Eucalyptus robusta Sm.
Myrtaceae -- Myrtle family
James R King and Roger G. Skolmen
Robusta eucalyptus, Eucalyptus robusta, is native to a narrow coastal area in southeastern Australia. The species is widely adaptable and has been introduced into many tropical, subtropical, and warm-temperate climates including Puerto Rico, southern Florida, coastal California, and Hawaii. It is naturalized only in southern Florida and Hawaii. Commonly called swamp-mahogany in Australia, it is usually called robusta eucalyptus in the United States (2,16), and beakpod eucalyptus in Puerto Rico (17).
The species was originally introduced as a candidate for timber production, fuel, watershed protection, and windbreaks. By 1960, more than 4650 ha (11,500 acres) of plantations were established in Hawaii. The species has been studied in Florida as a source of pulpwood (8).
Robusta eucalyptus is native along the Australian coast of New South Wales and southeast Queensland. It is found mainly in swamps and on the edges of coastal lagoons and rivers where it is subject to periodic flooding (5,9). The mean maximum temperature in the hottest month is 30° to 32° C (86° to 90° F); the mean minimum of the coldest month is about 3° to 5° C (37° to 41° F). Throughout the native range, from 5 to 10 light frosts occur each year (6).
In Hawaii, robusta eucalyptus grows well from near sea level to 1100 m (3,600 ft) where annual rainfall ranges from 1000 mm (40 in) to 6350 mm (250 in) and temperatures rarely if ever reach freezing.
Robusta eucalyptus in Florida grows mainly in the southern portion of the State where frosts may occur annually. Mean annual rainfall averages 1320 min (52 in) with 70 to 80 percent of rain falling during the May to October wet season.
In Puerto Rico the species makes its best growth in mountain regions about 460 m (1,500 ft) where annual rainfall averages 2540 mm (100 in) (17).
In southern California and along coastal northern California, plantings of robusta eucalyptus have been subject to several unseasonal cold spells (11,20,21) where temperatures reached -9° C (16° F). In every instance severe foliage damage was initially observed (more than 80 percent of the crown foliage killed), but the stems recovered within 3 months.
Although robusta eucalyptus can recover from occasional severe frost damage, the limiting variable in its distribution seems to be low temperature. If the temperature drops below -9° C (16° F) annually, introduced robusta eucalyptus will seldom be successful. In Yunnan Province, China, -7° C (19° F) damaged robusta eucalyptus, but to a lesser extent than E. globulus (4).
Robusta eucalyptus grows well on a variety of soils, ranging from its native intermittently flooded sites (6,9) to the hot summer-dry soils of California's Central Valley (11).
In Florida, typical soils are poorly drained, acid, fine sands with hardpans at depths proportional to the depth of the seasonally high water table. Robusta eucalyptus does best on the least poorly drained of these soils, which are typical of arenic and aeric haplaquods of the order Spodosols (7).
Most robusta eucalyptus in Hawaii are planted on sites considered too steep for agriculture-usually slopes of 10 to 20 percent. On the older islands of Kauai, Oahu, Molokai, and Maui, trees were planted predominantly on Oxisols and Ultisols. On the youngest island, Hawaii, plantings are mainly on Histosols and Inceptisols. All these soils are formed on basaltic parent materials, either volcanic ash or lava rocks. Soils are low in nitrogen and phosphorus and often strongly acidic. The lava substrate may be in either almost continuous sheets or in highly fractured porous clinkers. Soil drainage, therefore, varies from very poor to extremely rapid in very short distances.
In its native range the species is dominant in some areas and is often found in pure stands. Associated trees may include kinogum eucalyptus (Eucalyptus resinifera), bloodwood eucalyptus (E. gummifera), forest redgum eucalyptus (E. tereticornis), longleaf casuarina (Casuarina glauca), and various species of Melaleuca (8).
Throughout the 1930's, when most of the tree planting was done in Hawaii, robusta eucalyptus was used to overplant failed plantations. Consequently, because robusta eucalyptus could survive on a wide variety of sites, it is found in many mixed plantings. Some common associates with robusta eucalyptus are saligna eucalyptus (Eucalyptus saligna), tallowwood eucalyptus (E. microcorys), melaleuca (Melaleuca quinquenervia), horsetail casuarina (Casuarina equisetifolia), and silk-oak (Grevilla robusta). Treefern (Cibotium spp.) is also quite common in the understory of planted stands. One report refers to a pure stand of robusta eucalyptus being heavily invaded by Javanese podocarpus (Podocarpus cupressina). On wetter- sites on the island of Hawaii, robusta eucalyptus stands often develop a dense, almost impenetrable, understory of strawberry guava (Psidium cattleianum).
