Co-occurring invasive plant species are increasingly common, and there is evidence that invader interactions affect future invasions. Plant-soil feedbacks (PSF) are one mechanism explaining successful plant invasions; non-native plants tend to create positive feedbacks when grown in soils cultivated by natives or conspecifics. However, we know little about PSFs between co-occurring invaders, even though understanding feedbacks might inform us about how co-occurring species interact and affect communities. To study this problem, we used three naturally co-occurring invasive woody species Ligustrum sinense and Lonicera maackii, both with a long history at our site, and Rhamnus dauverica, a recent addition to the local flora, to test feedbacks within and among co-occurring invaders. We ask: 1-What are the feedbacks between two common, co-occurring invasive plants? And 2-How do soils cultivated by these invasive plants influence the growth of a newly arrived invader? We grew the two long-term resident invaders individually to create “invaded” conditioned soils, and then added seeds of the two resident species and the most recent invader in 3 soil treatments: Ligustrum-conditioned, Lonicera-conditioned, and uninvaded soils (live field inocula). We analyzed final plant biomass, plant height, and number of leaves and calculated PSFs among each invader across soil treatments.
Results/Conclusions
We detected no feedbacks for the earliest invader, Ligustrum sinense, across all three soil types. Lonicera maackii, however, performed better in uninvaded soils compared to invaded soils. Lonicera individuals grown in uninvaded soils had 23% more aboveground biomass and 15% more belowground biomass than individuals grown in Lonicera conditioned soils, and 43% more aboveground biomass and 41% more belowground biomass than in soils conditioned by Ligustrum. This translated into small, but significant, negative feedbacks for Lonicera in invaded soils. Conversely, the newly arrived invader Rhamnus dauverica showed significant positive feedbacks in invaded soils compared to uninvaded soils. When grown in uninvaded soils, Rhamnus had 50% less aboveground biomass and 35% less belowground biomass than in soils conditioned by Ligustrum, and 42% less aboveground and 12% less belowground biomass than in soils conditioned by Lonicera. These results suggest that, in addition to inhibition, facilitation can occur among non-native species and may promote further invasion. Also individual feedbacks vary between invasive plants, and with time we may see changes in abundance and composition of co-occurring invaders.