Eucalyptus grandis Hill ex Maiden
Rose Gum Eucalyptus
Myrtaceae -- Myrtle family
George Meskimen and John K. Francis
Eucalyptus grandis is native to the east coast of Australia. Its common name is rose gum or flooded gum (a misnomer). Rose gum is one of the premier forest species in the Australian States of Queensland and New South Wales where it grows 43 to 55 m tall (140 to 180 ft) and 122 to 183 cm (48 to 72 in) in diameter (15). Its form is excellent with tall, straight, clean holes up to two-thirds of the total height. The bark is thin and deciduous, shedding in strips to expose a smooth surface marked with flowing patterns of silvery white, slaty gray, terra cotta, or light green. Occasionally a "stocking" of light-gray, platelike or fissured bark persists over the basal I to 2 m (3 to 6 ft) on the trunk.
Rose gum is one of the most important commercial eucalypts, with more than one-half million hectares (1.3 million acres) planted in tropical and subtropical areas on four continents. Massive planting programs have been carried out in the Republic of South Africa and Brazil, and there are substantial plantings in Angola, Argentina, India, Uruguay, Zaire, Zambia, and Zimbabwe (21). In southwest Florida rose gum may be an emerging commercial species for plantations. It has been successfully tested for pulpwood and fuel; and its wood has potential for poles, pallets, veneer, and other products. In California, Hawaii, and Puerto Rico, rose gum appears in some species trials and landscaping.
Over its central range, rose gum grows on alluvial or volcanic loams in valleys and flats within 160 krn (100 mi) of the coast, straddling the Queensland-New South Wales border from latitude 26 to 33° S. Two outlier populations extend the range to the Atherton Tablelands at latitude 13° S. (10,15).
In Florida, intensive research on rose gum began in 1961 and operational planting in 1972. Through the 1980 planting season, it was commercially planted on 5,650 ha (14,000 acres) in Glades, Hendry, and Charlotte Counties in southwest Florida between latitude 26° 31' and 27° 02' N. and between longitude 81° 31' and 81° 48' W. Outside that zone there are numerous potential planting areas in south Florida.
The climate in the Australian native range of rose gum is humid subtropical with mean minimum temperatures during the coldest month ranging from 2 to 10° C (36 to 50° F) and mean maximums near 29° C (85° F) during the hottest month. Rainfall averages 1020 to 1780 mm (40 to 70 in); it is concentrated in the summer, but monthly precipitation during the dry season is at least 20 mm (0.8 in) (10,21). Coastal areas are generally frost-free, but higher altitude, inland areas experience occasional frosts (6).
Southwest Florida is humid and subtropical. Summers are long, rainy, and warm; winters are dry and mild but with the threat of damaging frost. Mean annual rainfall ranges from 1270 to 1400 mm (50 to 55 in). Monthly precipitation during the rainy season, June through September, averages about 180 to 200 mm (7 to 8 in). Rainfall during the dry season, November through April or May, averages 50 mm (2 in) per month (40). Dry-season rainfall is unreliable, however. Daily maximum temperatures from late May through September exceed 32° C (90° F) on most days but rarely reach 38° C (100° F). During the coldest month, daily maximum temperatures average near 24° C (75° F) and daily minimums near 11° C (52° F) (27). But swift continental cold fronts change balmy afternoons into dangerously cold nights. The lowest temperatures recorded in each of 30 winters averaged -4.4° C (24° F) (22).
This species grows on flats or lower slopes of deep, fertile valleys in Queensland and New South Wales. It grows best on moist, well drained, deep, loamy soils of alluvial or volcanic origin (6). Clayey soils are acceptable if they are well drained (23).
