Eucalyptus saligna Sm.
Saligna Eucalyptus
Myrtaceae -- Myrtle family
Roger G. Skolmen
Saligna eucalyptus (Eucalyptus saligna), also called Sydney bluegum, is a fast growing tree, valuable in plantation forestry. It grows in several warm temperate to subtropical countries, such as Brazil and the Republic of South Africa, and the state of Hawaii.
The name Eucalyptus saligna was given to type specimens in 1797. Another very similar but distinct species, found within the same geographic range, Eucalyptus grandis, was not named until 1918 (12). Before 1918, many introductions were made worldwide of seed collected from "E. saligna" that bore the characteristics of the type later to be called E. grandis. In most countries where introductions were made, therefore, considerable mixed planting and hybridization of the two species are present. Thus, in Hawaii, most saligna eucalyptus stands contain trees with a range of characteristics intermediate between those of E. saligna and E. grandis. Eucalyptus grandis is now preferred in South Africa because it self-prunes more readily and has smaller branches (28); and in Brazil because it is resistant to a canker disease and can be propagated vegetatively (6). Eucalyptus saligna has grown well where the climate is cooler; for example, in northern New Zealand (12) and in the uplands of Hawaii. Recent provenance tests of the two species in Hawaii suggest that E. grandis would be a better choice than E. saligna for most sites (26).
Saligna eucalyptus is native to the east coast of Australia from Bateman's Bay (lat. 36° S.) in southern New South Wales to the southeastern corner of Queensland (lat. 27° S.) (13). In the United States, it has been introduced into Florida, California, and Hawaii. In Hawaii it reproduces at the edges of planted stands. Although it was introduced into Hawaii in the late 1800's, the tree was not planted extensively until the 1960's, when it became the principal tree used for forestation.
In Australia, the tree grows from sea level to 300 In (1,000 ft) in the South and to 1220 m (4,000 ft) in the North. The climate within this range is warm-temperate to subtropical, with winter frosts to -15° C (5° F) at the higher elevations (12). In New Zealand, seedlings were frost tolerant to a minimum temperature of -7° C (21° F) (21). Rainfall is evenly distributed, or has a summer maximum, and ranges from 890 to 1270 mm (35 to 50 in) annually (13).
In Hawaii, saligna eucalyptus grows well between elevations of about 150 m (500 ft) and 1100 m (3,600 ft) where the temperature is never below 4° C (40° F). One stand is at 1980 m (6,500 ft) where light winter frosts occasionally occur, and the average daytime temperature is about 16° C (60° F). Most of the saligna eucalyptus stands have been planted between 300 and 610 m (1,000 and 2,000 ft) elevation in locations with evenly distributed or winter maximum rainfall of 1520 to 7620 mm (60 to 300 in) annually. The tree achieves its best growth on sites with about 2540 mm (100 in) annual rainfall, rather than on wetter sites, possibly because sunlight is greatly reduced by the cloud cover on wetter sites.
In the northern part of its range in Australia, saligna eucalyptus extends to the slopes and ridges. In northern New South Wales and Queensland, it is usually on the slopes, while the closely related rosegum eucalyptus (Eucalyptus grandis) is usually near or at the valley bottoms. Saligna eucalyptus does best on clay loams derived from shales and requires good drainage (13).
In Hawaii, saligna eucalyptus has been planted extensively on Histosols and Inceptisols on the island of Hawaii, and also on the Oxisols and Ultisols of Maui, Molokai, Oahu, and Kauai. These soils have in common moderate to strong acidity, low to very low available nitrogen and phosphorus, and rapid to very rapid drainage. All are formed on basaltic parent material, either volcanic ash or rock. In other respects they differ considerably, but all are unsuited or only marginally suited for agriculture. Slopes are usually 10 to 20 percent.
In Australia, saligna eucalyptus is usually found in mixture with tallowwood eucalyptus (Eucalyptus microcorys) and blackbutt eucalyptus (E. pilularis), the main coastal species of New South Wales, and is also found associated with several other eucalypts. It seldom grows in pure stands, whereas the closely related rosegum eucalyptus is typically found in pure stands (13). The common names used follow those of Bryan and Walker (2).
