Beyond single invaders: Disentangling the effects of co-invading alien herbs on sandy old-fields

Aims: Invasive alien plants can severely impact ecosystem diversity and function. While individual species’ effects are often studied, the interaction between multiple invasive species is less understood. This study examines how Asclepias syriaca and Solidago spp. (including Solidago gigantea and S. canadensis) influence taxonomic and functional diversity in sandy old-fields. The aims are to: (1) assess the individual and combined impacts of Asclepias and Solidago on resident plant diversity, and (2) determine whether interactions between these species alter their effects on the invaded community. Location Gödöllő Hills, Central Hungary. Methods I sampled 80 plots (2m × 2m) with varying levels of Asclepias and Solidago cover. Plant species cover was visually estimated, and trait data were obtained from databases. Species richness, community completeness, and community-weighted means (CWM) for traits like plant height and seed mass were calculated, along with Rao functional diversity. Generalized linear and mixed models were used to evaluate the effect of invasive cover on resident diversity and traits, both with and without considering the invasives' traits in community metrics. Results Solidago cover significantly reduced species richness and community completeness, while Asclepias showed no significant effect. When invasive species traits were excluded from the analysis, only the CWM and Rao diversity of the bud bank score were significantly related to Solidago cover. Including the invasives’ traits revealed that their trait values influenced community metrics significantly. Asclepias and Solidago cover were negatively correlated, but did not alter each other’s effects. Conclusions Solidago has a stronger negative impact on resident species richness than Asclepias, which appears neutral. The lack of interaction between these invaders suggests their individual impacts dominate over potential synergies or antagonisms.