How plant-soil interactions affect current and future invasions: Field and greenhouse plant-soil feedback results for three co-occurring southeastern invasive shrubs

Background/Question/Methods

Approximately one-third of forest invaders in the southeastern United States are woody shrub species. Many forests are invaded by multiple woody shrub species and little is known about the interactions of co-occurring invasive shrubs. Plant-soil feedbacks (PSF) are one mechanism explaining successful invasions. However, we know little about PSFs between co-occurring invaders, even though understanding feedbacks might inform us about how co-occurring species interact. To study this problem, I used three naturally co-occurring invasive woody species Ligustrum sinense, Lonicera maackii, and Rhamnus dauverica to test feedbacks within and among co-occurring invaders. Using field and greenhouse experiments I ask: (1) What are the feedbacks between three co-occurring invasive woody shrubs; and (2) are results from greenhouse and field plant-soil feedback studies similar? In the greenhouse, we conditioned uninvaded field soil with one individual of each invasive species and then grew individuals of each species in conditioned soil, sterilized soil, and uninvaded forest soils. In the field, we planted 1,080 seedlings of each invasive shrub underneath the canopies of the three invaders. We analyzed final plant biomass, plant height, and number of leaves, and calculated PSFs among each invader across soil treatments.

Results/Conclusions

We found evidence of facilitation among invasive species, which may promote further invasion and change abundance of woody invaders. In the greenhouse, we detected no feedbacks for Ligustrum, positive feedbacks for Lonicera in conspecific-conditioned soils, and positive feedbacks for Rhamnus in soils conditioned by other invaders. 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. In the field, we found that seedling survival in the first 15 weeks varied by species (F=34.45, p<0.001) but not by plot type (F=0.02, p=0.89). Across all plots, Rhamnus survival was 70.3% while Ligustrum and Lonicera survival was 35.8 % and 38.3%, respectively. However, in individual species models, Rhamnus survival in Rhamnus plots was 82%, which was significantly higher than in Lonicera (61%) or Ligustrum (70%) plots (F=3.20, p = 0.05). Survival of Lonicera and Ligustrum did not differ between plots.