Melaleuca quinquenervia (Cav.) S. T Blake
Myrtaceae -- Myrtle family
T. F. Geary and S. L. Woodall
Melaleuca (Melaleuca quinquenervia), also known as cajeput-tree, punktree, paperbark-tree, five-veined paperbark, or bottlebrush, is an evergreen tree from Australia brought to this country as an ornamental because of its showy "bottlebrush" flowers (3,11,12). It has been planted widely in tropical and subtropical regions. In Florida it has escaped cultivation and become naturalized in low areas and cypress swamps where it has an invasive habit. Any commercial use of the tree for timber products or biomass fuel is hampered by the quality of its corky bark. It is a productive honey tree, but stands of melaleuca are of dubious value to wildlife.
Melaleuca's native range is along the coast of eastern Australia from Sydney northward. It is native also in New Caledonia, Papua, New Guinea, and Irian Jaya. Melaleuca grows in swampy ground and on creek banks, and even on hillsides if ground water remains close to the surface. In its native habitat, melaleuca grows to 25 m (82 ft) tall and is typically found in almost pure stands or with a few associates, such as Casuarina glauca, Eucalyptus robusta, and E. tereticornis (2,3,10,22).
In melaleuca's Australian habitat, soils are nutrient deficient and flooded or wet for most of the year; summer rainfall dominates; light frost (-l° to -3° C; 30° to 27° F) occurs in most years in the south; spring is associated with brief to acute water stress; and fire and water-table fluctuations are major factors governing plant distribution (3,4,22). These conditions are similar to those of southern Florida (24) and help explain melaleuca's aggressive spread there.
In the continental United States, melaleuca is naturalized on a significant scale only in southern Florida. In Hawaii (20), a million trees have been planted in Hawaii State Forest Reserves alone, but natural regeneration is very localized. Planted melaleuca is common in southern California and is occasionally found in extreme southern Texas; it is uncommon in Puerto Rico (13).
Melaleuca was introduced to southern Florida in the early 1900's (14). By 1980 it dominated the stands in which it grew, or, where no other trees existed, it had a minimum stocking of 17 percent on 16 000 ha (40,000 acres) (8, p. 1-8). Scattered individuals and clusters of melaleuca trees grow on an additional 170 000 ha (420,000 acres) from the northern edge of Lake Okeechobee southward. Rare and isolated small pockets of natural regeneration are found in central Florida. Much of the melaleuca is in and around urban areas and it is grown as an ornamental as far north as Gainesville.
Southern Florida's climate is transitional between tropical wet-and-dry and subtropical humid and is similar to the climate of melaleuca's native habitat in Australia (21). The rainy season normally begins in June and ends in September in Florida. Occasional sudden freezing temperatures, which can be expected from late November to early March (15), and dry-season rainfall, both of which result from the passing of continental cold fronts, distinguish southern Florida's climate from tropical wet-and-dry.
In Hawaii, rainfall is evenly distributed or has a winter maximum. Good growth of melaleuca occurs at mean annual temperatures from 24° to 18° C (75° to 65° F), but trees grow in even cooler temperatures at high elevations (20). Trees grow well in rainfall of 1020 min (40 in) at lower elevations (20), and 5080 mm (200 in) at higher elevations.
Most of southern Florida is less than 8 in (25 ft) above sea level. The land is level to very gently sloping, and a freshwater table is close to the soil surface. In general, soils supporting melaleuca are in the suborders Psammaquents, Aquods, and Saprists (sometimes marly) of the orders Entisol, Spodosol, and Histosol, respectively (23). Many soils are shallow and underlain by limestone. In Hawaii, melaleuca is found from sea level to 1400 in (4,500 ft) elevation (20). It grows fairly well on all Hawaiian soils, including calcareous beach sand, but does best on Inceptisols (Dystrandrepts), Ultisols, and Oxisols developed on basalt ash or lava rock of pH 4.5 to 5.5, and under rainfall of 2030 to 5080 min (80 to 200 in) per year.
Most natural vegetation of southern Florida can be invaded to some extent by melaleuca (8, p. 9-15). It is often found growing in the cover type Pondcypress (Society of American Foresters Type 100) and in South Florida Slash Pine (Type 111); to a lesser extent it can be found in Baldcypress (Type 101) (6). The ecotone between slash pine and either cypress variety is readily invaded. Melaleuca also exists in some locations with the naturalized species Brazil peppertree (Schinus terebinthifolia) and Australian pine (Casuarina spp.). It is even found with button-mangrove (Conocarpus erectus) just inland of the tidal zone of Mangrove (Type 106). However, melaleuca invasion is less prominent on forested sites than on marshes and wet savannas.