The rose gum plantations in Florida lie in a physiographic region known as the Western Flatlands (11). Topography is nearly flat; elevations change almost imperceptibly from coastal sea level to 6, 12, or rarely 18 m (20 to 60 ft) elevation inland. Soils are almost exclusively members of the sandy, siliceous hyperthermic Haplaquods. Derived from marine deposits, these soils are mainly sands, strongly acid, poorly drained, and underlain by spodic horizons that are commonly impervious to root penetration and water drainage. The combination of high seasonal rainfall, flat topography, and low elevation results in high water tables, shallow root zones, and local inundation during the rainy season. Conversely, during the dry season these sandy soils rapidly become moisture deficient.
In its native range, rose gum grows in tall, open forests associated with the eucalypts E. intermedia, E. pilularis, E. microcorys, E. resinifera, and E. saligna, as well as Syncarpia glomulifera, Tristania conferta, and Casuarina torulosa. Rose gum also commonly grows on the fringes of and occasionally within rainforest (6). In Florida, rose gum plantations are most frequently established on palmetto prairies. The characteristic vegetation of palmetto prairies consists of a ground cover of Serenoa repens, Aristida stricta, Andropogon spp., Myrica pusilla, M. cerifera, Ilex glabra, and Quercus minima, with the scattered trees, Sabal palmetto, Quercus virginiana, and Pinus palustris (12).
Flowering and Fruiting- Rose gum, like all eucalypts, bears perfect flowers. Buds form in axillary umbels with usually seven buds per cluster. Each flower consists of a central style surrounded by stamens standing about 8 mm (0.3 in) tall and forming a bloom about 20 mm (0.8 in) in diameter. The puffy clusters of creamy white blooms are attractive and conspicuous but not of horticultural quality.
The main blooming season is mid-August through late September, but some trees start blooming early in August and the latest finish in mid-November. This blooming season of late summer-early fall matches the low-elevation blooming season in South Africa but contrasts with the late fall-early winter bloom at high elevations (18) and the midwinter bloom in Australia (5). Each tree blooms serially over a period of 5 to 10 weeks, with an average of only 12 percent of a tree's bud crop in prime bloom during a given week.
Foraging insects, particularly honeybees, pollinate the flowers. In an individual flower, the stigma is not receptive until after pollen shed, but because each tree blooms serially, there is, unfortunately, ample opportunity for self-fertilization. In a South African seed orchard, selfing occurred with a frequency of 10 to 38 percent, caused 10 recognizable detrimental abnormalities, and depressed the height growth of outplanted seedlings 8 to 49 percent compared to crossed progenies (19). Flowering precocity is strongly inherited; a few families bloom at plantation-age 1 year, many more at age 2, and 97 percent of the orchard at age 3.
From 2 to 3 weeks after blooming, the stamens and style wither and fall away, leaving a woody, urn-shaped seed capsule closed by four to six valve covers. The capsules are about 8 mm long by 6 mm in diameter (0.3 by 0.25 in). Most umbels carry five to seven capsules to maturity.
Seed Production and Dissemination- Seed capsules are mature for harvest 6 or 7 months after flowering. However, the capsules remain closed on the tree for at least 1 year after maturity, so it is possible to gather two seed crops at a time by harvesting in alternate years. Seed capsules should be harvested by severing the umbel stalks; the alternative of clipping seed-bearing twigs reduces the next flower crop.
The valves of the capsules dry out, open, and release seeds. Capsules scattered loosely on a dry surface release their seeds after about 2 hours in full sun. Commercial bulk lots can be extracted in about I week in chambers equipped with open-mesh shelves, heating from 30 to 35° C (86 to 95° F), forced-air circulation, and dehumidification.
Individual trees bear from 3 to 25 sound seeds per capsule, with an average near 8 (20) and a much greater mass of infertile ovules called "chaff." Fertile seeds are tiny, only about I min (0.04 in) in diameter. Chaff particles are lighter colored and only minutely smaller and lighter than seeds. Seed cleaning involves sorting by size and shape through multiple sieves, then separating by weight in a pneumatic separator.
