Invasive × Sorbaronia fallax nothosubsp. mitschurinii affects temperate Scots pine forest biodiversity and functioning

Invasive species affect biodiversity and ecosystem functioning, but their impacts along the abundance gradient has been insufficiently recognized. Here we aimed to assess the impacts of invasive × Sorbaronia fallax nothosubsp. mitschurinii (hereafter × Sorbaronia) spread into temperate Scots pine forests on understory vegetation biodiversity and ecosystem functioning. We hypothesized that × Sorbaronia will affect light transmittance into the forest floor, chemical composition of soil and litter, stand and litter production, and understory vegetation. We assessed the set of experimental plots along the invasion gradient in homogenous soil conditions in Western Poland. Around the initial propagule source (abandoned plantation) we established 66 study plots in a systematic design. Using generalized linear mixed-effects models and redundancy analysis we assessed the response of studied predictors to × Sorbaronia biomass. We showed that the studied taxon decreased light transmittance as well as understory cover and species richness of vascular plants. We also confirmed an increase in soil P and litter P, K, and Ca contents. Therefore × Sorbaronia modified the biogeochemical cycles of temperate Scots pine forests. We did not find an effect of the studied taxon on carbon sequestration, due to no impacts on the litter and soil C pools, or on stand productivity. Revealed impacts are scalable with invasive taxon abundance, expressed as aboveground biomass. The impacts of × Sorbaronia are similar to the impacts of the functionally and phylogenetically close widespread invasive species. The studied taxon has a high potential to negatively affect biodiversity and alter the functioning of temperate Scots pine forests, fulfilling the criteria of ‘transformers’ as an invasive taxon. Our results provide quantitative evidence for nature conservation and risk assessments related to the spread of the studied taxa.