Cajeput tree (Melaleuca quinquenervia)

Introduction:

The broad-leaved paperbark tree or melaleuca (Melaleuca quinquenervia) can reach heights of 25 meters and hold up to 9 million viable seeds in a massive canopy-held seed bank. This fire-resistant wetland-invader aggressively displaces native sawgrass and pine communities in south Florida, alters soil chemistry and modifies Everglades ecosystem processes. Melaleuca is notoriously difficult to control, however, bio-control (integrated with herbicidal and other methods) holds a promising alternative to traditional control methods.

Description:

The Australian broad-leaved paperbark tree (herein referred to as “melaleuca”) is a member of the Myrtaceae family, which comprises about 4000 to 5000 tropical and temperate species. Recent research incorporating DNA studies has resulted in an improved classification of the family. Earlier classifications in which a primary division was based on the possession of dry or fleshy fruit are not congruent with the evolutionary relationships shown by analysis of DNA sequence data.Melaleuca, which now includes Callistemon, belongs to the tribe Melaleuceae.

Melaleuca can reach 25 meters in height and grow to 90 centimeters in diameter. It is easily recognised by its spongy flaking bark (white, cream, orange-cream, fawn-grey or dark grey in colour (Craven, in press), lanceolate five-veined leaves, and clusters of woody seed capsules. Its white papery bark resembles birch and its white flower clusters resemble bottlebrush. Its white tufted inflorescences are indeterminate, two to five centimeters long and arranged in bottlebrush-like spikes . Flowers of M. quinquenervia, like most Myrtaceae, have numerous stamens on a cup-shaped hypanthium surrounding the ovary. Myrtaceae leaves are simple and entire and the plants are usually aromatic , some have an intense citrus-like or eucalyptus oil odour when crushed .Within the spike-like inflorescence, flowers are clustered in threes; and secrete nectar, which collects within the base of the hypanthium; the stamens are arranged in five bundles each bundle consisting of five to ten fused stamens; the petals and filaments are usually white or creamish. The capsular fruits, 2.7-4mm long, may persist for several years.

Occurs in:

Agricultural areas, natural forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed, scrub/shrublands, urban areas, wetlands.

Habitat description:

In its native range melaleuca occurs in seasonally and permanently inundated wetlands along the eastern coast of Queensland and New South Wales, Australia (11ºS to 34ºS). Australian habitats that support melaleuca populations include low-lying coastal wetlands behind heath-dominated headlands, riparian zones, brackish estuaries, mangrove swamps, Melaleuca swamp forest, monsoon scrub, littoral rainforest, grassland, open forest, low shrubland on coastal dunes and lagoon margins. In its invaded territories, melaleuca can infest relatively drier areas and invades a variety of forested and non-forested natural communities, including: freshwater marshes, wet grasslands, sawgrass prairies, disturbed cypress forests, wet pine flatwoods, Miami rock ridge pinelands, longleaf-slash pine, hardwood hammocks, salt marshes and mangroves.

Management:

Current management methods for melaleuca include herbicides, manual removal of plants, prescribed fires and bio-control.

Preventative Measures: Preventative measures are the best form of weed control. Education on the potential threats posed by melaleuca on invaded ecosystems should be targeted at the nursery industry and the general public.

Monitoring and Mapping: Model projections suggest there is considerable scope for further invasion of melaleuca under current climate conditions, with the highest risk areas occurring in Southeast Asia, the Caribbean, South and Central America and the Gulf coast in southern USA.

Physical:Mechanical removal using heavy equipment is not appropriate in most natural areas because of disturbances to soils and non-target native vegetation; however,

this method of control can be applied along canal and utility rights-of-way.

Physical: Physical methods also include the use of prescribed fire and of flooding More information is needed on the timing of prescribed burning, and constraints to this method include impacts on non-target species, the triggering of mass seed release by trees and liability concerns.

Chemical/Herbicidal Control: Exotic woody vegetation is most frequently managed by herbicides. Hexazinone and tebuthiuron are most effective in the control of melaleuca, however, they are no longer allowed to be applied directly to water in Florida. Current chemical control recommendations for melaleuca include low volume applications of glyphosate for control of saplings, and aerial or individual stem (girdle) applications of imazapyr alone, or in combination with glyphosate for mature trees.

Biological control: The lack of a long-lived soil seed bank makes M. quinquenervia vulnerable to herbivore-mediated reductions in fitness and delays in reproductive maturation. As canopy-held seed banks continue to diminish over time, seedling suppression is predicted to have long-term effects on plant density. Two bio-control agents, the Australian melaleuca snout weevil (Oxyops vitiosa) and the Australian melaleuca psyllid (Boreioglycaspis melaleucae), have been approved by the USDA for use against melaleuca and have been released in the field. Research is being conducted on at least six other potential bio-control agents, including leaf, stem tip, and flower bud feeded.

Legislative: Melaleuca is on both the United States’ Federal Noxious Weed List and the Florida Prohibited Aquatic Plant List (Class I Prohibited aquatic plant) (Florida Department of Environmental Quality).

Integrated management: As a result of the implementation of the integrated Melaleuca Management Plan 1999 almost 100 000 acres of natural area have been cleared of melaleuca.

The Areawide Management Evaluation of Melaleuca quinquenervia (TAME) aims to demonstrate the effectiveness of integrated control of melaleuca in invaded habitats in the United States and elsewhere.