In Florida, operational quantities of seeds can be harvested from an orchard at age 3.7 years and production increases annually to a plateau at about age 10. Seed production is reliable year to year, but there is great tree-to-tree variation in the quantity, purity, and viability of seed crops. The 1-year seed crop from some 7-year-old trees was as follows:
Seeds have been successfullly stored for 20 years by either freezing at -8° C (18°F) or refrigerating at 10° C (50° F). Rose gum seeds require no presowing treatment.
Seedling Development- Germination of rose gum is epigeal and takes place in 7 to 14 days after sowing (44). Moist, bare soil is required for natural regeneration; fire, erosion, and flood deposits provide satisfactory seedbeds. In commercial forests, the species is almost always regenerated by planting. Seedlings are usually raised to 20 to 30 cm (7.8 to 11.8 in) tall, which takes 3 to 5 months (29). Due to the sensitivity to desiccation, seedlings are normally grown in containers. Rigid containers with multiple cavities from which the seedlings are removed with roots and soil intact are almost always used in large operations. Seedlings are also grown in plastic nursery bags. In the absence of frost and drought, seedlings can be planted throughout the year. In many areas seedling production and planting must be carefully timed. In Florida, planting must coincide with summer rains to give sufficient time for seedlings to grow into hardy saplings before facing winter frosts.
Site Preparation and Planting in Florida- Success in establishing rose gum in plantations depends on double chopping or cross disking to greatly reduce vegetative competition, then broadcasting 1.12 t/ha (0.5 ton/acre) of ground rock phosphate to overcome a severe natural phosphorus deficiency, and finally bedding to elevate the seedlings out of the standing water during the first rainy season (25). Saw-palmetto rhizomes are thick, fibrous, and deeply rooted, requiring heavy equipment for chopping and bedding. Landowners prepare sites in the spring when the deepening drought enhances the kill from chopping or disking. Also, spring offers little time or moisture for weed competition to colonize the beds before summer planting starts.
Planting crews use high-clearance wheel tractors to straddle the beds and pull planting machines fitted with racks holding four cartons containing a total of 1,400 to 2,000 containerized seedlings. This number is sufficient to traverse several rows as long as I kin (0.6 mi). Recommended planting density is 1,495 seedlings per hectare (605 seedlings/acre) on beds spaced 3.7 in (12 ft) apart and seedlings spaced 1.8 in (6 ft) along the beds (26).
In the first 2 years of machine planting, survival averaged 86 percent in an extremely wet season and 75 percent in a dry year. Survival after machine planting probably averages about 84 percent. Hand planting routinely achieves at least 95 percent survival.
Vegetative Reproduction- After harvest, under favorable conditions, rose gum plantations regenerate by coppicing (sprouting) from the stumps. Two or three coppice rotations are commonly harvested before it is necessary to replant seedlings. Coppice shoots initially grow faster than seedlings, but that advantage is partially offset by stump mortality, which usually runs about 5 percent per rotation in South Africa (39). In many areas, coppice forms equally well regardless of the season of harvest. In Florida, however, it was shown that summer harvests (June-September) reduced coppicing ability.
Vegetative propagation has been a difficult challenge. Cuttings from tiny seedlings root readily, but rooting capability ceases before seedlings are 1 in (3 ft) tall because of natural rooting inhibitors produced by adult leaves (32). However, even in adult trees, cuttings from epicormic shoots induced at the base of the tree by felling or girdling retain the ability to root. Rooting success varies substantially among clones, and there are strong seasonal influences and exacting cultural requirements for each geographic area. The technique is particularly advantageous in multiplying outstanding hybrid individuals. Starting in the mid-1970's, some commercial plantations were propagated by rooted cuttings in Brazil (8,17), where the method is now used to establish major clonal plantations (9). Reproduction through tissue culture techniques has also been demonstrated (38).
Clonal seed orchards have been produced by grafting in South Africa, but delayed graft incompatibility is a common, debilitating problem. Incompatibility can be greatly reduced by grafting scions onto sibling or half-sibling rootstocks (45).
