Natural enemies thin melaleuca-canopy and help increase plant diversity in the melaleuca stands.

Authors: Rayamajhi, Min; Pratt, Paul; Center, Ted

Submitted to: Florida Exotic Pest Plant Council
Publication Type: Abstract Only
Publication Acceptance Date: 2/9/2008
Publication Date: 4/27/2008
Citation: Rayamajhi, M.B., Pratt, P.D., Center, T.D. 2008. Natural enemies thin melaleuca-canopy and help increase plant diversity in the melaleuca stands.. Florida Exotic Pest Plant Council.

Interpretive Summary: Melaleuca is a tree of Australian origin that became an invasive weed, displaced native plants, and form monoculture stand in many areas including the Everglades system in southern Florida. Therefore, control of this tree became essential to the conservation of native plants. It was assumed that the reduction in the density and canopy cover of melaleuca could help return native plant diversity in its monocultural stands. Natural enemies (insect and plant pathogens) that chronically feed and cause disease on melaleuca were considered environment friendly option to achieve the goal of reducing melaleuca populations and gradually increasing the plant diversity in the monoculture stands. After vigorous testing for their ability to feed on melaleuca only a weevil and a psyllid were imported from Australia and introduced to Florida in 1997 and 2002. These insects were able to cause substantially chronic defoliation of melaleuca trees. Two adventive natural enemies (lac-insects and a rust fungus) also became abundant during 2002 and intensified weevil and psyllid impact on melaleuca. As a result many melaleuca trees, especially of smaller dimension died and their density declined by up to 85% (within 8-year) where the populations of these natural enemies of melaleuca were well established. Remaining big trees in these sites lost up to 50% of foilages. Density decreased and loss of foliage facilitated light penetration to the forest floor. As a result other plant species returned to the once melaleuca monoculture sites. Within 5 years (2002 to 2005) of noticeable melaleuca canopy and density decline, a 2- to 4-fold increase in the number of plant species (mostly native) was evident in once melaleuca monoculture stand.

Technical Abstract: The Australian tree Melaleuca quinquenervia (Cav.) S.T. Blake (melaleuca) formed dense monocultural forests several decades after invading Florida and the Caribbean islands. These dominant forests have displaced native vegetation in sensitive wetland systems. We assumed that native plant diversity would increase following reductions in density of mature melaleuca stands in south Florida. We therefore examined data on changes in melaleuca densities, evidence of tree-canopy damage through biomass and litterfall analyses and plant species diversity derived from permanent plots that were monitored from 1997 to 2005. These plots were located within mature melaleuca stands in nonflooded and seasonally-flooded habitats. Two host-specific biological control agents of melaleuca, Oxyops vitiosa Pascoe and Boreioglycaspis Melaleucae (Moore), were introduced during 1997 and 2002, respectively. Also, an adventive rust fungus Puccinia psidii G. Wint and lac-scale insect Paratachardina pesudolobata Kondo & Gullan became abundant during the latter part of the study period. Aboveground biomass analyses showed reduced leaf (2.5-fold) and seed (7-fold) biomass from 1996 to 2003. Leaf-litter analyses for 1997 to 2005 period showed positive correlation between the melaleuca leaf damage in the canopy and species richness in the stand. Overall melaleuca density decline coincided with 2- to 4-fold increases in plant species diversity in the forest stand. The greatest declines in melaleuca density as well as the greatest increases in species diversity occurred in nonflooded as compared to seasonally-flooded habitats. The rapid increase in canopy-damage and reduction in melaleuca density during our 8-year study period may be attributed to the impact of natural enemies. Densities of other woody plants, particularly Myrica and Myrsine, which were sparsely represented in the understory also declined during the same period possibly due to infestation by the generalist lac-scale insect. These findings showed natural-enemy mediated reduction of melaleuca dominance which positively influenced plant diversity and facilitated the rehabilitation of degraded habitats.