Two co-occurring invasive woody shrubs alter soil properties and promote subdominant invasive species

1. Though co-occurrence of invasive plant species is common, few studies have compared the community and ecosystem impacts of invaders when they occur alone and when they co-occur. Prioritization of invasive species management efforts requires sufficient knowledge of impacts both among individual invasive species and among different sets of co-occurring invaders to target resources towards management of sites expected to undergo the largest change. Here, we observed differences in above- and below-ground impacts of two invasive woody shrubs, Lonicera maackii and Ligustrum sinense, among plots containing both shrubs (mixed), each species singly or lacking both species (control). We found additive and non-additive effects of these co-occurring invasives on plant communities and soil processes. Mixed plots contained two times more subdominant invasive plant species than L. maackii or L. sinense plots. Compared to control plots, mixed plots had three times the potential activity of -glucosidase, a carbon-degrading extracellular soil enzyme. L. maackii plots and mixed plots had less acidic soils, while L. sinense plots had higher soil moisture than control plot soils. Differences in soil properties among plots explained plant- and ground-dwelling arthropod community composition as well as the potential microbial function in soils. Synthesis and applications. Our study highlights the importance of explicitly studying the impacts of co-occurring invasive plant species singly and together. Though Lonicera maackii and Ligustrum sinense have similar effects on ecosystem structure and function when growing alone, our data show that two functionally similar invaders can have non-additive impacts on ecosystems. These results suggest that sites with both species should be prioritized for invasive plant management over sites containing only one of these species. Furthermore, this study provides a valuable template for future studies exploring how and when invasion by co-occurring species alters above- and below-ground function in ecosystems with different traits.