Flowering and Fruiting- Robusta eucalyptus has perfect flowers that are insect pollinated. In Florida, California, and Hawaii, trees have been observed to flower by the end of the third growing season. The peak flowering season in Florida is from September to November (7), and the peak season in California is from January to March (11). In Hawaii and more tropical areas, new flowers may appear at almost any time of the year and individual trees occasionally bloom year-round.
The trees flower with 5 to 10 flowered axillary umbels. The sepals and petals are fused into a caplike structure (operculum) that drops off the tip of the flower bud at anthesis. The eucalypts are, in general, protandrous (23). The showy part of the cream-colored flower is actually the numerous filamentous stamens that surround the stigma.
The fruit is a vase-shaped dark green capsule 12 to 15 mm (0.5 to 0.6 in) long that contains many small seeds. The fruit ripens 5 to 7 months after flowering.
Seed Production and Dissemination- Seeds of robusta eucalyptus are small and like all eucalyptus contain no endosperm. The viable seed is difficult to separate from the chaff (unfertilized or aborted ovules) in the ripe flower capsules. There are 200 to 400 viable seeds per gram (5,700 to 11,300/oz) of seed and chaff (12).
Seed dispersal is largely by wind and may begin within 6 weeks after the seed capsule ripens. In Florida, most trees retain seeds in closed capsules for more than 1 year after ripening (7).
Seedling Development- Germination is epigeal (12). Robusta eucalyptus in Florida has occasionally reproduced naturally around abandoned homesteads, probably following fire on the native range. The seed source was usually an old amenity planting of robusta eucalyptus and the seedlings outgrew the disturbed native vegetation. The species does not invade recently abandoned agricultural fields because of the more intense competition from weeds (7).
Most robusta eucalyptus stands in Florida are being established through the planting of container-grown stock. Seedlings in Florida need several months to grow into frost-hardy saplings before facing their first frost. Early spring planting would be ideal, but soil moisture is deficient until summer rainfall begins. Thus mid-June through mid-August is the recommended planting period (7).
Most robusta eucalyptus stands in Hawaii have been established as single species plantings and, after logging or other disturbance, regenerate as pure stands of coppice and seedlings. Robusta eucalyptus has recently been used in biomass plantations. These were all made with container-grown
seedlings to assure the rapid early start needed to stay ahead of the wide variety of competing, aggressive vegetation (25). After planting, container-grown seedlings in Hawaii grow almost 30 cm (12 in) per month for the first few years.
Vegetative Reproduction- The majority of new stems in logged stands of robusta eucalyptus are of coppice origin. These coppice shoots arise from dormant buds in the cambium of the stump. All parts of the stem surface under the bark contain dormant buds that sprout rapidly after crown injury.
Robusta eucalyptus is one of the Eucalyptus species that produces lignotubers. A lignotuber consists of a mass of vegetative buds and contains substantial food reserves. It begins forming in the axils of the cotyledons and the first three pairs of the seedling leaves. Eventually these organs are overgrown by the main stem and remain as tuberous bulges just above the root crown.
When robusta eucalyptus is logged, therefore, the source of the coppice is usually the dormant buds in the stem cambium surrounding the stump. But if the entire stem is killed through fire, or in young seedlings through grazing, new coppice shoots may arise from the lignotubers (23). In a Florida test, robusta eucalyptus coppicing proved to be less influenced by season of cutting than either E. grandis or a hybrid E. grandis x robusta, but was reduced during the hot, dry summer (26).
No rooted cuttings of robusta eucalyptus have been used on a commercial scale, but cuttings taken from young seedlings and young coppice shoots have been successfully rooted (10).
Growth and Yield- In 1960, a study in eight different Hawaiian plantations of robusta eucalyptus gave the following growth data for plantations at elevations ranging from 395 to 730 m (1,300 to 2,400 ft), and trees aged 23 to 38 years, with 358 to 642 trees per hectare (145 to 260/acre) larger than 28 cm (11 in) in d.b.h. (14,22):
Basal area: 51 to 184 m²/ha (220 to 800 ft²/acre).
Height of dominants: 28 to 55 m (93 to 179 ft).
Mean annual growth per stand: 7 to 48 m³/ha (100 to 685 ft³/acre).
Mean annual growth for all eight stands: 26 m³/ha (370 ft³/acre).
One of Florida's first eucalyptus plantations of operational scale established with genetically improved seedlings was established in 1972 on a palmetto prairie site. Within this planting, a system of inventory plots was established to develop the data needed to determine optimum rotation length, expected yields, and other management guidelines. Although the planting is considered seriously understocked with 786 trees per hectare (318 trees/acre), measurements at 10.25 years estimate a mean annual yield of 16.7 m³/ha (238 ft³/acre). Mean height of all stems was 16.6 m (54.5 ft) and height of dominant class trees only was 21.3 m (70 ft). Stand volume in 1979 was 172 m³/ha (2,458 ft³/acre) (7,18).
