Invasion impact of two goldenrods (Solidago canadensis and S. gigantea) on species diversity of plant guilds and soil environment

Two perennial goldenrods, Solidago canadensis and S. gigantea (Asteraceae), are considered successful plant invaders with an exceptional ability to affect the biodiversity and environment of native ecosystems in Europe. However, their species-specific invasion impacts and associated ecological mechanisms shaping the species diversity of plant guilds and the soil conditions in grasslands remain poorly understood. Here, a spatial invasion-gradient approach with different invasion levels categorised by the percentage of goldenrod cover (heavily invaded > 50%, transitional invaded 20–50% and uninvaded control plots) was used to assess invasion impacts on species diversity of forbs, graminoids and bryophytes as well as on soil and vegetation properties in semi-natural grasslands of Slovakia (Central Europe). Multi-guild diversity assessment showed that graminoids were more tolerant than forbs of the competitive and stressful conditions in invaded plots. Invasion level reduced species richness of forbs in heavily invaded plots, likely owing to intensified competition and litter accumulation generated by dense Solidago stands. No invasion effect was found on graminoid and bryophyte species diversity metrics. Most variation in species diversity and ecological responses was addressed to the shared invasion impacts of both Solidago species, but several species-specific invasion effects on forb diversity indices and soil chemistry (soil reaction, nitrogen and potassium concentrations, C-to-N ratio) were also identified. The contrasting forb responses in Shannon and Simpson diversity indices corresponded to shifts in community evenness associated with species-specific effects of the invaders on soil nutrient cycling and productivity. Goldenrod invasions also altered community structure, primarily through shifts in abundance-dominance interactions rather than through substantial species replacement. Our results highlight the importance of recording invasion impacts across functional guilds and ecosystem components to better understand the mechanisms of plant invasion in semi-natural grasslands.