Most shrub, herb, and graminoid species in southern Florida are likely to be found in association with melaleuca. Common associates are saw-palmetto (Serenoa repens), three-awn wiregrass (Aristida stricta), southern bayberry (Myrica cerifera), sawgrass (Cladium jamaicense), buttonbush (Cephalanthus occidentalis), and sawfern (Blechnum serrulatum).
In Hawaii, natural regeneration occurs only at the edges of plantations, on road cuts, and in swampy, sparsely vegetated spots in forests (20). It is one of the few trees that survive planting and reproduce naturally on upland bogs that form when native forests are destroyed.
Flowering and Fruiting- In Florida, flowering typically begins by age 3 and seedlings less than I m (3 ft) tall may bloom (14). Showy flowers are borne in creamy-white "bottlebrush" spikes 3 to 8 cm (1 to 3 in) long. Flowering occurs in every month except February, March, and April. After flowering, twigs continue to elongate from the ends of spikes to produce leaves or more flowers. Individual trees bloom from two to five times a year, but pronounced, regionwide flowering occurs at least twice a year. Soil type may influence time of flowering, and heavy rainfall may trigger flowering. In Hawaii, melaleuca flowers throughout the year (20). The species is monoecious, flowers are complete, and pollination is by insects.
Seed Production and Dissemination- Melaleuca's reproductive potential is prodigious (14,25). On average, 30 sessile seed capsules are left by one flower spike; a branch may bear 8 to 12 of these seed-bearing sections, often alternating with foliage, along a single axis. The capsules are hard, woody, squat, cylindrical, and brown and are aggregated in tightly packed files around the branches. A tree can hold seeds for more than 10 years. The seeds are tiny (30,000/g or 850,000/oz); a single capsule contains 200 to 350 seeds. Seeds are not released at maturity, but fire, frost, wind breakage, natural pruning, or damage by people interrupts the capsules' vascular connections, causing them to dehisce. While large numbers of seeds are typically released after injury, seedfall may occur year round. Ninety-nine percent of the seeds fall within a radius 15 times the height of the seed tree (8, p. 17-21). Fallen seeds can be spread by flowing water.
Seedling Development- Germination is epigeal. Dense reproduction occurs when fire prepares a seedbed and causes trees to shed millions of seeds. Seedlings averaging 2 m (6.5 ft) tall may be as dense as 3.5 million per ha (1.4 million/acre). If seedlings are submerged by water for several months, they may survive and resume growth. Seedling height growth may occur every month of the year, but growth is most rapid in spring to early summer and late summer to early fall. Natural seedlings rarely grow more than 1 m (3 ft) tall during the first year. However, seedlings planted at a density of 10,000 per ha (4,050/acre) grew 2 m (6.5 ft) in 6 months on drained muck soil (5,7,8, p. 23-28, 14,17).
Vegetative Reproduction- Melaleuca stumps readily sprout, and felled tops can root under very moist conditions. Root suckering is rare but can be profuse when it occurs (1,14).
Growth and Yield- The difficulty in determining ages of melaleuca trees has limited growth analyses (7,8, p. 23-28). A representative sapling stand may have 34,500 saplings per hectare (14,000/acre), and some areas have up to 158,000 per ha (64,000/acre). In Florida swamps, melaleuca stands that appear mature (fig. 1) may have 7,000 to 20,000 melaleuca stems per hectare (2,900 to 8,100/acre), outside bark basal area up to 133 m²/ha (580 ft²/acre), and a volume outside bark of 770 m³ /ha (11,000 ft³/acre). Average heights in these stands range from 15 to 21 m (49 to 69 ft). Maximum height is 30 m (98 ft). Stands on shallow or better drained soils contain substantially less volume than the swamp stands, although the density of stems may be equally high.
Trees in Hawaiian plantations (20) at age 40 on good sites average 50 cm (20 in) in d.b.h. and 18 in (60 ft) tall at a spacing of 6 by 6 in (20 by 20 ft). The largest trees there reach 90 cm (36 in) in d.b.h. and 24 in (80 ft) in height.
