Melaleuca (Melaleuca quinquenervia), an invasive, non-native tree originating in Australia, has become well established in south Florida where it invades both disturbed sites and intact natural communities. Melaleuca is well adapted to fire and flooding, both of which are driving processes in the ecosystems of south Florida. Introduced in the early 1900’s it has quickly spread and covered almost a half million acres in south Florida as of 1993. The limiting factor in establishment seems to be adequate soil moisture to allow seedling establishment without prolonged flooding that causes seedling death. Once established melaleuca survives under a variety of moisture regimes regardless of soil type. Infestations tend to start as single outlier trees that become dense monocultures over time displacing native plants and animals. Animals do use melaleuca stands, but to a lesser extent than native habitats.

Melaleuca is extremely well adapted to fire. Thick bark and the ability to sprout from anywhere on the stem or branches not killed by fire provide the ability to recover after even intense fires. Fires also release large quantities of minute seeds from serotinous capsules into a fire prepared seedbed. Only the smallest individuals are killed by fire, often for only 1 year following germination or until saplings reach 1 m in height.

Melaleuca has altered the fire regime of south Florida by altering fire behavior. Melaleuca foliage is extremely flammable due to the arrangement of foliage and to its chemical content. The papery bark of melaleuca acts as a ladder fuel carrying fires into the canopy. Dense melaleuca stands are the only vegetation type in south Florida that will support and perpetuate crown fires. Native vegetation often succumbs to the intense crown fires that melaleuca supports. Canopy species such as pine and cypress, though adapted to surface fires, are often top killed by intense melaleuca crown fires and subsequently outcompeted.

Prescribed fire managers charged with maintaining the health of Florida’s natural communities must continue burning, but must take measures to prevent fire from accelerating the spread of melaleuca. This approach is most effective if it is aimed at containing starting infestations (i.e., scattered melaleuca within natural lands). Fire can be used as one of several tools to help control melaleuca if applied judiciously. Fire managers must avoid fire triggered seed release into fire prepared seedbeds. Instead, fire managers must use an alternate method, often herbicides, to trigger seed release into intact fuel beds and subsequently burn seedlings and saplings while they are in a fire-susceptible stage. Fire is not a recommended management tool for dense melaleuca stands.

Prescribed fire managers, particularly in south Florida, must consider the effects of non-native, invasive species on their burn program and conversely the effect of their burn program on invasive species. Melaleuca poses several problems with regards to burning programs. Fire alone often aids the spread of Melaleuca by triggering massive seed release (Myers and Belles 1995, Laroche 1999). Land stewards managing lands infested with melaleuca often ask two questions: 1- How can my burn program designed to benefit and perpetuate native plant communities be carried forward without promoting the spread of melaleuca? 2- How can a burn program be used to aid the elimination of melaleuca? The answer to both questions involves active control measures in addition to fire.

The melaleuca management plan (Laroche 1999) has adopted a strategy of containment of infestations. The first course of action is to eliminate outlying individuals that may serve as foci for seed release and population spread. This is an important point in burn programs because fire triggers seed release. When managing areas with outlying individuals managers are advised to:

• Apply herbicides (see Laroche 1999 for recommendations and further details) to trigger seed release into an intact fuel bed. Germination will be less successful than in a newly burned area and fire may be applied at a later date to kill seedlings while in a fire susceptible stage. Monitor the site for germination, which is triggered by adequate moisture levels. Monitor the cohort(s) of seedlings for growth rate and apply fire before seedlings leave the fire-susceptible size classes. Virtually all seedlings less than 6 months old and 90% of seedlings less than 2 years old can be killed by fire. Growth rates vary greatly from site to site necessitating monitoring. The best kill rates of seedlings can be achieved before they reach 1 m in height (Myers and Belles 1995).
• If herbicide application is not possible prior to burning due to other management concerns or because of budgetary constraints (see the Florida Department of Environmental Protection invasive plant management program for funding assistance on public conservation lands), then there are several options:
  • Protect outlier trees from fire and exclude thickets of melaleuca from burn blocks to avoid seed release (Wade 1981).
  • If it is not possible to exclude all outlier trees, conduct burning at a time when seed will be released just prior to conditions unfavorable for seedling survival. For example:
    • Apply fire early in the wet season prior to rising water such that seedlings will germinate and then be flooded.
    • Apply fire prior to the onset of a lengthy drought period, at the end of the wet season such that seeds will germinate on a moist seedbed then the dry season ensues causing seedling mortality.
    • Chemically treat resprouting adults before they replenish seed and burn again as soon as fuel conditions warrant (Myers and Belles 1995).
    • This technique is ecologically risky and much less desirable because moisture regimes reliant on the weather cannot be controlled or predicted with accuracy.
    • Monitoring germination and seedling growth is important in the success of this method, but failure may occur because more than one episode of germination can occur depending on moisture conditions, germination may be delayed if normal rains fail to materialize, and if dry periods are interrupted with rainfall, seedlings may survive (Laroche 1999, Myers and Belles 1995).
    • In addition, without efforts to kill adult trees, they will resprout and will have replenished seed stores prior to the next fire cycle.
• Land stewards must be aware of other environmental factors that may trigger seed release from melaleuca such as frost, or herbicide treatments. In the case of frost, it is possible for managers to burn seedlings established within the intact fuel bed. Preferably, these tactics should be coupled with an integrated control strategy. Alternatively, the potential additional seed release stimulated by burning should be evaluated and the benefits weighed. Burning after herbicide treatments should be part of the integrated management plan.
• Control of the massive melaleuca regeneration following a wildfire will involve not only fire but also extensive chemical control efforts (See Myers and Belles 1995 or Laroche 1999 for details and recommendations).
• Fire is not a recommended management tool for dense stands of melaleuca. However, dense stands of melaleuca pose significant wildfire threats, particularly when they are in urban interface areas. Common control tactics utilized in the southeast are often unsuccessful in control of crown fires (Wade 1981).

