Species Description: Melaleuca, Melaleuca quinquenervia, is a fast-growing and hardy tree with a slender crown and a layered, peeling paper-like white bark somewhat resembling that of a birch tree. Branches occur at irregular intervals off of the main trunk.
Melaleuca leaves are 10-15 cm long, evergreen, lanceolate, simple and short-stalked, parallel-veined, alternate in arrangement, and aromatic when crushed. The flowers are small and white, with multiple stamens, arranged in bottlebrush-shaped clusters at the branch tips. The fruits are small, round woody capsules containing 200-300 seeds (Langeland and Burks 1998, FLEPPC 1999, IFAS
Potentially Misidentified Species: Melaleuca are unlikely to be mistaken for any other species where it grows in Florida. The river birch (Betula nigra), whose papery bark is somewhat similar to that of melaleuca, is restricted to north Florida counties in which M. quinquenervia does not occur (University of Florida 2006).
Regional Occurrence: Melaleuca quinquenervia is a non-indigenous species whose native range includes Australia, New Guinea and the Solomon Islands (Langland and Burks 1998).
In Florida, melaleuca is restricted to the southern half of the state, and counties at the southern end of the state are most vulnerable. The northern limit of its potential range is commonly cited as Indian River County on the east coast and Pinellas County on the Gulf coast. However, melaleuca has been found naturalized as far north as Hernando, Lake, and Brevard counties, and even up into Volusia County on the east coast (Wunderlin et al. 1995, FLEPPC 2005).
IRL Distribution: Melaleuca quinquenervia is established in all six counties in the IRL watershed.
Age, Size, Lifespan: Melaleuca quinquenervia can grow to around 33 m in Florida at a rate of approximately 1-2 m/year (Langeland ad Burks 1998).
Abundance: Melaleuca quinquenervia is considered the most prominent of 60 non-native plant species presently invading the natural areas of south Florida. The species has invaded more than 200,000 hectares (nearly a half-million acres) in South Florida by 1994, including significant acreage within the Everglades (Langland and Burks 1998, Mazzotti et all 2002).
Reproduction: Melaleuca quinquenervia primarily propagates by sexual seed production. The species matures rapidly, and is capable of flowering within three (and as little as two) years of germination and as frequently as five times each year (Meskimen 1962, Laroche 1994b). In South Florida, the species blooms primarily during the winter months (November-January), although it is capable of some flowering throughout the year. Flowering is asynchronous both among trees and among the flowers of a single specimen (FLEPPC 1999).
Large M. quinquenervia specimens have a very high reproductive potential, and up to 20 million seeds per year are stored in the seed capsules of a single tree. Seed capsules must be dried out before they can release seeds to the environment. Seeds can remain viable within seed capsules for several (at least 10) years (Meskimen 1962, Langland and Burks 1998).
Physical damage such as broken or cut branches will trigger the rapid release of seeds from capsules on the injured branch. If a tree is felled or experiences a hot-burning fire this will trigger the shedding of all seeds within a few days (Woodall 1983).
Embryology: Percent seed viability is reported to be fairly low at 10-20%, although the seeds that are viable may remain so in their seed capsules for years (Meskimen 1962). Because trees can harbor several million seeds, the viable seedstock on each tree can reach a level of several hundred thousand seeds.
Manipulative studies suggest that the viability of released seeds is reduced by 50% after 8 months in the ground (FLEPPC 1999). Germination occurs in moist soils within several days of wetting, and inundated seeds can germinate or can remain viable for up to six months (Myers 1975, Lockhart 1996).
Temperature: Frost appears to limit northward expansion in Florida, although young trees may be the most susceptible to freezing; mature melaleuca have been observed to survive record-breaking freeze events. Freezing is yet another environmental stress factor known to trigger mass seed release (Woodall 1981).
Hydrology: Melaleuca quinquenervia responds optimally in seasonally wet sites, but is also able to flourish in standing water as well as in well-drained upland soils (Laroche 1994b). Wetter communities are usually more susceptible than drier sites to initial invasion and unusually wet years extend the flowering and growing season in M. quinquenervia (FLEPPC 1999).
Salinity is believed to be a limiting factor, although melaleuca is capable of growing in mangrove and backmangrove areas in both its natural and introduced range (Myers 1975, Center and Dray 1986).