In Hawaii, saligna eucalyptus has been planted in mixture with three species of eucalyptus-tallowwood, robusta (Eucalyptus robusta), and rosegum-with melaleuca (Melaleuca quinquenervia), Formosa koa (Acacia confusa), horsetail casuarina (Casuarina equisetifolia), silk-oak (Grevillea robusta), and a host of other species. On most sites, it has outgrown and shaded out or badly suppressed all of these species except the equally fast growing rosegum eucalyptus and the tolerant Formosa koa. In closed stands, about the only understory species found are strawberry guava (Psidium cattleianum) and occasional treefern (Cibotium spp.).
Saligna eucalyptus grown in Hawaii regenerates naturally on bare soil immediately after logging, or on cultivated land adjacent to planted stands. It rarely becomes established in undisturbed grass or brush cover and never in its own shade. Coppicing of stumps is variable. Usually about half to two-thirds of the stumps will sprout (26). Age, weather, and (probably) heredity influence coppicing. The tree also usually produces a mass of special bud tissue at the groundline known as a lignotuber. The lignotuber will sprout if the stem is killed back by fire or other injury.
Flowering and Fruiting- Saligna eucalyptus trees begin to flower at 3 to 4 years of age. Flowering in Hawaii is most prolific during January to March but occurs to some extent year round. In Australia, the tree also flowers from January to March; in California, from April to June. Flowers that consist of numerous stamen filaments surrounding a single shorter pistil occur in umbels of 4 to 9 flowers. Before opening, the flower buds are about 10 mm (0.4 in) long and 5 min (0.2 in) in diameter with a short stalk (pedicel), and a blunt, rather pointed cap (operculum) enclosing the stamens. Flowers are perfect. The opened flowers are yellowish white and are insect pollinated. Pollen is generally shed before the style becomes receptive, so selfing is rare.
The fruit is a dark-brown, bell-shaped capsule 0.8 em (0.3 in) long and 0.5 cm (0.2 in) in diameter. It is short stalked and has four pointed, rim level, or slightly exserted valves. The capsule ripens about 6 months after flowering but opens to release seed 1 or 2 months after ripening (12,13,20).
Seed Production and Dissemination- Seeds are black, irregularly shaped, and about 1.3 min (0.05 in) in diameter. They are released along with a large amount of reddish-brown chaff when the capsule valves open. There are 460 viable seeds per gram (13,000/oz) of seed plus chaff (20).
Seeds are naturally dispersed by wind. They can be collected from ripe capsules dried to open after picking. Some unopened ripe capsules are always present on trees in Hawaii but are most common in August and September. Fresh seeds germinate readily in 10 to 20 days without pregermination treatment. Seeds can be stored in airtight containers for several years at 0° to 5° C (32° to 41° F) (20).
Seedling Development- The seedling has obcordate (inverse heart-shaped) cotyledons that are home epigeously as in all eucalypts. Juvenile leaves are opposite for 3 or 4 pairs, then become alternate, short stalked and lanceolate, and 2.5 by 5.0 cm (1 by 2 in) in size. The adult leaves are alternate, stalked and lanceolate, tapering to a long point, 2.3 by 15 cm (1 by 6 in) in size (13).
In Hawaii, nursery-grown seedlings in containers reach plantable size in 4 to 5 months. Although seedlings are hardy and will survive bare or open-rooted planting, planting of container-grown stock provides more assurance of success if the weather is dry just after planting. Under adverse conditions newly planted seedlings often desiccate and suffer leaf-drop, but such plants usually sprout from the lower stem and recover. When this dieback slows growth, additional weeding or maintenance usually is required to clear competition (32).
Around the world, seeds usually are germinated in flats containing light-textured medium, and seedlings are transplanted into other containers after 6 to 8 weeks when a third pair of leaves begins to appear (12). Seeds also are sown directly into beds or tubes, but thinning of seedlings is usually required with this method because the small seeds are difficult to handle individually. Thinning requirements can be overcome by using pelletized seed and seeding devices (15,31).
In Hawaii, saligna eucalyptus seedlings have been grown extensively in open beds. Because of their rapid growth, these seedlings usually are root pruned at 15-cm (6-in) depth at 6 months and top pruned at 8 months to a 30-cm (12-in) height. Bare root stock frequently has not survived well after field planting, and Hawaii's practice has now changed to growing seedlings only in polyethylene tubes (30).
