Eucalyptus Gobulus Labill. Bluegum Eucalyptus Myrtaceae Myrtle Family Roger G

Eucalyptus gobulus Labill. Bluegum Eucalyptus Myrtaceae Myrtle family Roger G. Skolmen and F. Thomas Ledig Bluegum eucalyptus (Eucalyptus globulus), also worldwide have been to locations with mild, called Tasmanian bluegum, is one of the world’s best temperate climates, or to high, cool elevations in known eucalyptus trees. It is the “type” species for the tropical areas (8). The ideal climate is said to be that genus in California, Spain, Portugal, Chile, and many of the eastern coast of Portugal, with no severe dry other locations. One of the first tree species intro- season, mean annual rainfall 900 mm (35 in), and duced to other countries from Australia, it is now the minimum temperature never below -7” C (20’ F). In most extensively planted eucalyptus in the world. coastal California, the tree does well in only 530 mm (21 in) rainfall accompanied by a pronounced dry Habitat season, primarily because frequent fogs compensate for lack of rain. A similar situation is found in Chile Native Range where deep fertile soils as well as fogs mitigate the effect of low, seasonal precipitation (8). In Hawaii, Four subspecies are recognized. The type tree, sub- bluegum eucalyptus does best in plantations at about species globulus, is largely confined to the southeast 1200 mm (4,000 ft) where the rainfall is 1270 mm coast of Tasmania but also grows in small pockets on (50 in) annually and is evenly distributed or has a the west coast of Tasmania, on islands in the Bass winter maximum. Seasonality of rainfall is not of Strait north of Tasmania, and on Cape Otway and critical importance to the species. Although it Wilson’s Promontory in southern Victoria, Australia generally grows well in countries with a Mediter- (9). Other subspecies are found northward in Victoria ranean or cold season maximum rainfall, it grows and New South Wales (13). well also in summer rainfall climates of Ethiopia and The species was introduced into California in 1856 Argentina (8). (1) and into Hawaii about 1865 (18) and has become naturalized in both States. It is also fairly common Soils and Topography as an ornamental in Arizona but is not naturalized there. In California, it is now primarily used in line Bluegum eucalyptus grows well on a wide range of plantings along roads and as windbreaks, but soils. It requires good drainage, low salinity, and a formerly, extensive plantations were established. soil depth of about 0.6 m (2 ft) or more. Other limit- Plantings total about 16 000 ha (40,000 acres) (17). ing factors are few (8). In locations with a The planted range in California extends from Hum- pronounced dry season, such as California, the tree boldt County in the north to San Diego County in the grows best on deep alluvial soils because of the south, with best growth in the coastal fog belt in the greater moisture supply. vicinity of San Francisco. Numerous plantings are seen In Hawaii, the tree grows very well on Typic and in the Central Valley from Redding, south through Hydric Dystrandepts, soils of the latosolic brown Fresno to Bakersfield, and San Bernardino. Hawaii has forest great soil group. These soils are generally 0.9 about 5000 ha (12,000 acres&almost all of them on m (3 R) deep, acid in reaction, and formed on volcanic the islands of Hawaii and Maui. In California and ash. In California, the tree grows well on a much Hawaii the tree regenerates within and near the edges wider range of soils than in Hawaii, from the Ultisols of plantations. In some areas of Hawaii it spreads fast and Alfisols developed on deeply weathered sedimen- enough to be considered a pest by ranchers. tary deposits and sandstone to Inceptisols and Aridisols Recently, the species has also been planted in its developed on a wide variety of parent materials. native Tasmania where it is an important pulpwood In Portugal, almost 15 percent of the land area is source (22). planted to this species. Most stands are on soils developed from sandstone and limestone, which have Climate been badly degraded by cultivation since ancient times. Best yields occur on the heavy texture clay- Although bluegum eucalyptus has great climatic loams and clays (11). adaptability, the most successful introductions Associated Forest Cover The authors are Principal Silviculturist (retired) and Supervisory Research Geneticist, Pacific Southwest Forest and Range Ex- In its native habitat, bluegum eucalyptus grows in periment Station, Berkeley, CA. pure stands and in mixture with messmate stringy- 299 Eucalyptus globulus bark eucalyptus (Eucalyptus obliqua), mountain-ash by cattle or sheep (fig. 1) 03). This condition not only eucalyptus (E. regnans), manna eucalyptus (E. caused it to be a popular tree for planting in open uiminalis), black peppermint eucalyptus (E. amyg- grasslands years ago, but