The influence of two wildfires and biological control agents on the population dynamics of Melaleuca quinquenervia in a seasonally inundated wetland

Authors: Tipping, Philip; MARTIN, MELISSA - Natural Resources Conservation Service (NRCS, USDA); FOLEY, JEREMIAH - Virginia Tech; PIERCE, RYAN - Alan Plummer Associates, Inc; GETTYS, LYN - University Of Florida

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2021
Publication Date: 2/22/2021
Citation: Tipping, P.W., Martin, M., Foley, J., Pierce, R., Gettys, L. 2021. The influence of two wildfires and biological control agents on the population dynamics of Melaleuca quinquenervia in a seasonally inundated wetland. Invasive Plant Science and Management. 14(1):3-8. https://doi.org/10.1017/inp.2021.4.
DOI: https://doi.org/10.1017/inp.2021.4

Interpretive Summary: The invasive capacity of Melaleuca quinquenervia may be reduced as evidenced by a 99% reduction in the size of recruitment cohorts following wildfires in 1998 and 2007 in the same area. In the past, prior to the implementation of classical biological control in 1997, mass recruitment events post-fire promoted this species to dominate landscapes. Today, seedlings, saplings, and large trees are now attacked continuously in Florida by three biological control agents. The density of the 1998 recruits was reduced by 96.3% and none had produced seed capsules by 2005. In addition to reducing the aerial seedbanks of reproductive trees, the biological control agents reduced the rate of growth and height of the recruited seedlings following the second fire in 2007. After three years, the natural mortality of new recruits (89.4%) increased slightly to 96.6% when attacked by the biological control agents. Recruits were 63% taller during the same period when protected from the agents using insecticides. There was no recruitment during the next three years despite seed rain and periodically favorable hydrological conditions. Newly recruited seedlings face not only high levels of natural mortality, but also constant herbivory from biological control agents, the combination of which will reduce their numbers, slow their growth, and inhibit the survivors from achieving the capacity to reproduce for many years. Land managers should target mature reproductive trees rather than cohorts of recruits to exert maximum suppression on M. quinquenervia populations.

Technical Abstract: The potential of Melaleuca quinquenervia (Cav.) S.T. Blake to re-invade cleared areas was evaluated over a 13 y period that included two wildfires and the introduction of biological control agents. The first wildfire occurred in 1998 and was followed by a mean of 591.5 recruited seedlings m-2. Recruits from that fire were cleared seven years later in July 2005 for a second experiment to evaluate seedling recruitment into cleared areas. Seed rain, seedling recruitment and mortality, and sapling growth rates were monitored in plots located around large individual reproductive trees. A second natural wildfire in 2007 burned through those plots leading to increases in seed rain followed by a pulse in recruitment of 21.04 seedlings m-2, 96.5% fewer than after the 1998 fire. Recruits in half of the plots around each tree were then treated with regular applications of an insecticide to restrict herbivory by biological control agents, while herbivory was not restricted in the other two plots. There was no difference in seedling mortality between treatments 1083 days post fire (2007) with 96.6% seedling mortality in the unrestricted herbivory treatment and 89.4% mortality in the restricted herbivory treatment. Recruits subjected to the restricted herbivory treatment grew taller than those in the unrestricted herbivory treatment, 101.3 cm vs. 37.4 cm. Many of the recruits were attacked by the biological control agents that slowed their growth. Although solitary M. quinquenervia trees retain some capacity to re-invade areas under specific circumstances, there was a clear downward trend in their overall invasiveness at this site with progressively smaller recruitment cohorts plus significant and damaging herbivory from biological control agents. Land managers should prioritize removing large reproductive trees over treating recently recruited populations which can be left for many years to the biological control agents to suppress before any additional treatment would be needed.