On favorable sites in Hawaii, planted seedlings grow to about 3 m (10 ft) in height in 1 year, and 3 to 5 in (10 to 16 ft) per year for the next 10 years. After clear cutting of a 44-year-old saligna eucalyptus plantation, natural seedlings that became established grew to saplings that averaged 9 cm (3.5 in) in d.b.h., and 11 rn (36 ft) in height, 22 months after logging. Several of these saplings were 18 m (59 ft) tall.
Vegetative Reproduction- Saligna eucalyptus can sprout prolifically from dormant buds located in the cambium throughout the stem. After a tree is cut, shoots sprout from many points on the remaining bark surface. Those highest on the stump suppress those lower down and, if not broken off by wind or by weak attachment, become coppice stems that overgrow the stump (12).
Sprouts will also grow from the lignotuber, a mass of bud tissue at or just below the groundline. Lignotubers are found on saligna eucalyptus from all but its northernmost provenances, but not on rosegum eucalyptus (12). In managing saligna eucalyptus for coppice, it is desirable to cut stumps 12 cm (5 in) or less in height, so that the sprouts will develop from near the lignotuber. Such sprouts generally are more firmly attached but are frequently suppressed by sprouts arising from higher on the stump. Lignotubers persist when stems are killed by shading, thinning, or fire and often sprout vigorously after a mature stand is cut (8).
Rooting of cuttings of saligna eucalyptus had been difficult (16) until a method was developed at the Aracruz Co. in Brazil (7). The method consists of collecting coppice sprouts that are just beginning to harden and keeping them constantly moist while 2-leaf-pair cuttings are prepared and end-dipped in rooting hormone. The cuttings are placed under intermittent mist in individual containers. In Hawaii, saligna eucalyptus has been easier to root than E. grandis, although most success elsewhere has been with E. grandis (3,7). However, just as was found for E. grandis in Brazil (6), cutting rootability is variable among coppice from individual saligna eucalyptus trees.
Tissue culture propagation has also been successful in Hawaii. The techniques used with saligna eucalyptus are essentially those reported by Boulay (1) for other eucalyptus species. Terminal and lateral shoot tips of greenhouse-grown rooted cuttings are multiplied, separated, and rooted in sterile culture, and afterwards grown to normal size in a mist chamber. A number of propagules of saligna eucalyptus produced by tissue culture are now being compared in clonal progeny tests.
Grafting success has been reported for saligna eucalyptus (24,27). Cleft, side, splice, and bottle grafting were all used successfully, but the tests were not observed for a long enough period to determine the extent of long-term incompatibility, a problem with many species of Eucalyptus.
Growth and Yield- Saligna eucalyptus is a fast growing tree, well suited for producing high yields of wood fiber on short rotations. Measurements of a plantation spacing study on a good site at Kaumahina, Maui (29) provide an example. Four spacings were tested: 2.4 by 2.4 m, 3.0 by 3.0 in, 3.7 by 3.7 m, and 4.3 by 4.3 m (8, 10, 12, and 14 ft). At 2 years, trees averaged 9.6 cm (3.8 in) in d.b.h. and 10.7 rn (35 ft) in height. At 5 years, they had grown to 20.8 cm (8.2 in) in d.b.h. and 22.9 in (75 ft). Mean annual volume increment had already peaked at the two closer spacings in the study and was rapidly leveling out at the wider spacings. At 15 years, the trees in this study averaged 26.7 cm (10.5 in) in d.b.h. and 39 in (129 ft) tall. The largest tree was 61 cm (24 in) in d.b.h. and 49 rn (161 ft) tall. At 5 years, the trees at 2.4 by 2.4 m (8 by 8 ft) had produced 294 m³/ha (4,200 ft³/acre), or 58.8 m³/ha (840 ft³/acre) per year. At 15 years, these trees yielded 683 m³/ha (9,759 ft³/acre), or 45.6 m³/ha (651 ft³/acre) per year. Trees at 4.3 by 4.3 m (14 by 14 ft) yielded 33.1 m³/ha (473 ft³/acre) per year.