it permits natural seed- dalina), and white peppermint eucalyptus (E. pul- lings to survive in the presence of grazing animals chella). Although, for the most part, it has been alongside the planted stands. The tree reproduces by planted in pure plantations in countries where it has seeding into openings in planted stands and into been introduced, it has also been planted in mixture. fields adjacent to plantations. In California, it has most commonly been mixed with Seed stored in the soil under older stands often forest redgum eucalyptus (E. tereticornis) and river germinates prolifically following logging and the redgum eucalyptus (E. camaldulensis) (19). In resultant natural reproduction interferes with the Hawaii, it has been planted in mixture with many management of the coppice stand (21). other eucalypts. Most of the dense bluegum eucalyptus stands in Flowering and Fruiting-Bluegum eucalyptus California and Hawaii are noted for being almost in California flowers from November to April, the wet devoid of understory vegetation, except for a few season (15). In Hawaii, some trees flower throughout hardy grasses. Although this condition is most likely the year, but flowering is heaviest in February to related to the rather dry climate that provides the March. The flower buds have a warty cap or oper- best site for the species, it has also been shown that culum about 2.5 cm (1 in) in diameter, which falls the leaves of the tree produce water soluble off, allowing the very numerous stamen filaments to phytotoxins that can prevent radicle growth of many extend in shaving-brush fashion above the cup- herbaceous plants (7). In Hawaii, firetree (Myrica shaped base (hypanthium). The yellowish white faya) is a species that sometimes invades bluegum flowers are pollinated by insects, hummingbirds, and eucalyptus stands. The noxious passion fruit vine other pollen and nectar feeders. As in almost all (Passiflora mollissima) has also been found thriving eucalyptus, pollen is usually viable before the stigma in a young coppice stand. becomes receptive (8). The fruit, a distinctive top- shaped woody capsule 15 mm (0.6 in) long and 2 cm Life History (1 in) in diameter, ripens in October to March in California, about 11 months after flowering (15). In Reproduction and Early Growth Hawaii the fruit ripens throughout the year. Bluegum eucalyptus has a considerable competi- Seed Production and Dissemination- tive advantage as compared with most other tree Bluegum eucalyptus seeds are relatively large for a species in that its juvenile foliage is seldom browsed eucalyptus. There are 18 to 320 seeds per gram (500 to 9,1OO/oz) of seeds and chaff, or about 460 clean seeds per gram (13,OOO/oz) (2,5,15). Capsules release seed immediately on ripening and the seed is dis- persed by wind. Calculated dispersal distance from a 40-m (131-ft) height, with winds of 10 km/hr (6 mikr), was only 20 m (66 ft). Newly released seeds germinate within a few weeks if conditions are suitable. Trees usually begin to produce seeds at 4 to 5 years and yield heavy seed crops in most locations at 3- to 5-year intervals (23). Seeds can be stored for long periods in air-tight containers at 0” to 3” C (32” to 38” FL Seedling Development-Newly germinated seedlings have inverse heart-shaped cotyledons, borne epigeously. The stems of seedlings, especially those grown in the shade, are usually square in cross section, often for as much as 3 to 5 m (10 to 16 ft) of stem length. These square stems usually have Figure 1-Bluegum eucalyptus, showing how it readily seeds into prominent ridges or “wings” at the corners. Juvenile grasslands adjacent to planted stands. It spreads rapidly (if not leaves, which are opposite and broadly lanceolate, 9 controlled) because cattle do not graze the seedlings. by 9 cm (3.5 by 3.5 in), may persist for more than a 300 Eucalyptus globulus year (9). Trees in coppice stands 6 m (20 ft) or more tus is normally carried for three coppice rotations in height are often entirely in the juvenile leaf form. after the first, or seedling rotation. Rotations range These juvenile leaves bear a bluish gray, waxy bloom from 5 to 10 years in different countries and sites. and are the reason for the common name of the tree, Undesirable shoots are usually removed during the bluegum. first 2 years of a coppice crop, but thinning is nor- Nursery-grown seedlings in containers reach mally not done. In Portugal, coppice stands are some- plantable size, about 30 to 40 cm (12 to 16 in> high, times managed by the system of “coppice with stand- in 3 to 4 months. Seedlings can be established if ards” so that a sawtimber crop of the straightest and planted with bare roots, but success is highly de- best trees is retained between coppice harvests to be pendent on favorable wet weather after planting.

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