The need to understand the effect co-occurring species have on one another is becoming increasingly important due to increased invasive plant richness. Invasional meltdown refers to the process that invasive species facilitate one another’s invasion in various ways, possibly increasing the magnitude of the impact. Here, we specifically examined invasional meltdown between two common invasive trees and one native tree of the southeastern U.S. We used a fully factorial design that consisted of planting combinations of each species interspecifically, intraspecifically, and alone. We randomly assigned these planting combinations to one of two fertilization treatments (fertilized and non-fertilized) in two blocks. The overall design was 9 planting combinations x 8 replicates block-1 x 2 blocks. We used Three Way ANOVA’s to examine if there were differences in species, neighbor, and fertilizer treatments for the dependent variables: total biomass, shoot, and root biomass, along with the competition index lnRR (natural log of the response ratio) and nodule production of Albizia and Robinia when grown in different planting combinations. lnRR = natural log of Pcontrol / Pmixture. For the biomass analyses, all variables were natural log transformed prior to analysis to meet the assumptions of normality.
Results/Conclusions
Three Way ANOVA’s indicate that there were no significant differences in the interactions of species, neighbor, and fertilizer treatments. Total biomass, shoot, and root biomass showed significant differences in neighbor and fertilized treatments. When examining the competitive index among these three species, no factor was significant. Nodule production did show significant differences among species and neighbor x fertilizer treatments. This finding contradicts what much of the literature suggests, in that nodule producing species produce more nodules when nutrients are limiting. Our research suggests that when under increased nitrogen applications, species will produce more nodules regardless of the neighbor. Further, these results suggest that given the current state of our environment, with increasing nitrogen added to the atmosphere, invasive nodulating species may up-regulate nodule production, which could potentially facilitate other invasives into areas where they would not normally invade.