Planted trees in Puerto Rico have reached 27.4 m (90 ft) in height and 41 cm (16 in) in d.b.h. in 15 years (17). Coppice stands often outproduce seedling stands. A 10-year-old coppice stand in Hawaii produced 140 m³/ha (2,000 ft³/acre), while an adjacent 12-year-old seedling stand yielded only 96 m³ /ha (1,372 ft³/acre) (3).
Rooting Habit- The most distinctive characteristic of the rooting habit of robusta eucalyptus in Hawaii is the tree's ability, in moist areas, to initiate adventitious roots from buds on the stem at heights of 6 to 12 m (20 to 40 ft) (fig. 2) (13). These roots grow downward through the moist bark and into the soil. As the root grows in diameter, it sometimes breaks free from the soft bark and appears as an aerial root. The lower stems of occasional robusta eucalyptus become completely encased in an interwoven mass of these aerial roots, some of them 20 cm (8 in) in diameter (14). The species rarely displays this habit in its native range or in more temperate climates. Adventitious roots, however, have been noted on a robusta eucalyptus in the Sydney Botanical Garden in Australia, and near Rio de Janeiro (15). Although some layering from the stem may occur as noted earlier, most roots originate below the lignotubers and occupy the entire available soil profile on well-drained sites. Robusta eucalyptus is usually quite windfirm on deeper soils and is often used for windbreaks in Hawaii.
Reaction to Competition- Robusta eucalyptus is classed as intolerant of shade. Where planted in alternate rows with saligna eucalyptus it is invariably overtopped, suppressed, and usually dies within 30 years. In Hawaii, robusta eucalyptus is planted on prepared sites and usually grows faster than weedy competitors invading the site. On extremely refractory sites robusta eucalyptus is considered the species of last resort because of its remarkable ability to survive and grow.
Damaging Agents-Robusta eucalyptus is remarkably free of serious insects or diseases when grown in the United States. Cylindrocladium scoparium has caused serious losses of seedlings in Florida (1). However, this fungus can now be successfully controlled by fumigation of soil and containers with methyl bromide before sowing and a followup treatment with benomyl spray. The major cause of damage to robusta eucalyptus stands in Hawaii is wind (14). Violent windstorms have snapped stems and uprooted trees. Uprooting damage can be particularly severe when stands are established in shallow soils overlaying a solid mantle of lava rock. Naturally, such shallow soils should be avoided and planting concentrated on soils or fractured bedrock where roots can penetrate to greater depths.
In Florida, robusta eucalyptus plantings at about age 5 may develop a condition called "robusta breakup." Patches of young trees may develop a bend in the main stem or on primary branches. Breakage may also occur along the main stem or primary branches, and the wood at the point of breakage may appear dry and brash. No primary pathogens or pests have been associated with this breakage. Minor element deficiencies are suspected but are not proven as the cause. Adjacent stands of rosegum eucalyptus (Eucalyptus grandis) appear unaffected (7).
Robusta eucalyptus has found use in urban forestry and as farm windbreaks because of its dark shiny leaves and its generally dense crown. Twigs and branches continually die off and fall to the ground, however, so that the tree is rather hazardous for use in parklands, campgrounds, or even gardens. On the island of Kauai, an older roadside planting of robusta eucalyptus, though most attractive, is maintained at a high cost for road cleanup.
We know of no published data on population differences in robusta eucalyptus. Studies (see "Races") using seed collections from Australia could be suitable for grouping and analyzing by particular provenances, but such analyses have not been reported.
In 1975, foresters in southern Florida established a genetic base population of 352 collections of Eucalyptus robusta from individual selected trees in Australia, advanced generation families from two previous generations of selection in Florida, as well as selections from Florida's naturalized stands. This base population was subsequently selected and rogued to form a seedling seed orchard that produces seeds of a bona fide land race of E. robusta for southern Florida. This seed orchard was also a source of genetic material for an effort to develop E. grandis and E. robusta hybrids adapted to Florida conditions (7,19).
Several natural hybrids involving Eucalyptus robusta have been reported (24). All of the known interspecific hybrids are between E. robusta and other species of the subgenus Symphomyrtus. Several have been assigned recognized botanical names. They are E. botryoides var. platycarpa (E. botryoides x robusta), E. grandis var. grandiflora (E. grandis x robusta), E. longifolia var. multiflora (E. longifolia x robusta), E. kirtoniana (E. robusta x tereticornis), E. patentinervis, E. insizwaensis (E. robusta x globulus, probably), and an unnamed hybrid (E. robusta x saligna, probably).