We studied 11 widespread herbaceous invasive alien species of East-Central Europe and their 16 impact metrics (resident plant communities’ ecological characteristics, trait composition, functional diversity, and soil parameters) by sampling invaded and similar, uninvaded sites (space-for-time substitution method). Our aim was to (1) investigate the detailed ecological impacts of invasive plants on native plant communities; (2) explore the type of cover-impact relationships across impact metrics and their consistency across species; (3) study whether the cover-impact relationship depends on functional traits of invasive species. We present the data table with the 11 invasive species: the status of the sites (invaded, uninvaded), the cover of invasive species at plot level, the invasive species traits (lifespan, height, SLA, seed mass, clonal spread, flowering duration), community characteristics (species richness and diversity, native vegetation cover and bare ground cover at plot level), trait composition of native plant communities (native vegetation height, CWM height, CWM SLA, CWM seed mass, CWM clonal spread), functional diversity (functional richness, functional evenness, functional divergence, functional distance, RaoQ) and soil properties (N, P, organic C, pH).

We surveyed various regions of Hungary and Romania, where one of the chosen invasive species achieved dominance in the recipient communities. The chosen invasive species were: Ambrosia artemisiifolia L., Asclepias syriaca L., Erigeron annuus L. (Desf.), Erigeron canadensis L., Gaillardia aristata Pursh, Helianthus tuberosus L., Impatiens glandulifera Royle, Reynoutria japonica Houtt., Rudbeckia laciniata L., Solidago gigantea Aiton, Symphyotrichum lanceolatum Nesom. agg. To infer the effects of an invasive species on native plant communities, we used the space-for-time substitution approach. We chose seven pairs of sites for each invasive species: ‘invaded sites’, where the invasive species had considerable cover, and ‘uninvaded sites’, where the invader had no significant cover (< 5%) or was absent. 

In all sites, we sampled the vegetation in three 3 m × 3 m plots per site. Fieldwork was carried out between 2017 and 2019. Percent cover of native vegetation, bare ground, and all vascular plant species was visually estimated. To characterize the effect of invasive species on the trait composition of the resident plant communities, we measured the height of native vegetation and the specific leaf area (SLA) of native species, while the seed mass and clonality of native plant species were extracted from trait databases. We collected soil samples from five randomly selected cores per site and were mixed and analysed for pH, organic carbon (C) in humus, phosphorus (P) and all nitrogen (N) content.