Melaleuca is well adapted to fire and can recover after fire both vegetatively and sexually. Melaleuca often survives even intense fires. Melaleuca, common name paper bark, has white, flaky, spongy bark that can reach more than 3” thick at maturity. Like many other fire resistant trees, this bark insulates the vascular cambium from heat generated by fires. Melaleuca can sprout from any point below that killed by an intense fire: along the stem, at the root collar or from roots (Wade 1981, Bodle et al. 1994, Myers and Belles 1995). Production of epicormic branches along the bole in response to injury by fire can enhance reproductive potential by creating large numbers of new twigs and thus, many flowering sites.

Melaleuca has a very high reproductive potential in which fire plays a major role (Wade et al 1980). Seed release from serotinous capsules is triggered by fire. Melaleuca can flower within 3 years of germination and may flower up to 5 times per year (Meskimen 1962 in Laroche 1999). Minute seeds are held in persistent woody capsules along branches. Each capsule can hold 200 to 300 seeds (Meskimen 1962 in Laroche 1999) and each tree holds many thousands of capsules. Capsules open when they desiccate (often when the vascular connection with the parent tree is broken). This can be in response to radial growth, to shading and death of lower branches, to frost, to herbicides, and to fire (Myers and Belles 1995).

Neither seed capsules nor seeds are generally endangered by fire. Opening of capsules takes several days following desiccation and seed capsules are not usually consumed by fire because they are dense woody structures and because crown fires typically do not have long residence times (Wade 1981).

Seed rain after fire is affected by fire type and by the stand structure. Most seed (95%) is released within 5 weeks of a fire; however, several thousand seed per square meter continued to fall 5 weeks after fire (Myers and Belles 1995). Extremely dense stands tend to produce fewer seeds than scattered open- grown individuals or stands with mature emergent seed-baring trees (Myers and Belles 1995). It has been estimated that a 10m tall open-grown tree can release around 20 million seeds (Alexander and Hofstetter 1975 in Myers and Belles 1995). An intense crown fire that kills branches will release more seed than a surface fire that causes little vegetative mortality (Myers and Belles 1995, Wade 1981). Mature trees can replenish stored seed within 2 years (Myers and Belles 1995).

Seedling establishment and survival is affected by fire, flooding and drought. More germination occurs on burned sites than unburned sites, i.e., fire prepares a seed bed (Myers and Belles 1995) and episodes of germination tend to be triggered by the presence of adequate moisture levels. Seeds may remain dormant on a site for months. It appears that seed viability may be limited after 1 year. Prolonged periods of flooding and dry conditions cause seedling mortality. Fires kill 90% of seedlings less than 20cm tall. Some saplings are killed by fire until they reach 2-3 m tall at which point saplings survive fires (Myers and Belles 1995).

Fires may also aid melaleuca dispersal, establishment and persistence. Fire has been postulated to remove vegetation that hinders the transport of tiny seeds by water, thus increasing dispersal distances via water. Fire removes competing vegetation. Fire also creates a safe site for melaleuca seedlings because recent fires “fireproof” areas by removing fuels. Fire also liberates nutrients for use by plants (Wade 1981). Late dry season fires (April and May, when the most acreage is burned in the Everglades) allow Melaleuca to get its seed on the ground before most other dominant tree species.

Melaleuca is a true fire-type species. It not only persists and reproduces after fire, but its presence promotes fire. Melaleuca foliage is highly flammable. It contains volatile oils that promote burning. The papery bark of melaleuca acts as a ladder fuel that can carry fire into the canopy. Further, the arrangement of aerial fuels can create intense crown fires. The bark can also act as a firebrand as it is transported in the smoke plume starting spot fires (Wade 1981).