Growth and Yield- Growth of rose gum on short rotations is rapid. Mean height growth of 2 m/yr (6.5 ft) is common (29), and a rate of 4 m/yr (13 ft) has been reported (35). Mean yields are about 27 m³/ha/yr (386 ft³/acre/yr) (21). Wood increment on the best sites is even more impressive:
| Volume of uncleaned seeds | Weight of uncleaned seeds | Healthy germinants | Healthy germinats per tree | ||||
| (liters) | (quarts) | (kg) | (lb) | (/gram) | (/ounce) | ||
| 1.7 | 1.8 | 0.99 | 2.18 | 688 | 19,504 | 694,115 | |
average 3.5 m (11.5 ft) annually for the first 4 years, then taper off to average 2.4 in (7.9 ft) over an 8-year rotation. Adequately stocked commercial plantations have not reached harvest age, but preliminary growth data support the following planning assumptions:
An 8-year seedling rotation.
Trees averaging about 18 in (60 ft) tall at age 8 years.
Annual yield for volume and weight in the following range:
| Volume | Dry Weight | |||
| m³/ha | ft³/acre | t/ha | tons/acre | |
| Pessimistic | 12.9 | 184 | 5.4 | 2.4 |
| Realistic | 16.1 | 230 | 6.9 | 3.1 |
| Optimistic | 19.3 | 276 | 8.3 | 3.7 |
These indicated annual yields are well below world standards, and are probably due to south Florida's infertile soils and seasonally high water tables. Yields include about 18 percent bark by volume and 14 percent by weight. Density averages about 0.45 g/cm³ (0.026 oz/in³) for wood and about 0.32 g/cm³ (0.018 oz/in³) for bark. Moisture content is about 0.50 g/cm³ (0.029 oz/in³) for wood and 0.72 g/cm³ (0.042 oz/in³) for bark (14).
Rooting Habit- Natural seedlings develop a pronounced taproot with few laterals if conditions permit. The roots of containerized seedlings more or less assume their natural form after being outplanted, regardless of previous restrictions (3). Rose gum does not develop lignotubers (6).
The soils of south Florida drastically sculpture the root systems of rose gum trees. These spodosols feature a thin, sandy A, horizon with meager accumulations of nutrients and organic matter. Below lies a strongly leached A2 horizon of white, sterile sand. The A2 changes abruptly to a B2h, the spodic horizon, consisting of fine sand accreted with organic and aluminum compounds. In some areas this spodic hardpan perches water tables and resists root penetration both physically and by aluminum toxicity (4). Typically, taproots penetrate about 50 cm deep (20 in), then divide into two or more smaller taproots that terminate barely into the spodic horizon (4). Excavated trees show an abrupt fringe of dead lateral and feeder roots in the A2 horizon, apparently anoxia mortality from perched or raising water tables during the summer rainy season (4). This shallow anoxic zone restricts the exploitable soil during the prime summer growing season, and the roots cannot penetrate the spodic horizon to pursue the retreating water table during the long dry season.
Reaction to Competition- Rose gum is intolerant of shade. Seedlings can only develop in full or nearly full sunlight; trees must maintain a dominant or codominant canopy position to long survive. Suppressed trees quickly die and intermediate trees must grow to an overstory position or eventually lose vigor and die.
Despite their startling growth capacity, newly planted rose gum seedlings compete poorly with weedy vegetation, tolerating neither root competition nor shading. In Florida, they need 3 months reasonably free of competition to grow about 1.5 in (5 ft) tall and dominate the site. The local planting effort concentrates on virgin prairies and cutover pineland where chopping and bedding control the native ground cover sufficiently to permit rose gum establishment without post-planting weed control. However, early vegetative competition often retards growth and probably contributes to large tree-to-tree variation within stands. Post-planting weed control by herbicides and cultivation is beneficial (37). An increase in volume at 5 years of 48 to 55 percent was obtained by combinations of cultivation and herbicide spraying during the first 24 months (30). Competing vegetation also contributes to the fire danger. Rose gum completely occupies suitable sites in Florida with adequate stocking by plantation-age 2.5 years and herbaceous ground cover mostly disappears. On poor sites, intense competition may continue for 5 years.
