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Melaleuca Quinquenervia Dominated Forests in Florida: Analyses of Natural-Enemy Impacts on Stand Dynamics

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Melaleuca Quinquenervia Dominated Forests in Florida: Analyses of Natural-Enemy Impacts on Stand Dynamics Plant Ecol (2007) 192:119–132 DOI 10.1007/s11258-006-9231-3 ORIGINAL PAPER Melaleuca quinquenervia dominated forests in Florida: analyses of natural-enemy impacts on stand dynamics Min B. Rayamajhi Æ Thai K. Van Æ Paul D. Pratt Æ Ted D. Center Æ Phillip W. Tipping Received: 4 April 2006 / Accepted: 23 October 2006 / Published online: 28 November 2006 Ó Springer Science+Business Media B.V. 2006 Abstract Melaleuca quinquenervia (melaleuca) absolute density and basal area/ha of melaleuca is a native of Australia but has become an invasive during the natural-enemy impacted period coin- plant in Florida, USA. We conducted a long-term cided with the increased incidence of the popula- demographic study of melaleuca in three sections tions of plant-feeding insects and fungi. The mean (central, transitional, and peripheral) of monocul- diameter at breast height continued to increase in ture stands located in Florida, and quantified all sections of the stands throughout the study absolute density, diameter at breast height and period. An increasing trend in basal area prior to basal area of trees by section at three sites. natural-enemy impact was reversed after increase Additionally, we monitored the impacts of natural in natural-enemy abundance and noticeable im- enemy (insects and fungi) on melaleuca popula- pact in all three sections of the stands. These tions which became apparent after 2001. Both findings lend support to a growing body of absolute density and basal area, from before literature that implicates natural enemies as (1997–2001) and after noticeable natural-enemy increasingly important density-independent regu- impact (2002–2005), were compared. Prior to the lators of M. quinquenervia populations. natural-enemy impact, absolute density of melal- euca trees declined primarily due to self-thinning Keywords Basal area Á Biological control Á and associated losses of small trees, but diameter Boreioglycaspis melaleucae Á Density Á Diameter at breast height increased, as did the basal area. at breast height Á Oxyops vitiosa Á Paratachardina Later during the period when natural enemies lobata Á Puccinia psidii prevailed, absolute density declined at a signifi- cantly greater rate across all sections but was highest at the periphery. The decrease in mean Introduction The U.S. Government’s right to retain a non-exclusive, Herbivores and pathogens, as integral compo- royalty-free license in and to any copyright is nents in natural systems, can play a role in acknowledged determining which plant species successfully nat- M. B. Rayamajhi (&) Á T. K. Van Á P. D. Pratt Á uralize or spread subsequent to introduction T. D. Center Á P. W. Tipping (Kosola et al. 2001; Crawley 1989). Only a few USDA – Agriculture Research Service, Invasive Plant papers report dynamics of plant invasions as Research Laboratory, 3225 College Ave., Fort Lauderdale, FL 33314, USA influenced by plant-feeding insects or plant e-mail: [email protected] pathogens, with the majority coming from the 123 120 Plant Ecol (2007) 192:119–132 biological control literature (Morris 1999; Paynter Florida Everglades systems (Hofstetter 1991; 2005). Such information could help in the formu- Bodle et al. 1994). In doing so, it has displaced lation of strategies for restoration of habitats native vegetation, degraded wildlife habitat, cre- degraded by invading plants. ated fire hazards, and caused human health Tree densities in both managed and natural problems (Morton 1969; Diamond et al. 1991). stands tend to decrease in a density-dependent The extent of melaleuca infestations in southern manner due to competition for resources such as Florida has been estimated at 0.20–0.61 million space, light source, and nutrients which leads to ha, an area encompassing 7–20% of the total shelf-thinning (Davies 2001; Zhang et al. 2005). region, and many remaining natural areas within Often, stand characteristics, such as stand basal this region are considered vulnerable to contin- area and biomass, increases (Binkley et al. 2004) ued invasion (Bodle et al. 1994). According to a despite decreases in stand density caused by the 1997 estimation, melaleuca still occurs in ca 0.17 mortality of suppressed trees. In general, these million ha of southern Florida (Bodle and Van trends in the stand attributes are negatively affected 1999) despite chemical and mechanical control if and when natural enemies colonize the system efforts (Laroche 1999). Agencies charged with the (Straw et al. 2005). Herein, we present a study that responsibilities of managing natural areas have reports the effects of a mixture of natural enemies shown interest in long-term environment-friendly (including two recently introduced biological con- methods to restore and maintain the ecosystems trol insects) on an exotic invasive plant. Australian of southern Florida (Bodle et al. 1994; Molnar native Melaleuca quinquenervia (Cav.) S.T. Blake et al. 1991). Therefore, an integrated control (hereafter ‘‘melaleuca’’) trees, considered to be one strategy that deploys multiple biological control of the most aggressive invaders of south Florida agents to supplement chemical and mechanical environments (Hofstetter 1991; Bodle et al. 1994), methods has been advocated for melaleuca con- is ideal for studying natural-enemy impact on the trol in Florida (Bodle et al. 1994; Turner et al. performance of an invasive plant. 