Summary1. Understanding the factors that drive commonness and rarity of plant species and whether these factors differ for alien and native species are key questions in ecology. If a species is to become common in a community, incoming propagules must first be able to establish. The latter could be determined by competition with resident plants, the impacts of herbivores and soil biota, or a combination of these factors. 2. We aimed to tease apart the roles that these factors play in determining establishment success in grassland communities of 10 alien and 10 native plant species that are either common or rare in Germany, and from four families. In a two-year multisite field experiment, we assessed the establishment success of seeds and seedlings separately, under all factorial combinations of low vs. high disturbance (mowing vs mowing and tilling of the upper soil layer), suppression or not of pathogens (biocide application) and, for seedlings only, reduction or not of herbivores (net-cages). 3. Native species showed greater establishment success than alien species across all treatments, regardless of their commonness. Moreover, establishment success of all species was positively affected by disturbance. Aliens showed lower establishment success in undisturbed sites with biocide application. Release of the undisturbed resident community from pathogens by biocide application might explain this lower establishment success of aliens. These findings were consistent for establishment from either seeds or seedlings, although less significantly so for seedlings, suggesting a more important role of pathogens in very early stages of establishment after germination. Herbivore exclusion did play a limited role in seedling establishment success. 4. Synthesis: In conclusion, we found that less disturbed grassland communities exhibited strong biotic resistance to establishment success of species, whether alien or native. However, we also found evidence that alien species may benefit weakly from soilborne enemy release, but that this advantage over native species is lost when the latter are also released by biocide application. Thus, disturbance was the major driver for plant species establishment success and effects of pathogens on alien plant establishment may only play a minor role.

The phylogenetic distance of an introduced plant species to a resident native community may play a role in determining its establishment success. While Darwin’s naturalization hypothesis predicts a positive relationship, the preadaptation hypothesis predicts a negative relationship. Rigorous tests of this now so‐called Darwin’s naturalization conundrum require not only information on establishment successes but also of failures, which is frequently not available. Such essential information, however, can be provided by experimental introductions. Here, we analysed three datasets from two field experiments in Germany and Switzerland. In the Swiss experiment, alien and native grassland species were introduced as seeds only with and without disturbance (tilling). In the German experiment, alien and native grassland species were introduced both as seeds and as seedlings with and without disturbance (tilling) and with and without fungicide application. For the seedling‐introduction experiment, there was an additional herbivore‐exclusion treatment. Phylogenetic distance affected establishment in the three datasets differently, with success peaking at intermediate distances for the seed datasets, but decreasing with increasing distances in the seedling dataset. Disturbance favoured seedling survival, most likely by weakening the resident community. Synthesis. By analysing experimental introductions, we show that the relationship between phylogenetic distance and establishment, at least for seedling emergence, may actually be nonlinear with an optimum at intermediate distances. Therefore, Darwin’s naturalization hypothesis and the preadaptation hypothesis need not be in conflict. Rather, the mechanisms underlying them can operate simultaneously or alternately depending on the life stage and on the environmental conditions of the resident community.

Dust seeds are the smallest seeds in angiosperms weighing just about a few micrograms. Th ese seeds are characteristic of most orchids, and several studies have been performed on seed features, fecundity and dispersal of orchid dust seeds. In this study we examine seed features, seed production and seed dispersal in another plant group with dust seeds, the Pyroleae (Monotropoideae, Ericaceae), focusing on six species: Pyrola chlorantha , P. minor , P. rotundifolia , Chimaphila umbellata , Moneses unifl ora and Orthilia secunda . Seed production per capsule among these species was in the range between ca 1000 and 7800, and seed production per capsule bearing shoot was in the range between ca 7000 and 60 000. Combining our results with published information on pollen-ovule ratios suggests that these Pyroleae species have a generally effi cient pollination system. Th e most fecund species was P. minor , the only species among the investigated that is probably largely self-pollinating. Th e investigated Pyroleae species have a seed production comparable to the less fecund orchid species. We studied seed dispersal in the fi eld in one of the species, P. chlorantha . Despite the extremely small and potentially buoyant seeds, the vast majority of seeds are deposited close to the seed source, within a few meters. Further studies on the recruitment ecology of the investigated Pyroleae species are currently under way.

The success of invasive alien and common native species may be explained by the same underlying mechanisms. Differences in intraspecific competition as well as differences in plant-soil feedback have been put forward as potential determinants of plant success. We teased apart the relative roles of competition and plant-soil feedback in a greenhouse experiment with 30 common and rare alien and native species from nine plant families. We tested whether plant biomass decreased less for common than rare species, regardless of origin, when grown at higher relative frequencies (1, 3 or 6 out of 9 plants per pot) in a community and in soil previously conditioned by the same species at different frequencies (0, 1, 3 or 6 out of 9 plants per pot) in an orthogonal design for these two factors. Plant survival decreased slightly, but non-significantly, for all species when grown in soil previously occupied by conspecifics. Among surviving plants, we found a decrease in biomass with increasing intraspecific competition across all species (regardless of origin or commonness), and alien species were more negatively affected by previous high plant frequency than native species, but only marginally significantly so. Our findings suggest that, while intraspecific competition limits individual biomass in a density-dependent manner, these effects do not depend on species origin or commonness. Notably, alien species but not natives showed a decrease in performance when grown in soil pre-conditioned with a higher frequency of conspecifics. In conclusion, soil-borne pathogen accumulation might be weak in its effects on plant performance compared to intraspecific competition, with neither being clearly linked to species commonness.