Rooting Habit- The root system of melaleuca is adapted to fluctuating water tables. The surface root network is complemented by abundant vertical sinker roots that extend at least to the water table's deepest annual level. During periods of surface flooding, "water roots" proliferate from permanent surface roots and submerged portions of the stem (14).
Reaction to Competition- Melaleuca rarely has to compete directly with other tree species in Florida because it mainly invades sparsely vegetated ecotones, prairies, marshes, and fire-damaged forests. It is classed as intolerant of shade. Melaleuca's presence in pine and cypress stands can cause an otherwise innocuous fire to become a crown fire that damages melaleuca only superficially but can kill the coniferous competition (8, p. 2935). Massive seed release typically follows, allowing melaleuca to preempt the site and form an almost pure stand. Pure stands with closed canopy strongly inhibit the development of understory vegetation, including advance reproduction of melaleuca seedlings.
Landscaping and lowering of water tables have accelerated the spread of melaleuca in Florida and increased the area that can be invaded easily. Melaleuca is a common ornamental in southern and central Florida; seed trees have, thereby, become widely distributed. Drainage and excessive use of ground waters shortens the annual hydroperiod, the effect being a substantial increase in large destructive wildfires (24). A general drying of the environment places most native wetland plants at a disadvantage relative to melaleuca, which successfully combines the tolerance of fire and seasonally low ground water levels with adaptations to seasonal flooding. In Hawaii, fire is not common and sites with impeded drainage do not dominate; therefore, melaleuca has little competitive advantage.
Damaging Agents- Melaleuca seems to be unusually free of disease, even in its native habitat (9,22). Although many insects, nematodes, and fungi have been found on melaleuca in Florida, none seriously damages the trees (8, p. 125-128). Severe freezes defoliate and kill branches of mature melaleuca even in extreme southern Florida, but trees generally recover by epicormic sprouting. Even when the cambium is killed to the ground line, sprouts arise from the root collar. Seedling kill from freezes, however, probably limits significant amounts of natural regeneration north of Lake Okeechobee. Melaleuca is rarely killed by fire; fire-damaged trees quickly recover by prolific epicormic sprouting.
In Florida, melaleuca is a common ornamental, but to many, an undesirable one because of its reputation for causing acute respiratory problems. Volatile substances produced by the tree have been implicated (16). Oils in the foliage and bark emit a medicinal fragrance; the nectiferous flowers emit an unpleasant, musty odor. However, clinical studies found neither the tree's vapors nor its pollen to be virulent irritants or allergens (8, p. 101-115). Respiratory problems attributed to melaleuca do not occur in Australia (10) or in Hawaii (20).
Melaleuca is not used in Florida or Hawaii for traditional timber products because its bark-to-wood ratio is high, the average stem diameter small, and the form poor. However, the wood is a suitable timber for such uses as pulp and cabinetry; the bark has potential uses as an amendment to plant potting mixes and in packaging and insulation (8, p. 37-68). The entire tree can be used as a biomass fuel but it is more difficult to use than most other species because of its powdery, low-density bark (8, p. 69-78). The leaves contain an essential oil (niaouli oil) that is extracted and sold commercially in New Caledonia (2,19). The virtually identical cajeput oil is derived from Melaleuca cajeputi in Indonesia. In Hawaii the tree was planted to conserve soil on deforested sites, and the tree has had many other uses in its native habitat (18,20,22).
The abundant flower crops of this insect-pollinated species are essential to Florida's large apiary industry (8, p. 79-80). However, melaleuca in Florida is viewed by many as an environmental threat that transcends its commercial value. Native vegetation is displaced and pure stands have dubious value to wildlife (8, p. 81-89, 91-98). Its consumption of ground water is suspected to substantially exceed that of native vegetation (8, p. 117-123). Buildings in melaleuca stands are exposed to a serious fire hazard (8, p. 29-35).
Melaleuca was initially introduced into Florida as seeds and probably originated from only a few trees in New South Wales, Australia (14). Records for these initial and possibly subsequent introductions are inadequate for determining provenance.
Racial differences have not been observed in Florida. In Hawaii, at least eight other melaleuca species are present on a minor scale (20). Melaleuca quinquenervia for many years was lumped with nine other species under the name M. leucadendron (L.) L., confusing the literature considerably (2).