1998). Melaleuca trees are erect, exceeding 20 m in Biological control of melaleuca was imple- height (Hofstetter 1991) with multi-layered, thick mented beginning in 1997 with the release of white or grayish papery bark that insulates the Oxyops vitiosa Pascoe, a specialist weevil whose trunk and branches (Holliday 1989; Bodle et al. larvae and adults feed on foliage and bud tissues 1994). Inflorescences are indeterminate, 2–5 cm (Center et al. 2000). Impacts caused by this long, and arranged in bottlebrush-like spikes weevil to melaleuca populations began to appear (Holliday 1989). Persistent capsular fruits develop in research plots during 2001. A second biological from flowers and are arranged in a series of control agent, the melaleuca psyllid [Boreioglyca- clusters, which may remain attached to the spis melaleucae (Moore), attacking foliage and trunks, branches, or twigs for several years inflorescence], was released during spring 2002 (Meskimen 1962). These capsules release copious (Center et al. in press). In addition, infestations of quantities of seeds when their vascular connec- the lobate lac scale [Paratachardina lobata lobata tions are disrupted by increased bark thickness or (Chamberlin) attacking green bark and foliage stresses such as fire, frost, mechanical damage, tissues] now impact melaleuca. This scale insect, herbicide treatments, or self-pruning of branches which is a true generalist known to infest numer- (Woodall 1982; Hofstetter 1991). Because of the ous plant species in several phylogenetically massive seed release from reproductively active unrelated plant families, became very prevalent trees, extremely dense melaleuca forests are in the south Florida landscape during the spring common (Hofstetter 1991; Van et al. 2000). of 2002 (Pemberton 2003). An unidentified sooty Standing biomass of 129–263 metric tons/ha has mold (indiscriminately covering foliage and green been reported for melaleuca stands in the United stems) is also associated with heavy infestations States and Australia (Van et al. 2000). of this scale. In addition, the rust fungus Puccinia Melaleuca has aggressively invaded many psidii Pascoe (attacking succulent stems and south Florida ecosystems, including portions of foliage), which is specific to plants in the family 123 Plant Ecol (2007) 192:119–132 121 Myrtaceae (Laundon and Waterson 1965; Marlatt evaluations of the impact of introduced natural and Kimbrough 1979), was recently discovered enemies on these attributes. These three sites heavily infecting the young foliage of melaleuca represented typical melaleuca habitats (nonfloo- trees (Rayachhetry et al. 1997). Attributes such ded and seasonally-flooded) in south Florida. as growth, reproductive performance, survival, Sites designated as ‘‘nonflooded’’ became inun- and rate of invasion of new areas determine dated intermittently for a few hours to several melaleuca’s invasiveness while stand density and days during or following periods of heavy rain but basal area determine its dominance at a given were not continuously flooded nor flooded every location. Based on the ability of the current year (hence not seasonally flooded). Soils in all natural enemies of melaleuca to inflict leaf- study areas were dominated by poorly drained damage, defoliation, twig dieback, smaller tree organic muck soils generally classified as histosols mortality (Rayachhetry et al. 2001; Pratt et al. (Brown et al. 1991). Understory vegetation in 2003, 2004a, b, 2005; Franks et al. 2006; Morath research stands at the onset of this study in 1997 et al. 2006) and the general patterns of self- was absent or, if present, was sparsely confined to thinning of trees of different stature, we hypoth- a few plants that occur sporadically under canopy esize that natural-enemy impacts in melaleuca gaps and at the invasion fronts of developing dominated stands will result in (1) a decreasing melaleuca stands. overall density in peripheral plots that have an abundance of smaller trees, (2) an increasing Plot establishment and data collection mean diameter throughout the stand due to mortality of smaller trees, and (3) a decreasing The research stands were typical melaleuca
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