These figures are comparable to those of Eucalyptus grandis/E. saligna grown in other countries. In Kenya, a mean annual increment over 5-year periods of 21 m³/ha (300 ft³/acre) for the seedling crop followed by 32 m³/ha (457 ft³/acre) for the first coppice crop was obtained (11). Other mean annual increment figures cited for E. grandis are 14 to 45 m³/ha (200 to 643 ft³/acre) in Uganda, 28 m³/ha (400 ft³/acre) in Zambia, 50 m³/ha (715 ft³/acre) at 14 years in Argentina, and 22 m³/ha (314 ft³/acre) in New South Wales, the native habitat of both species (12).
In two 4-year-old stands in Hawaii, annual increment averaged 13 and 36 m³/ha (185 and 515 ft³/acre). The faster growing stand yielded wood with a specific gravity of 0.41 for an estimated annual dry-weight yield of stem wood of 15 tonnes/ha (6.7 tons/acre).
The tallest tree in Hawaii, thought to be the tallest hardwood in the United States, is a saligna eucalyptus. When last measured in 1979, the tree was about 50 years old, 137 cm. (54 in) in d.b.h. and 82.3 m (270 ft) tall.
Rooting Habit- Saligna eucalyptus develops roots throughout the soil profile so that it is quite windfirm on deep soils, but easily windthrown on shallow soils. It does not produce a taproot. Roots are primarily from the stem below the lignotuber, although layering sometimes occurs a short distance from the lignotuber on buried stems. In plantations in Hawaii that are not subject to periodic short drought, about two-thirds of the root system is confined to the upper 61 cm (24 in) of soil where most of the available nutrients are found. In plantations subject to occasional drying of the surface soil, the shallow roots are killed and a deeper root system develops.
Reaction to Competition- Because the tree is such a fast starter, planted seedlings can frequently grow faster than surrounding grass and herbaceous vegetation and shade it out. This is particularly true if the seedlings have an intact root system when planted, as in modern tube container planting, so that little or no "shock" occurs to delay new growth after planting. At the upper elevational boundaries of sugarcane fields, saligna eucalyptus grown from seed in the soil at the time of cane harvest actually outgrew the sugarcane ratoon crop.
In Hawaii, original plantings are made on completely cleared land. Pre-emergent herbicides, though effective, have rarely been used. If pre-emergents are not used, one cleaning around trees that require it is made after 3 months and, depending on the site, a second cleaning may sometimes be made at 6 months. Further weeding is seldom necessary. Coppice growth of saligna eucalyptus is so rapid that competing plants are rarely a problem after cutting.
Tests in Hawaii show that the leguminous tree Albizia falcataria outgrows saligna eucalyptus on some sites when planted row on row with both species equally fertilized. It is one of the few woody plants known that can grow this fast on sites that are suited for saligna eucalyptus. The trials of mixing the legume with E. saligna produced increased yields of the eucalypt on some wet sites, but reduced yields on other, drier sites (10).
In South Africa, thinning schedules have been developed for trees planted at 1330/ha (538/acre) that call for thinning 25 percent of the stems present at 6 years when the stems removed are 13 cm (5 in) in diameter, and 25 percent again at 10 years when they are 20 cm (8 in) (12). These thinnings are continued at 3- to 5-year intervals until a sawtimber harvest is made at age 30. In the interim, all stumps are allowed to coppice to keep the site free of competition and to supply fuelwood crops.
Saligna eucalyptus is classed as very intolerant of shade and the slower growing trees in a stand quickly become suppressed. In Hawaii, crown closure is usually complete and crown differentiation begins in 3 years in stands planted at 3 by 3 in (10 by 10 ft). In coppice stands where numerous stems grow from every stump, crown differentiation begins as soon as sprouts appear. Many studies have shown that the maximum yield of wood is obtained by not thinning coppice at all (12). However, if larger diameter and straighter stems are desired, thinning to one to three stems per stump is desirable.