Competitive relationships develop early among the plantation trees and stay well defined throughout the 8-year rotation. In Florida's first commercial plantation, 75 percent of the sample trees that were in the top quartile for height at age 2.5 years were still in the top quartile at age 8.5 years; and 73 percent of the trees in the bottom quartile at age 2.5 were either dead or still in the bottom quartile at 8.5 years. Similarly, out of 131 sample crop trees at 8.5 years, 85 percent had been predicted as crop trees at 2.5 years; of 37 sample trees that were culls or dead at 8.5 years, 68 percent had been predicted as culls at 2.5 years.
Damaging Agents- The greatest threat to rose gum survival is a lapse in soil moisture at outplanting time. If this occurs, managers must be prepared to stop planting until the rains begin again. Severe nursery losses have been suffered from a stem-girdling fungal canker caused by Cylindrocladium scoparium, but alternating sprays of chlorothalonil and benomyl prevent or control it (1).
Severe frost damages rose gum saplings even in the commercial plantation zone, but they sprout and regrow vigorously. Frost has cost a season's growth several times but never a plantation. South Florida usually has inversion freezes, with lower temperatures at ground level than at 2 m (6.6 ft). There is a strong positive correlation between freeze resistance and rapid early growth; resistant trees develop larger stems with thicker, insulative bark close to the ground, and also elevate tender terminal tissue into the higher, warmer air. Each additional year of growth reduces the risk of frost damage. Since planting research began in 1961, severe damage has been suffered 1 out of 3 years by seedlings in their first winter, I out of 5 years by saplings in their second winter, and only 1 out of 19 years by trees in their third winter or older.
Local plantations are suffering an increasing incidence of basal cankers caused by the fungus Cryphonectria cubensis. Canker incidence in the oldest pilot-scale plantation increased from 15 percent at age 7 to 50 percent at age 11 (2). Infected trees in Florida do not seem debilitated, but mortality has been serious in Brazil (30 percent) and Surinam (50 percent). Basal cankers may appear in trees less that 2 years old.
Lightning occurs with an unusually high frequency in southwest Florida (36). Over an 8.5-year rotation on a 67-ha (165-acre) plantation, 4.4 percent of the sample trees suffered lightning strikes or splashes and 2.5 percent actually died as a result.
No hurricane has struck southwest Florida since commercial planting began, but there is a 10-percent hurricane probability for any given year. A hurricane would cause serious windthrow, as hurricane Allen did to Eucalyptus in Jamaica (41).
Termites which devour seedlings during the first years, have been a serious problem in rose gum plantations in India (28). Also worth mentioning is the serious tendency of rose gum logs to end-split. Losses can be kept at a minimum by milling within 3 days of cutting, bucking to lengths as long as possible, and sawing carefully (33).
The sapwood of rose gum is pale pink and the heartwood light to dark red. The wood is straight grained, coarse textured, and moderately strong (6). It is moderately durable at best, but the sapwood is generally resistant to Lyctus spp. borers (6,7). The specific gravity varies from 0.62 to 0.80 (6,7,23). Rose gum timber is used for general construction, joinery, plywood, panelling, boat building, flooring, utility poles, mine timbers, and posts (6,7).
In 1972, rose gum wood harvested from an 8.5-ha (21-acre) research planting supported a commercial-scale trial run in a Florida pulpmill. A mixture of 70 percent rose gum and 30 percent native hardwood was manufactured into quality facial tissue with excellent properties of strength and softness (42). Earlier laboratory tests showed that debarked rose gum wood gave screened Yields of kraft bleached paper pulp equal to those of representative native hardwood furnish at comparable kappa numbers. Brightness of bleached pulp was equal or superior to native hardwood controls at equivalent or slightly higher processing costs (14). However, strength properties of handsheets were generally inferior to those of native hardwood controls.
