Crown area predicts total biomass for Rhodomyrtus tomentosa, an invasive shrub in Florida

Authors: Smith, Melissa; Pratt, Paul; Rayamajhi, Min

Submitted to: Invasive Plant Science and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2022
Publication Date: 3/15/2022
Citation: Smith, M., Pratt, P.D., Rayamajhi, M.B. 2022. Crown area predicts total biomass for Rhodomyrtus tomentosa, an invasive shrub in Florida. Invasive Plant Science and Management. 15(1):61-66. https://doi.org/10.1017/inp.2022.8.
DOI: https://doi.org/10.1017/inp.2022.8

Interpretive Summary: Downy rose myrtle, Rhodomyrtus tomentosa, is a persistent invasive plant in south Florida's pine understories. In order to accurately gauge the invasion and its impact, a first step is to determine the amount of biomass contributed by downy rose myrtle. We harvested multiple plants form multiple sites over multiple years to build a biomass prediction model based easily derivable plant metrics. We found that height and crown area (the two-dimensional area of the shrub crown based on two radii) were good predictors of biomass. We also found that biomass could be well predicted by crown area alone using a linear equation. Finally, no plants less than one meter tall produced fruit and for each centimeter increase in height, downy rose myrtle produces 0.32 more fruits. This has management implications because it suggests that tall shrubs should be removed to reduce both biomass and fruit production.

Technical Abstract: Predictive models of biomass derived from non-lethal measurements greatly assist in the inventory of natural areas. Where invasive species are concerned, this assessment is further needed to ascertain impacts of invasions and efficacy of management strategies. Furthermore, a rapid inventory allows for multiple assessments of impact over larger areas. Downy rose myrtle, Rhodomyrtus tomentosa (Aiton) Hassk, is an invasive shrub in Florida and Hawaii that is native to Southeastern Asia. Rhodomyrtus tomentosa was imported into Florida in the early 20th century through the ornamental plant trade and produces pink flowers and edible purple globe fruits. This woody shrub is particularly problematic in the understory of Florida’s mesic pine forests where it forms dense, impenetrable thickets. To more accurately characterize the populations in Florida and build a predictive model for biomass that could be used to inform control methods, we established a network of sites from which we harvested individuals over three years. Based on these measurements, we built a simple predictive model for R. tomentosa biomass utilizing a forward stepwise regression method. Additionally, because Rhodomyrtus tomentosa spreads by seed production, we sought to determine if seed production can also be predicted through this method. Crown area and height predict dry biomass well when incorporated into a linear model. We also found that crown area alone can accurately predict biomass in a linear model. We found that number of fruits positively correlates to plant height, but only after plants reach reproductive size – plants under one meter do not produce fruit. Here we demonstrate that two simple measurements of plant size - height and crown area - can accurately predict biomass in R. tomentosa in Florida and guide control methods by focusing on removing individuals larger than one meter tall.