Fire Tolerance: Young seedlings are usually killed by fire, but older individuals withstand fire well (Meskimen 1962, Myers 1975, 1983). Frequent fires facilitate the establishment of Melaleuca quinquenervia by triggering seed release, killing back competitors, and preparing the site for germination of seedlings (Myers, 1983, FLEPPC 1999). Melaleuca invasion alters community fire regimes, as when it displaces herbaceous species such as sawgrass, Cladium jamaicense (Flowers 1991).
Trophic Mode: Autotrophic (photosynthetic).
Associated Species: Within its native range Melaleuca quinquenervia thrives in low-lying wet areas that experience periodic fires. These are conditions similar to those found in a number of south Florida habitats. Florida habitats that have been invaded by melaleuca include natural systems such as lake margins, pastures, pine flatwoods, mesic prairies, sawgrass marshes, and cypress stands, as well as marginal habitats like roadsides, rights-of-way, and ditch banks. Habitat disturbance may facilitate invasion by this pioneering species but it is not a prerequisite condition (FLEPPC 1999).
In general, melaleuca stands in South Florida are reported to be little-used by native wildlife. Schortemeyer et al. (1981) reported an exception in a number of bird species (including anhingas, egrets, and herons) who were observed to utilize melaleuca stands where artificially deep water and extended hydroperiods had killed or damaged a significant number of native trees.
Invasion History: Melaleuca was brought to Florida through multiple introduction events in the early 20th century, originally for use as a landscape ornamental tree and also as a source of wood. The first two introduction sites were in Broward and Lee counties, where M. quinquenervia seeds transported from Australia were planted. In the 1930s, M. quinquenervia trees were planted as soil stabilizers on canal levees bordering the southern end of Lake Okeechobee and also in Big Cypress National Preserve. Seeds were also intentionally scattered from airplanes over the Everglades in the 1930s to facilitate the rapid establishment of melaleuca forests (Austin 1978).
The popularity of melaleuca as an ornamental species, as a windbreak, and for fence rows further facilitated spread of the species in Florida. Environmentally-mediated spread of melaleuca through South Florida and deep into the interior of the Everglades was also greatly facilitated by propagule transport via wind and water (FLEPPC 1999). Even decades later, the general South Florida distribution of melaleuca centers around those areas where the tree was originally introduced (Mazzotti et al. 2002).
As recently as 1970, M. quinquenervia continued to be recommended as "one of Florida's best landscape trees" (Langland and Burks 1998).
The speed at which melaleuca spreads and comes to dominate new areas has been described as explosive (Cost and Carver 1981, Hofstetter 1991). As little as 25 years is required for a one square-mile area to progress from 5% to 95% M. quinquenervia infestation (Laroche and Ferriter 1992). It poses a serious threat to ongoing Everglades restoration and preservation efforts, and a threat to South Florida's other natural areas as well (FLEPPC 1999).
Potential to Compete With Natives: Melaleuca is principally an invader of disturbed sites, but undisturbed native Florida habitats may exhibit a degree of resistance to invasion and establishment (Ewel et al. 1976). Canal banks, pine savanna, managed pineland margins, prairie and cypress marshes and sawgrass prairies are among the disturbed and undisturbed habitats susceptible to M. quinquenervia (Myers, 1975, Richardson 1977, DiStefano and Fisher 1983, Duever et al. 1986, Laroche and Ferriter 1992). Where it does become established, M. quinquenervia can form dense monotypic stands capable of displacing native plants (Richardson 1977).
Langland and Burks (1998) note that melaleuca is now recognized internationally as a threat to the Florida Everglades, a World Heritage Site and International Biosphere Reserve.
Possible Economic Consequences of Invasion: Although melaleuca appears to have minor positive economic benefit in Florida as a plant utilized by commercially managed honeybees, the negative impacts of invasion by this species are of far greater consequence.
Diamond et al. (1991) published a cost-benefit analysis for south Florida in which they contrasted a modest estimated annual benefit to the beekeeping and pollination service industry of around $15 million with an estimated $168.6 million a year gained through eco-tourism and other industries that would be lost in the event of complete infestation by melaleuca of the Everglades and other south Florida wetlands.
The World Conservation Union's Invasive Species Specialist Group (ISSG) lists melaleuca as among "100 of the world's worst invasive alien species" and recognizes the species as a potentially major driver of ecosystem change.
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