Damaging Agents- Saligna eucalyptus grown in plantations in many parts of the world is susceptible to the eucalyptus canker disease, Cryphonectria cubensis. The disease kills young trees, deforms stems, and causes basal cankers that reduce the coppicing ability of stumps (19). Rosegum eucalyptus is somewhat resistant and Eucalyptus urophylla, perhaps, is immune to the disease, so these species are now being used in place of E. saligna in many Brazilian plantings. In Hawaii, the disease is present only on the island of Kauai. It attacks E. grandis in Florida but is not causing serious damage (18).
In Western Australia, two other canker diseases, Botryosphaeria ribis and Endothia havaensis, were determined to be pathogenic on E. saligna planted there, while another, Cytospora eucalypticola, was present but less damaging (14).
Phoracantha semipunctata, a wood-boring insect, degrades wood and reduces growth of eucalyptus in many places, including Hawaii, but is only a serious problem in trees that are stressed by severe drought. In Australia, saligna eucalyptus is subject to damage by Spondyliaspis psyllids, which predispose the trees to attack by the wood-boring beetle Xyleborus truncatus (22).
In Hawaii, wind damage is a severe problem. In January 1980, a severe windstorm caused severe blowdown in 75 percent of the saligna eucalyptus stands planted during the 1960's (17).
In Hawaii, saligna eucalyptus has been used to some extent for sawtimber, but only with considerable difficulty and expense. Most of the milling and lumber quality problems are those associated with growth stress-severe end-splitting of logs, spring of cants during sawing, compression failures, and brashness of the wood near the pith (25). Because of this, the tree is now planted primarily for early harvest as pulpwood, or, if it proves economic in the near future, as industrial fuelwood to replace oil.
Elsewhere in the world, particularly in South Africa and Brazil, the trees and their close relative, E. grandis, are grown extensively for pulp, poles, and fuel.
In an attempt to solve the problem of confused and probably mixed introductions of Eucalyptus saligna and E. grandis, differences between them have been noted for mature trees in South Africa (12), as follows:
| E. saligna | E. grandis | |
| Bark: smooth type bluish; rough type on lower stem | Bark: smooth type white; rough type often extends up stem | |
| Flowering (South Africa): January to April | Flowering (South Africa); July to December | |
| Valves of fruit: 3 or 4 pointed, straight or spreading | Valves of fruit: 4 to 6 blunt, incurved | |
| Root crown: Lignotuberous | Root crown: Not lignotuberous | |
| Branches: Persistent under shade | Not persistent under shade |
These characteristics vary among provenances of each species. The northernmost provenances of saligna eucalyptus, for example, do not have lignotubers (12). When grown in some locations, for example, Hawaii, flowering seasons overlap and trees probably hybridize extensively. Among 6-year-old trees of provenances collected in Australia growing side-by-side at two locations in Hawaii, no consistent differences were observed between E. saligna and E. grandis in leaves, bark, or branching habit (26).
Saligna eucalyptus produces denser wood than E. grandis, but in Hawaii (26), and also in the Republic of South Africa (9) where yields of the two species growing on the same sites have been compared, the best performing E. grandis provenances for a particular site produce a higher total weight yield than E. saligna, despite the wood density difference.
Because of the wide international interest and the problems of hybridization and identification of the two species, a comparison of E. saligna and E. grandis populations representative of the entire range of each species was made in Australia (4). Distinct differences were found in seedling and mature-tree morphology and allozyme frequencies between core populations of the two, but intermediate types were found in some remote locations. Core mature saligna eucalyptus had smaller seed, upright valves (4 per fruit), and non-glaucous fruit and branchlets as compared with E. grandis, which had larger seed, incurved valves in 5's, glaucous fruit and branchlets. Saligna eucalyptus seedlings had lignotubers and were glaucous; not so, E. grandis. Saligna eucalyptus seedlings also had smaller cotyledons and narrower, longer leaves. The allozyme patterns found for native populations in Australia showed species differences and were later compared to patterns found for populations collected in the Republic of South Africa, which were thought to be hybridized (5). All the South African trees sampled fell within the allozyme patterns found in Australia for E. grandis, even though several were morphologically suspect.
In addition to the Eucalyptus grandis/E. saligna complex, E. saligna crosses with E. robusta, bangalay eucalyptus (E. botryoides), and probably with forest redgum eucalyptus (E. tereticornis) (12,28). In the southern part of its natural range, a region of introgression of E. saligna with E. botryoides exists (23).