A pyrolysis test was conducted to determine the energy value that could be recovered from whole-tree chips of 9-year-old rose gum grown in southwest Florida. Seventy percent of the energy contained in the dry chips could be recovered as char and oil, which could be transported and stored. Twenty-one percent of the trees' energy value was converted to noncondensed volatile oil and low-energy gas that could only be used on site or sold to an adjacent user (34).
Rose gum is most closely related to E. deanei and E. saligna (6). No subspecies or varieties are recognized.
Rose gum trees planted in southwest Florida constitute a land race developed through three generations of selection and progeny testing in the local environment. Because of recurrent selection for local adaptation, the trees perform better than progenies of outstanding trees selected in Australia, South Africa, or elsewhere.
A long-range breeding system calls for importing as many rose gum seed lots as possible-preferably collections from selected single trees in Australia-but some bulk lots and many lots from exotic populations outside Australia have been included. Each seed lot (family) contributes about 60 seedlings to a large outplanting called the genetic base population. All individuals of all families are completely randomized in single-tree plots. Trees are measured for growth rate and scored for cold hardiness, stem straightness, branch habit, and general adaptation. At 2.7 years (one-third of the rotation age), the trees are selected and the rest are rogued to convert the base population to a seedling seed orchard. The best families usually contribute three or four selects to the seedling seed orchard; most families contribute only one or two; and about one-third of the families drop out of the breeding population for lack of any worthy candidates.
Select trees exchange pollen in the first massive bloom at age 3+ years. The following spring the resultant seed is collected and used to establish the next generation's base population, which also is the progeny test of the seedling seed orchard. Thus, a generation of selection is completed in 4 years. Progeny test results identify the best commercial seed-orchard trees as well as poor seed trees to be rogued from the orchard.
Each generation of selection enhances the land race's adaptation to local conditions, but new families must be imported to broaden the genetic base and minimize inbreeding depression.
In the current genetic base population, first-generation Australian families average 7.5 dm³ (0.26 ft³) of stem volume at 2.5 years. Compared with those Australian families, second-generation Florida families average 95 percent more stem volume (14.6 dm³ or 0.52 ft³); third-generation families, 127 percent more (17.0 dm³ or 0.60 ft³); and fourth-generation families, 163 percent more (19.7 dm³ or 0.70 ft³). A study designed to measure realized gain compares the following three populations, each in replicated block plantings:
Premier-Six advanced-generation families (average 3.5 generations of selection) that are top-ranked for the combined traits of volume production, cold hardiness, form, and coppicing.
Commercial-The 33 advanced-generation families (average 2.9 generations) included in the seed mix for the 1979 commercial season.
Ancestral-Four imported seed lots from which all six premier families descend and 21 of the 33 commercial families.
At age 1.5 years both premier and commercial trees significantly exceeded the height of ancestral trees by 23 and 13 percent, respectively. The three populations all differed significantly in their cold hardiness. The premier families suffered mainly foliar damage, while the commercial and ancestral families suffered increasingly severe damage to foliage and woody parts.
The Florida land race is predominantly, but not purely, E. grandis. A few seed orchard trees and scattered offspring display recognizable admixtures of traits from E. robusta, E. tereticornis, and E. camaldulensis. Given synchronous flowering and proximity, rose gum can hybridize with many eucalyptus species. Some of the resultant F1, hybrids are superior to either parent species for certain exotic forest environments, but F2 and later generations show classic segregation and hybrid breakdown. F, hybrids between E. grandis and E. tereticornis seem especially promising in India (43) and Zambia (16). In Espirito Santo, Brazil, hybrids between E. grandis and E. urophylla are planted as clonal stands from rooted cuttings (8). In south Florida, there is evidence that F, hybrids between E. grandis and E. robusta could be more productive than either pure species.