Melaleuca stands have created a novel fire environment in south Florida. Dense melaleuca stands are the only vegetation type in South Florida that will support a crown fire. Melaleuca stands have been characterized as having a mixed fire regime, i.e., able to support a high intensity fire with low mortality (Myers 2000). In addition, melaleuca stands can support a variety of fire behaviors ranging from low intensity surface fires after which little seed is released to high intensity crown fires after which huge amounts of seed are released (Myers 2000, Wade1981). Vegetation adapted to the natural fire regime in South Florida, one of frequent surface fires, does not fare well with the addition of crown fires. High intensity crown fires often kill pine or cypress in forested communities invaded by melaleuca. Dense stands of melaleuca shade out herbaceous vegetation and reproduction from other canopy species (Myers 1983) (Wade et al. 1980).

Melaleuca tends to perpetuate itself. Crown fires create a sunny environment on the forest floor by removing the canopy. Melaleuca has been found to germinate better in sun than in shade (Meskimen 1962). In addition, loss of foliage reduces transpiration, thereby increasing soil moisture, which is a requirement for successful melaleuca establishment. Established trees then resprout (Wade 1981). Elimination of native vegetation and creation of nearly monospecific melaleuca stands results.

Melaleuca (Melaleuca quinquenervia), a native of Australia, has become well established and invasive in southern Florida, USA. It occurs in most counties south of Hernando, Lake and Brevard Counties (Wunderlin 2000). Melaleuca invades disturbed sites such as fallow farm land, urban areas, canal banks, and spoil piles, but the larger ecological problem is that it invades in-tact natural plant communities such as pine flatwoods, Everglades tree islands, cypress forest edges, freshwater marshes, sawgrass marshes, and the non-saline end of mangroves (references within Mazotti et al. 1997, and Laroche 1999). (See Everglades, and Slash Pine for a description of plant communities)

Melaleuca is well adapted to fire and flooding, both of which are driving processes in the ecosystems of south Florida (See Everglades and Slash Pine). Melaleuca was introduced into Florida in the early 1900’s (Laroche 1999) at two coastal locations, one in SE Florida (Broward County) and the other in SW Florida (Lee County). In the 1930’s it was planted along the south rim of Lake Okechobee and in the Big Cypress National Preserve (Laroche 1999). Since then its expansion has been rapid. Though melaleuca seeds don’t typically spread far, once it has gained a foothold, it can claim a site forming monospecific stands. For instance, a 1 square mile plot of sawgrass marsh 5% infested with melaleuca was observed to reach 95% melaleuca in 25 years. As of 1993 over 450,000 acres of south Florida were infested with melaleuca with the heaviest infestations around areas of original introductions (Laroche 1999). Large-scale coordinated efforts have gone into the control of melaleuca in south Florida because of the ecological threat melaleuca poses (See also Exotic Plants). These efforts have been successful on publicly owned conservation lands and as a result, the acreage of Melaleuca has decreased since 1993.

Melaleuca can establish and thrive in a variety of soil types and along a gradient of moisture regimes. Melaleuca tends to invade wet areas more than dry areas (Hofstetter 1991 in Myers and Belles 1995). Even exceptionally wet or dry sites may become invaded by melaleuca in dry or wet years as the soil moisture regime becomes favorable for melaleuca seedling establishment (Bodle et al.1994). Seedlings cannot withstand long periods of flooding or excessive soil drying. Once Melaleuca is established, however, it is able to withstand a variety of moisture conditions. Soil type does not seem to limit the establishment of melaleuca. It can become established on mineral soils, organic soils or marl soils, though it tends to do better on acid sands or organics rather than marl (Myers 1983).

Melaleuca infestations often start as scattered single outlier trees (Wade et al 1980) that become dense monocultures with closed canopies and little understory (Wade et al. 1980). Melaleuca is well known to displace native vegetation and reduce diversity of plant and animal communities (Laroche 1999). Plant species associated with Melaleuca are generally remnant plants that were representative species of the natural community melaleuca invaded. Melaleuca shades herbaceous vegetation, and tends to eliminate other woody vegetation (see Fire Regime). Shade tolerant ferns such as swamp fern (Blechnum serrulatum) are common associates of melaleuca (Sowder and Woodall 1985).

Animal communities associated with melaleuca tend to be less diverse and have lower abundance than those associated with native plant communities. Melaleuca heads (tree islands composed of tall, mature melaleuca trees at the center ringed with saplings and seedlings) are used by some rodents: cotton mice (Peromyscus gossypinus), short tailed shrews (Blarina carolinensis), and a few cotton rats (Sigmodon hispidus) (Mazotti et al. 1981, Ostrenko and Mazzotti 1981), Though Peromyscus were more abundant in melaleuca heads than in sawgrass prairies, the diversity of rodents in melaleuca heads was not representative of other native habitats (Mazotti et al. 1981 Ostrenko and Mazzotti 1981.). A few birds have been observed using melaleuca heads. Snail kites will use melaleuca trees as perches when other native vegetation is not available. Other birds will use melaleuca for nesting in the absence of other native vegetation (Schortermeyer et al. 1981).