijkar, S., 1991. Handbook: producing rooted cuttings of Eucalyptus camaldulensis. Association of South East Asian Nations(ASEAN)-Canada Forest Tree Seed Centre Project, Muak-Lak, Saraburi, Thailand.
Midgley, S.J., Eldridge, K.G. & Doran, J.C., 1989. Genetic resources of Eucalyptus camaldulensis. Commonwealth Forestry Review 68: 295-308.
Onyewotu, L.D.Z. & Stigter, C.J., 1995. Eucalyptus - its reputation and its roots. Millet and a eucalyptus shelterbelt in northern Nigeria. Agroforestry Today 7: 7-8.

J.C. Doran & W. Wongkaew

Doran, J.C. & Wongkaew, W., 1997. Eucalyptus camaldulensis Dehnh.. In: Faridah Hanum, I & van der Maesen, L.J.G. (Editors): Plant Resources of South-East Asia No 11: Auxiliary plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea