Summary• Exotic plant invasions can alter ecosystem processes, particularly if the invasive species are functionally different from native species. We investigated whether such alterations can be explained by differences in functional traits between native and invasive plants of the same functional group or by differences in functional group affiliation.• We compared six invasive forbs in Europe with six native forbs and six native graminoids in leaf and whole-plant traits, plasticity in response to nutrient supply and interspecific competition, litter decomposition rate, effects on soil nutrient availability, and allelopathy. All traits were measured in a series of pot experiments, and leaf traits additionally in the field.• Invasive forbs differed from native forbs for only a few traits; they had less leaf chlorophyll and lower phosphorus (P) uptake from soil, but they tended to have a stronger allelopathic effect. The invasive forbs differed in many traits from the native graminoids, their leaves had lower tissue densities and a shorter life span, their litter decomposed faster and they had a lower nitrogen-use efficiency.• Our results suggest that invasive forbs have the potential to alter ecosystem properties when invading graminoid-dominated and displacing native graminoids but not when displacing native forbs.

Question: Invasive alien plants can affect biomass production and rates of biogeochemical cycling. Do the direction and intensity of such effects depend upon the functional traits of native and alien species and upon the properties of the invaded habitat, with the same alien species having differing impacts in different habitats?Location: Lowlands of Switzerland.Methods: Fourteen grassland and wetland sites invaded by Solidago gigantea and widely differing in biomass production and soil P availability were surveyed. To determine whether the impact of the species was related to site fertility, we compared the invaded and native vegetation in terms of biomass, species composition, plant traits and soil properties.Results: S. gigantea generally increased the above-ground biomass production of the vegetation and soil C content, while reducing nutrient concentrations in biomass and N availability in the soil. However, it had no significant effect on plant species richness, soil respiration, soil pH and P availability. Leaves of S. gigantea had a greater C content than those of native species; other leaf traits and root phosphatase activity did not differ significantly. Conclusions:Our results suggest that a conservative nutrient-use strategy allows S. gigantea to invade a broad range of habitats. The observed effects of invasion did not vary according to biomass production of the invaded sites, but some effects did depend on soil P availability, being more pronounced at more P-rich sites. Thus, the full range of invaded habitats should be considered in studying the potential impact of plant invasions on ecosystem processes.

Summary1. Plant-soil feedbacks can influence the success of non-native plant invasions. We investigated if these feedbacks and the underlying invasion effects on soil microbes and nutrients depend on the species composition of the invaded vegetation, and whether these effects are related to differences in the invasibility of native plant communities. 2. We carried out a mesocosm experiment simulating the invasion of Solidago gigantea into three wetland plant communities (Molinion, Magnocaricion and Filipendulion), each composed of five plant species but differing in productivity. To study plant-soil feedbacks, we used different soil inoculum types from invaded and non-invaded field sites of the corresponding communities and a reference site. Invasion success was assessed by measuring the biomass of S. gigantea after three growing seasons and by analysing soil properties several times during the experiment. 3. Invasion success varied significantly among communities and soil inoculum types. Solidago gigantea produced more biomass in the Molinion than in the two more productive communities. In all three communities, it exhibited a negative feedback upon itself, producing 31-46% less biomass when the substrate was inoculated with soil from a stand invaded with S. gigantea. 4. The presence of S. gigantea did not influence total biomass in any community nor N and P availability in soil. However, it led to a decrease in soil bacterial and an increase in soil fungal biomasses. These changes were similar in the three communities and unrelated to the biomass of S. gigantea biomass in the invaded communities. 5. Synthesis. The experimental comparison between effects of an invasive plant species on soil properties in different native communities showed similar effects despite pronounced differences in the ability of the invasive species to grow in the different communities. In this system, plant-soil interactions may thus affect invasion, but not explain differences in the invasibility of different communities. The invasive species increased soil fungal biomass, particularly in its own soil, compared to native species and experienced a negative feedback, suggesting that the course of its invasion might be affected by species-specific soil pathogens.