Relationships between plant functional traits and community invasibility vary across invaders and abiotic conditions

Sarah Kimball, Center for Environmental Biology, UC Irvine, Jennifer Funk, Plant Sciences, University of California Davis, CA, Monica Nguyen, Chapman University, Megan Lulow, UC Irvine, Travis E. Huxman, Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA and Gregory Vose, Ecology and Evolutionary Biology, University of California Irvine, Irvine, CA

Background/Question/Methods
Functional traits of individual plant species influence community composition through assembly processes. Theory predicts trait convergence due to environmental filtering, and trait divergence due to biotic filtering and competition for resources. Learning how non-native, invasive species overcome these filters provides information regarding mechanisms of invasibility. In a large restoration project, different mixtures of native plants were added to the landscape and non-native establishment was measured to investigate relationships between plant traits and invasibility. Specifically, we asked what mix of natives was most resistant to invasion, how functional traits of native species related to invasibility, and whether relationships between traits and invasion varied depending on environmental or biotic conditions.
We measured cover, height, and biomass of invasive species in plots and used ANOVA to evaluate the role of native plant mix in invader performance in two different years on slopes with different aspects. We measured several functional traits of species and calculated community-weighted trait means, functional diversity, and the difference between invasive and native functional traits for each plot. We used multiple regression to evaluate whether community-weighted trait values influenced invader performance, and ANCOVA to determine whether the relationship between invader performance and community-weighted traits varied depending on slope aspect.
Results/Conclusions
Invasive species had lower performance in plots containing native shrubs and higher performance in plots containing native perennial bunchgrasses. Plots containing fast-growing natives had more of the fast-growing invasive, Brassica nigra. This supports trait convergence and environmental filtering. Plots containing natives with stress tolerant traits tended to be more heavily invaded by the invasive, Salsola tragus. Differences in water-use efficiency values of S. tragus and the native community were positively correlated with S. tragus performance, indicating biotic filtering. Several community trait metrics significantly influenced invader performance, although the direction of the response was not consistent across years and invaders. The environmental context, specific trait, identity of the invader, and native community restored were all important factors that interacted to influence the strength and direction of the relationships between native community traits and invasibility. We identified a complexity in the relative importance of trait convergence and divergence that is not consistent across varying resource availability or depending on trait spectra. Future research may focus on phenology, as temporal distribution of functional traits may clarify relationships to invasibility. It may be possible for land stewards to identify native functional groups that are more likely to prevent invasion in restoration.