Potential interactive effects between invasive Lumbricus terrestris earthworms and the invasive plant Alliaria petiolata on a native plant Podophyllum peltatum in northeastern Ohio, USA.

We test whether the invasive earthworm Lumbricus terrestris and leaf litter of the invasive herbaceous plant Alliaria petiolata interact to influence the native plant, Podophyllum peltatum, using both observational field data and a multi-year experiment. We hypothesized invader interactive effects on the native plant might result from either changes in allelochemical distribution in the soil or nutrient availability mediated by the invasive earthworm pulling leaf litter down into the soil. Within the field data we found that Alliaria petiolata presence and higher soil nitrogen correlated with reduced Podophyllum peltatum cover, and no evidence for an invader-invader interaction. Within the factorial experiment, we found a super-additive effect of the two invaders on plant biomass only when activated carbon was present. In the absence of activated carbon, there were no differences in Podophyllum peltatum biomass across treatments. In the presence of activated carbon, Podophyllum peltatum biomass was significantly reduced by the presence of both Lumbricus terrestris and Alliaria petiolata leaf litter. The absence of an effect of Alliaria petiolata leaves without activated carbon, combined with a failure to detect arbuscular mycorrhizal colonization, suggests that indirect effects of allelochemicals on arbuscular mycorrhizal fungi were not the primary driver of treatment responses. Rather direct nutrient availability might influence a potential interaction between these invaders. Leaf nitrogen content was higher and leaf CO2 concentration was lower in the presence of Lumbricus terrestris, but treatment did not influence maximum photosynthetic rate. While the field data do not suggest a negative interaction between these invaders, the experiment suggests that such an interaction is possible with greater environmental stress, such as increasing nitrogen deposition. Further, even plants with rapid physiological responses to increased nitrogen availability may have other physiological limits on growth that prevent them from compensating from the harm caused by multiple invaders.

Tests of alternative evolutionary models are needed to enhance our understanding of biological invasions.

Contents Summary 701 I. Introduction 701 II. Why we need an explicitly evolutionary perspective 702 III. A case study invasion experiment 702 IV. The way forward 703 V. Conclusions 705 Acknowledgements 706 References 706 SUMMARY: Comparing models of trait evolution might generate new insights into the role of evolutionary history in biological invasions. Assumptions underlying Darwin's naturalization conundrum suggest that close relatives are functionally similar. However, newer work is suggesting more complex relationships between phylogenetic and functional distance. We present an example in which communities of close relatives are functionally divergent in leaf traits and have greater invader biomass. Such an approach leads to new questions, such as: When might selection lead to divergence between close relatives? For example, a history of sympatry might correspond with divergence. We suggest that moving beyond a simplistic version of Darwin's naturalization conundrum as alternative hypotheses will lead to a more nuanced view on how evolution has shaped biological invasions.

Spatial heterogeneity of plant-soil feedbacks increases per capita reproductive biomass of species at an establishment disadvantage.

Plant-soil feedbacks have been widely implicated as a driver of plant community diversity, and the coexistence prediction generated by a negative plant-soil feedback can be tested using the mutual invasibility criterion: if two populations are able to invade one another, this result is consistent with stable coexistence. We previously showed that two co-occurring Rumex species exhibit negative pairwise plant-soil feedbacks, predicting that plant-soil feedbacks could lead to their coexistence. However, whether plants are able to reproduce when at an establishment disadvantage ("invasibility"), or what drivers in the soil might correlate with this pattern, are unknown. To address these questions, we created experimental plots with heterogeneous and homogeneous soils using field-collected conditioned soils from each of these Rumex species. We then allowed resident plants of each species to establish and added invader seeds of the congener to evaluate invasibility. Rumex congeners were mutually invasible, in that both species were able to establish and reproduce in the other's resident population. Invaders of both species had twice as much reproduction in heterogeneous compared to homogeneous soils; thus the spatial arrangement of plant-soil feedbacks may influence coexistence. Soil mixing had a non-additive effect on the soil bacterial and fungal communities, soil moisture, and phosphorous availability, suggesting that disturbance could dramatically alter soil legacy effects. Because the spatial arrangement of soil patches has coexistence implications, plant-soil feedback studies should move beyond studies of mean effects of single patch types, to consider how the spatial arrangement of patches in the field influences plant communities.

Functional trait values, not trait plasticity, drive the invasiveness of Rosa sp. in response to light availability.

PREMISE OF THE STUDY: Functional trait plasticity in resource capture traits has been suggested as an underlying mechanism promoting invasive species establishment and spread. Earlier studies on this mechanism treat invasiveness as a discrete characteristic (i.e., invasive vs. noninvasive) and do not consider the potential impacts of evolutionary history. In the present study, we used a continuous measure of invasiveness and a phylogenetic framework to quantify the relationship between functional trait expression, plasticity, and invasiveness in Rosa. METHODS: In a manipulative greenhouse experiment, we evaluated how light availability affects functional traits and their plasticity in Rosa sp. and the out-group species, Potentilla recta, which vary in their invasiveness. KEY RESULTS: Across functional traits, we found no significant relationship between plasticity and invasiveness. However, more invasive roses demonstrated an ability to produce a more branched plant architecture, promoting optimal light capture. Invasiveness also was linked with lower photosynthetic and stomatal conductance rates, leading to increased water-use efficiency (WUE) in more invasive roses. CONCLUSIONS: Our results suggest that functional trait values, rather than plasticity, promote invasive rose success, counter to earlier predictions about the role of plasticity in invasiveness. Furthermore, our study indicates that invasive roses demonstrate key functional traits, such as increased WUE, to promote their success in the high-light, edge habitats they commonly invade.